antman89iroc

Here is an article I found very interesting and is a must read if you are interested in valve train geometry.

http://www.sbintl.com/tech_library/a...m_geometry.pdf

In a nut shell is says that most of the rocker arm geometry we have been told, and specifically, how we determine pushrod length is wrong. The rocker arm must be at a 90* angle with the valve stem at half lift. It gives a very straight forward method of establishing push rod length. It's the first time this has been described in a way that can be measured rather than "black magic" and trial and error.

I am interested in hearing what those of you who have extensive experience with this subject have to say.

Orr89RocZ

iTrader: (20)

Thats what i do with my shaft rockers now because i can place the rocker anywhere i want for the most part. Stud mount setups are limited but this is a good practice to try if you can do it.

AlkyIROC

Same thing with my shaft rockers. There's a tool that comes with the rockers that allows you to determine where 1/2 lift is with some simple math. I can't lower the rocker pedestals but if they're too low, they can be shimmed up slightly. When I first got mine, the intake mounts were far too low for my long intake valves. I contacted T&D and they replaced them with some taller mounts for the cost of shipping. My shaft mount rockers also corrected pushrod angles. Corrected them so much that I had to modify a few holes through the head so that the pushrods would clear.

That's why when you are rolling the engine over to look for the witness mark, you're looking for the narrowest contact patch. It doesn't matter if it's directly in the center of the valve tip but too far to the edge of the tip is wrong also. The smaller the contact patch, the better.

With a SBC, the simple plastic pushrod length tool is the best thing to use to determine pushrod length. Tool is off the valve tip and the pushrod is too long. Tool doesn't touch the pushrod and the pushrod is too short. Plain and simple.

antman89iroc

Some of the things I found most interesting are how pushrod length/rocker position are related directly to lift, that the contact pattern is a crude way to attempt to obtain proper geometry and that contact pattern location isn't a priority. Using the pushrod length to "center" the contact patch would actually be the wrong move.

Mostly though, using closed valve positioning, proper geometry can be established by measuring tip and trunion axis locations and setting pushrod length to achieve that location. This method seems straight forward and analytical vs cranking the motor over until you get a narrow patter.

I also enjoyed the technical history and found it helpful in understanding what we are actually trying to achieve. The story about Harland Sharp making a miscalculation on his initial roller rocker design, and other companies copying his mistake was priceless.

This all came up for me because I am having some valve train issues. I went back and rechecked my setup and found my pushrods are .060" too short now. And these were set up by a professional shop and had nice, centered contact patterns.

z 28 jari

iTrader: (1)

Good video:

antman89iroc

That's a good video. I like how he used the stud/adjuster to "measure" the rocker position. Simple.

Abubaca

iTrader: (3)

I'm at work now, but I'm looking forward to reading the article and watching that video. I'm getting ready to measure for my pushrods in the next week or so, and I've never done it before. I'm pretty detail oriented, but like many things in the motor.....being off just a little can cause big problems. I won't lie...I'm a little anxious about it! ...anyhow, thanks for posting.

antman89iroc

Related to this, how much rocker-stud thread engagement should I consider the minimum before I go to a longer rocker stud? I've always used the 1 1/2 times the diameter of the bolt as a safe amount but it seems less is ok. Possibly stud diameter or even a little less. I'm adding lash caps and as the rocker goes up on the stud, the available threads gets shorter. Especially since the pushrod needs to be lengthened as well. Just wanting to keep it safe.

antman89iroc

I performed my setup just like the video but I have a concern. The rocker roller is very close to the exhaust side of the valve. I am going to attempt to check it at half lift to see how close it gets but it does have me thinking. I am using some .080 tall lash caps which makes the assembly higher and with the rocker stud angle pushes the tip to the exhaust side even more. I want this to be correct and have determine that before my pushrods were about .250 too short! I know the article I posted says don't worry about the contact position but I wonder if fudging down a bit would be a safe idea.

sofakingdom

iTrader: (1)

Yeah we see people post what Jim has written over the years, all the time; and it's true, to a point, as far as that goes.

First problem of course, is getting the whole "90°" thing right. Consider, you have rockers from 2 different mfrs that are IDENTICAL IN EVERY WAY, EXCEPT one of them has the push rod seat recessed ¼" farther up into the rocker than the other. HOW are you going to measure that?

Next problem is, even assuming you get the above thing right, when do you suppose that the wear on the guide mostly occurs? Would you say, it occurs mostly when the valve is on its seat? just off the seat? partway open? most of the way open? all the way open? I think you would agree, that would be related to the amount of force applied to the tip (which in turn is DIRECTLY proportional to lift) i.e. the tendency to wear is ALWAYS GREATEST at higher lift. So apart from the "last .0001 of lift" type of argument, where is it more important to have the rocker pushing straight down on the valve stem?

To answer the last post, I'll have to defer to an individual who was so intelligent that he is completely beyond even my imagination, let alone knowledge. He invented a technique for taking what we know, and pushing its boundaries a little more and a little more at a time, from experiments we CAN perform, to things there's NO WAY we can experience here on Earth. The man in question was Albert Einstein (you may have heard of him?) and his technique was the "thought experiment". Let's do one, except there'll be just a little bit of physical observation involved, but MOSTLY, we're going to be THINKING instead of reading magazine articles.

First notion we need to disabuse ourselves of, is that "max valve lift" is the be-all end-all of the whole deal. If you're building a drag racer that you replace the entire valve train every pass, then yeah, you could go that way; but most of us are out here in THE REAL WORLD, and we're going to be driving this thing on the street AT LEAST A LITTLE. Now, if you count engine rotations in a ¼ mile run, and compare that to engine rotations in a beer run to your local Kroger, I think you'll figure out IN A BIG HURRY that on the street AT ALL, even ONCE over the lifetime of an engine, ya gotta come off of the "max lift" kind of mentality, and adjust it for "not have to rebuild it every time I drive it". Capische? Longevity, reliability, non-self-destruction, is MORE important than "every possible .001 HP". So what would we want so as NOT to tear up in our street engine?

How about, something we can't even change out ourselves AT ALL, even if we tore the engine down and put in a new valve train every time we cranked the thing up? That would be, THE VALVE GUIDES. And the way they tear up, is SIDE LOADING. Which is, force applied to the valve PERPENDICULAR TO ITS LENGTH that tends to mash the valve stem up against the inner surface of the guide. I think you would agree, that would be a combination of, the amount of force applied to the tip (which in turn is of course DIRECTLY proportional to lift... spring tension), in combination with, the degree to which the force on the valve stem IS NOT applied directly along its length. In other words, the tendency to wear the guides is GREATEST at the combination (multiplied together) of full lift, and when the geometry is poorest. So let's direct our thought experiment in such a way as to find the best possible situation for THAT pair of parameters, that we just identified as THE MOST CRITICAL.

Place a pencil on your desk. (valve stem) Place the tip of a 2nd pencil (rocker arm tip) EXACTLY in the center of the eraser of the 1st pencil (roller in center of valve tip) and push EXACTLY in line with the 1st pencil (perfect geometry). The pencil goes straight (no side loading on the valve guide), right? OK, now move the tip of the 2nd pencil off out to the VERY EDGE of the eraser (rocker roller lands out toward the edge of the valve stem). Again, push EXACTLY IN LINE WITH the 1st pencil. Pencil STILL goes straight along its length (NO side loading), right? OK, do both of those things acoupla more times just to reinforce the picture clearly in your mind. Now, once you've become convinced that you're looking at reality and not some kind of a trick, place your 2nd pencil's tip in the VERY CENTER of the 1st pencil, and push the 2nd pencil at a 30° angle away from straight in line with it. (rocker is SLIDING across the valve stem tip instead of pushing straight along it) Which way does the 1st pencil go? Do it again until the picture is completely clear. Again, no tricks here, no "sleight of hand", no "it's a car so it's a giant mystery", no "it's a car so the laws of physics don't apply", no "I heard at the McDonalds parking lot last Friday night", no magazine articles. JUST REALITY.

So what do you want your valve train setup to do?

Answer is, IT DOESN'T FORKING MATTER where on the valve stem the rocker tip lands, as long as it's securely on there (doesn't concentrate too much force onto a small area of the tip and tear it up). I'd say, just as a rough guide, if it's within the middle 2/3 of the valve stem at all times ESPECIALLY NEAR FULL LIFT, everything is good. Because, when it's near ZERO lift, there's no force on anything to begin with, so if you're going to have error which you ALWAYS will at SOME point in the rocker sweep, THAT'S where you want to move it to.

So yeah, Jim's "mid lift" method is THEORETICALLY pretty damn good, the problem then being, how do you find it. Easiest way is, by using a check spring and a solidified lifter IDENTICAL TO the lifters you're going to install, and adjusting the push rod until you find the NARROWEST SWEEP ACROSS THE VALVE STEM TIP, then using a slightly shorter push rod than that. Emphasis on SLIGHTLY. Like, you'd round down to the next shorter .050" increment that they're available in, and if it's already really close to that, maybe one .050" increment farther. You MIGHT give up acoupla .001"s of lift, MAYBE, but you'll optimize the LIFETIME of every part in the whole system.

IGNORE where on the valve stem tip the rocker tip falls, as long as it doesn't seem to be hanging over the edge; and instead, focus on finding the NARROWEST SWEEP, and go from there.

Damon

[edit] What I typed below I must have done at almost the same time as Sofa. He raises a lot of the same points I do but in more detail.[edit]

1X stud diameter is OK for rocker nut thread engagement. Wouldn't go less than that. Ideally, you want the top of the lock nut to be flush with the top of the adjusting nut as this yields MAXIMUM thread engagement for BOTH. How to get: Get rocker geometry established properly. THEN choose your final stud length to set the lock nut at the right height to be flush with the adjusting nut. Longer stud = lock nut higher.

FYI- the mothership (GM) does NOT USE THE 1/2 LIFT METHOD for rocker geometry. They shoot for about 2/3 max lift. That final 1/3 of lift towards max the rocker barely moves across the valve tip. WHY? Because that way most of the rocker tip's travel across the valve tip happens at low lift when spring pressures are much less. Decreases wear on the parts, doesn't "flex" the valve stem as much at high RPMs.

If you've ever wondered why stock length sticks usually seem to be a little on the short side for most applications, this is why. It's INTENTIONAL AND BY DESIGN. Shoot for narrowest pattern and then take about 50 thou off the pushrod length and you'll be roughly in the ballpark.

If you are building a motor that's going to see a lot of street use, I'd recommend you follow GM's lead and set your valvetrain up like this. I do and my motors never have valvetrain problems. I'm very **** about valvetrain setup.

antman89iroc

Thanks for your input. Yeah, I suppose that because of lack of experience I sometimes question my own thinking. That's why I ask questions sometimes, like thread engagement on the stud, I'm just cross checking my own thinking. I agree that the contact point on the valve tip doesn't affect geometry but I wondered if others have had experience on "how close to the edge is pushing your luck".

I'm not looking for max power or even max lift. My goal is to setup a reliable, quiet (as possible) valve train. Using my brain and imagination it seems to me that if you jerk the valve open in the first 1/3 of lift that you would be slamming it shut in the last 1/3 of closing- noise and probably bouncing valves on seats. Which is exactly what I suspect was happening before when my noisy valve train pounded a pushrod into a rocker cup.

I sincerely appreciate your thoughts and comments, that's what I hoped to accomplish by bringing attention to the article, a different way of thinking. Sofa you are right, many time I forget to trust my own instincts and deduction and you have given my confidence a shot in the arm.

AlkyIROC

Look at the factory setup. The stamped steel rockers are highly inaccurate. Camshaft grind wasn't the same in every engine but all the SBC engines used the same length pushrods and rockers. They were quiet and lasted 100,000+ miles unless something failed like a failed lifter, wiped out lobe, worn chain set etc. It wasn't the best system but it worked just fine.

For a daily driver where maximum power isn't high on the requirements list but you've changed enough parts where some valvetrain geometry needs to be adjusted, perfect length is not required. If you can get anywhere near to perfect, it will be good enough.

skinny z

When it's all said and done, the best result is as narrow a sweep as possible and centred on the valve tip.
Trying to get my geometry sorted out meant that as I approached the ideal push rod length (via the half lift method) and the sweep became narrower, it also pushed the pattern towards the exhaust (as the OP experienced). So much so that at the ideal geometry a portion of the roller tip would be off the edge of the valve. I stress off.
Using Crower's .050" offset trunnion rocker positioned the rocker assembly closer to the intake side of the head and gave the necessary room to get the narrowest contact that also rode across the centre line of the valve stem. I'd say the sweep was about 30 to 40 thou when all was said and done. Not perfect but close.
I've known more than one engine assembler that gave in on getting his VGT just right because his particular selection of parts pushed to contact off the edge of the valve. Their remedy is to sacrifice the ideal.

skinny z

You might find some interesting reading here.

https://www.thirdgen.org/forums/tech...nclusions.html

antman89iroc

You just summed up my experience. I spent the weekend on this and the best pushrod length put the contact patch .125" from the edge of the lash cap which would be about .070" from the edge of the valve stem. I was too worried about running it with almost no safety margin that I put the original pushrods in my motor and my brand new pushrods back in their box. (BTW the rocker angle and contact patch with short pushrods is pretty much like what Comp recommends.)

With short pushrods the contact patch is very well centered with a ~.125" wide pattern.

With long pushrods the contact patch is darn near the exhaust side of the valve stem with a .070" wide pattern.

Here is something I found very interesting. At one point I had one short (Comp setup) and one long (mid-lift setup) installed in the same cylinder. When I rotated the engine there was a noticeable difference in effort to compress the springs. The long rod setup took less effort than the short rod did. Kinda makes me think I'd be better off getting off set rockers and running the longer pushrod than.

skinny z

The best result is as narrow a sweep as possible. .070" is wide compared to the less than .030" that's achievable with lifts in the mid 500s.
This is precisely why Crower makes the offset trunnion rocker. Every head and combination is a little different. In my case, due to the 1/10th longer than stock valves, there was no way I could get the contact to stay on the valve and get a narrow sweep. At one point, before the offset rockers, I gave up and settled for a wider contact and more off centre than I cared for. End result...guides done in a few thousand miles.

antman89iroc

Yeah, my guides are probably done too which is why I didn't want to push the contact too close to the edge, in case things move around when running.

Anyway, I've had trouble locating offset trunnion rollers. Do you have any part numbers or how did you search?

skinny z

Search: Crower Enduro Centerline Rollerized Rocker.
Let me know how you make out. It comes as a PDF file otherwise I'd copy and paste the link.

antman89iroc

Thanks, got it. Did you use the SST or aluminum?

skinny z

Steel all the way. I'd never use an aluminum rocker in my application.

AlkyIROC

My stud mounted rockers were SS. The shaft mounted rockers are aluminum but with no large hole through center of the rocker shaft, the stresses on the rocker are much less.

antman89iroc

I found the Crower off set rockers. They are pretty expensive. ~$450 alum ~$550 for the stainless. At that price point it starts to make the shaft rocker system look reasonable. Man if I had known this from the beginning I would have just gone with shaft rockers.

It makes me think though, I'm not doing anything super special. Dart block uncut, AFR heads uncut, .600 lift cam 155#/450# springs, it seems that an issue like this would come up for many users of AFR heads. Heck if they'd just back the stud holes up the .050-.100 it would make life a lot easier and cheaper. Still Comp recommends the "old method" of having the rocker tip on the intake side when closed, center at mid lift and on the exhaust side at full lift. This setup causes a .125" contact patch. I wonder if Comp recommends this same method with their shaft setups?

skinny z

If you think about it, you ARE doing something special. Aftermarket block and heads. Someone else's rocker arms and lifters. There are so many variables that a statement like AFR's is just that, a statement. I'm sure all of those cylinder heads sitting on the floor at the machine shop aren't there for no reason. Mostly guides I bet. And centred on the valve tip at mid lift is what you're looking for but that's secondary to having the narrowest sweep.
Enter the offset rocker. Or in many cases, a shaft mount system. The guys here that have the shaft setup have explained how there's a gauge supplied to help determine pushrod length. Even then, there's no guarantee that it will produce perfect results.

skinny z

And out of curiosity, whose lifters and rockers are you using?

antman89iroc

Well, fair enough, what I meant is that I'm not doing anything out of the ordinary for someone who's laying out enough cash for a set of AFR heads. My experience is probably typical and could be accounted for in stud placement.

I'm using a Comp hyd roller .600" lift cam ground on a .850" base circle (yes, verified) with Comp Hyd roller short travel retro lifters & Comp pro magnum steel full rollers 1.6. Afr 210 heads with spring upgrade for hyd roller and as-supplied valves. (I believe they may be +100 tall) and I'm using .080 lash caps because the valve tips were getting eaten up.

What really boggles my mind is that everything I can find from Comp recommends the old shoe rocker contact method where the contact moves across the tip and back. I'm not reinventing the wheel here and it just seems the head/rocker manufacturers would recognize this and account for it. Regardless, I do understand better what is going on here and do want to get the most stable, reliable valve train I can afford. I appreciate your comments and suggestions too.

skinny z

What Comp recommends falls in line with the mid lift method. Ideally, you want contact to start inboard, be centred at half lift and fully extended at max lift. I can see where an .080" lash cap in addition to a .100" taller valve would add to the overall problem of getting the right length pushrod in there. Keep in mind, that for every head AFR cranks out with a relocated rocker stud, there'll be a bunch of builders that need it on the original location. And I'm not sure how much AFR or any of the head manufacturers care about your valve train geometry once you've purchased their product.
On a similar note, why would your valve tips be getting damaged other than perhaps the rocker tip riding clear off the edge of the valve or some very large valve lash dimensions?

antman89iroc

I agree with you on the fact that my problem is just that, my problem.

However, I disagree in the Comp recommendation is what I understand as "mid lift". If mid lift achieves 90* to the valve stem at mid lift then the rocker tip would be coming back to the intake side for the second half of lift, wouldn't it?

In regard to my situation with valve tips I believe it is a compound problem. First, I have likely encountered some valve float. Also, because AFR relocates the intake valve and pushrod location the intake rocker sits at a noticeable angle. The arc of the rocker is now at compounded angles with the intake valve. Of course everyone with AFR (and most other aftermarket) head experience the same thing. However, the builder who set this up to begin with used the Comp method which left a wide contact patch. I believe this and the rocker angle probably caused or made more probable the valve float. I could have over revved the motor but I normally shift at 5800 which should be safe if everything is right to begin with.

skinny z

The rocker tip will continue to advance across the valve tip until max lift is reached. At that point, the direction is reversed and the rocked tip will retreat to the intake side. That will transmit maximum camshaft data to the valve and at the same time minimizes the sideways forces applied to the valve guide.
5800 should be an easy rpm limit on the valve train.

Damon

"The arc of the rocker is now at compounded angles with the intake valve."

Could you explain that a little further? I you're saying the valve tip and the rocker stud angles don't match, you have an issue. They HAVE TO travel parallel to eachother or it will put all the load on one side of the rocker's roller tip PLUS introduce frictional loading 90* to the direction the roller tip is capable of rolling. That's what you call "death" in valvetrain geometry.

I know AFR moves things around in subtle ways on their heads, but rolling a valve angle without also rolling the rocker stud angle to match should not be one of them. BBC guys know all about "rolled" valve angles, but on a SBC it's a much rarer bird.

AFR's website says NOTHING about "rolled" valve angles on the 210 heads.

antman89iroc

Sure, I may have used improper terms but this is how I inderstand it.

AFR moves the intake (and exhaust for that matter) over some amount from the stock location. At the same time they move the pushrod location some amount in the other direction. The result is an "offset" of about .250". (if you refer to jessel shaft rocker specs for these heads it will show how they account for this) Their literature even states to only use their guide plates for this reason. When rockers are installed there is a noticeable angle to the intake rockers. So from the front view there are the angles of the pushrod, rocker stud and valve stem. From the top view there is now an angle too. That is what I mean by compound angles. A lot of mfg do it but it screws up the rotation of the rocker and induces a scrub across the valve stem. I believe that is why shaft rockers have the offsets they do to allow for the offset but keep the rocker pivoting on the proper axis.

sofakingdom

iTrader: (1)

Comp's instructions might be correct for SOME engines, BUT NOT small block Chevy.

In the SBC, the valves are at a considerable angle compared to the studs. That is, they're not parallel. The stem is angled toward the stud substantially. As a result, as the stem gets pushed down, the tip gets farther and farther away from the stud. Even if the rocker pushed down in a straight line (which of course it doesn't; it moves in an arc) the contact point would move toward the intake side ALOT as it gets near full open. The arc motion just adds to the tendency for the contact point to move toward the intake side at full lift EVEN MORE.

Then, consider "ratio". All "rocker ratio" is, is the ratio of the distance between the valve stem and the stud, to the distance between the push rod and the stud. As you increase the "ratio", one OR BOTH of those distances has to change, since that's what ratio IS. To go from the SBC stock 1.43 or so, to the "nominal" "design" 1.5, REQUIRES that either the rocker be LONGER on the valve side, or SHORTER on the push rod side, OR BOTH, than "stock". To go to 1.6 requires even more of a change. Since you can only move the push rod seat in toward the stud just so far before it's up into the trunnion or ball seat, most 1.6 rockers lengthen the opposite side a bit... moving the contact point out towards the exh side, and making it IMPOSSIBLE for the contact point NOT to be out there. In other words, you will MASSIVELY SCREW UP the geometry EVEN BY TRYING to center the pattern in such a setup.

As always, the advice remains, find THE NARROWEST SWEEP of the rocker tip across the valve stem tip; and IGNORE where on the stem the pattern lands, as long as it's within reason. IT DOESN'T MATTER where it lands. IT DOES MATTER that it's as narrow as possible, indicating the least sidewards force on the stem & guide, and the most direct STRAIGHT pushing action resulting in the most lift.

antman89iroc

Sofa, that is exactly what my mind is telling me. The past two engines have been setup with the "center the contact" method and both have had problems.

What do you think about the valve/pushrod off sets? Is it worth trying to account for this in a stud mounted rocker? Some companies do offer rockers with push rod offsets. Otherwise the only way to properly account for all this is a shaft mount system. Which, if I were starting over is probably what I would do. I have just about spent that much on parts and will have done so by the time repairs and gaskets come into play.

This is one of the frustrating things for me. I have "assembled" a lot of motors but wanted it done "right" so I hired a professional builder. I took a lot of advice and bought quality parts as I could afford. I guess no one will pay attention to details like I will so lesson learned. 17K miles, mostly to work and back, and I'm at least pulling heads and will likely pull the motor so I can see for myself whether I need a .850" BC cam or will a larger one fit. I may "detune" the motor a bit just to ensure durability.

antman89iroc

Oh, one more thing Sofa, when you say "IGNORE where on the stem the pattern lands, as long as it's within reason" what do you consider within reason? If I were to use the crower offset rockers I could get it about .110 from the edge of a .313 valve stem. Otherwise, with my current Comp rockers, the contact pattern will be ~.060 from the edge. I was afraid that would be too close and if parts moved or flexed a bit the roller could come off the edge of the stem.

sofakingdom

iTrader: (1)

I would say, (in fact DID say) within the middle 2/3 of the stem or so. .060" from the edge is about as far as I'd call "good" for the center of it to fall. If it goes there at one end, especially near the zero-lift end of the arc, I'd say it's OK. It can't really fall off. The only problem would be if all the force was concentrated on a narrow bit of the stem.

As far as needing offset rockers, I'd have to see how yours line up; but most of the time you can handle that with the guide plates. I'm not familiar enough with those particular heads to know if they're too far offset for that.

antman89iroc

Well thank you for reiterating. I do appreciate the help. I didn't mean to cause you to repeat yourself.

I had already figured out what I considered the longest pushrod I could use with my current parts and get very close to 90* at mid lift and not fall off the edge of the valve. It ended up with about .065 wide and .060 from the edge of the stem. I added lash caps so it's about .145 from the edge of the cap. However I chickened out and reused the previous pushrods. I ran it for 20 or so miles but knew it wasn't right either so tonight I got the nerve up to run the longer pushrods. These are +.250" longer than the ones the builder used. (7.4" to 7.650") Man, it does make a difference. It idles with more "lope" than before and runs smoother. It had almost a vibration with the shorter rods that doesn't seem to be there now. I am going to put some miles on it and re-check everything.

skinny z

Do you have a picture of what the witness mark looked like?

sofakingdom

iTrader: (1)

A thing about push rod length that most people don't realize, starts with cam design.

To make a cam "bigger", they DON'T make the lobes "taller". Rather, since "lift" is THE DIFFERENCE BETWEEN the tip of the lobe and the "heel", they make the heel LOWER. The tip of the lobes, even on a STOCK cam, is pretty much always near about as big as it can possibly be and still be able to be installed; i.e. the same OD as the bearing journals. So even if no changes whatsoever are made to the valve end of the valve train - valves are stock length, the heads are stock, etc. etc. etc. - just the act of increasing the cam lift tends to require lengthening the push rods. A small base circle cam for a stroker, longer valves to accommodate more spring, and all that sort of stuff, exaggerates that. But then of course, since extending the valve upwards moves the stem tip closer to the stud, it also moves the contact point out toward the exh side, especially at low lift, FURTHER rendering the idea of "centering" the pattern irrelevant.

So yeah, I'm not surprised, in a general way, that optimizing the push rod length PROPERLY, by narrowing the pattern instead of centering it, made the engine run NOTICEABLY better. And of course I'm always glad to hear when somebody takes good sound advice and reports positive results.

skinny z

What would your advice be to the many (myself and the OP included) who, in their attempts to achieve the narrowest sweep possible, find that the contact DOES get too close to the valve tip edge? The choice by many at that point is to sacrifice the ideal geometry and run a shorter than optimum pushrod. It's been noted that in my case I opted for the offset trunnion and was able to get both a narrow sweep and be centred.

sofakingdom

iTrader: (1)

Yes indeed, there may be a point at which such a thing becomes necessary.

However, that's quite a different matter from using "centering" as the method for choosing push rod length.

AlkyIROC

On my BBC engine, I use Dart 360 race heads. Both valves are longer than stock to allow taller springs for the 0.800" lift on the camshaft and I run lash caps on the intakes because I use a 5/16" stem to reduce the valve weight. When I measured for pushrod length, the exhaust pushrods were very close to stock length in a BBC which totally baffled me as to why but my intake pushrods are around 0.400" longer than stock.

I also went with a larger diameter pushrod with thicker 0.120" walls instead of the normal 0.080" wall to increase stability. Had to modify a few pushrod passages through the head to allow the larger pushrods to pass through. The shaft rockers corrected rocker alignment over the valves but also moved the pushrods over from where they would normally run. Shaft rockers don't use pushrod guide plates.

antman89iroc

Lots of good info and tips here. Thanks a lot everyone.

I don't have updated pics now but will get some when I recheck everything. One thing I just thought about is that since I use hydraulic rollers and don't have one made solid I probably got a little bit wider pattern than if I had a solid lifter. When I set the rocker down for pattern check it touches while I am putting the 1/2 turn preload, during which time the rocker tip is moving slightly towards the intake side. Maybe not much and it may be overridden when going through lift, but worth mentioning.

I first used the technique shown on a video I saw where the rocker trunion and roller centerline is adjusted to 90* from the stem with the valve closed. Then, I turned the adjustment nut down 1/2 the lift amount by counting turns. It's easy for me since I have 20 threads per inch or .050" per turn. I rounded a little on the short side then cross checked by pattern. With lash caps it's more complicated to figure the actual valve stem location but the "ideal" length put the contact patch very close to the edge of the valve stem (~.125" from the edge of the lash cap) I also compromise for pattern location by using a slightly shorter than ideal to get further from the edge for safety margin. Actually, the pushrod length I used is perfect for the valve height without the lash cap. I may be changing things around later anyway since I want to optimize valve timing and use the largest base circle cam possible. Then, after the cam is optimized if the pattern is off center I will likely use the Crower rockers with the back set trunion. I spoke with Crower tech and they offer the .050 backset rocker and said they could custom build with a .090 backset if needed. I can also see a need for the stiffest pushrod available. Thick wall .100 are what I will buy when I change lengths.

During all of this testing I found it interesting that I could tell a difference in effort required to compress the valve springs with long or short pushrods. The shorter rods seemed to put the trunion so low that I believe it was almost in a bind. It's easy to imagine how that would make it harder to raise the lifter. When running the pushrod would almost "pop" the valve off the seat, and probably let it slam shut.

All the advice and input was invaluable, but I learned a lot just swapping pushrods and using my adjustable pushrod and observing the effects. There's a lot less "black magic" to it now. Funny how that goes.

skinny z

Here's something you'll probably find interesting. It's from a series of tests I did a few years back. Using lines that intersect the various points and valve angle helped to visualize the effects of a short pushrod, 7.15" vs a long pushrod, 7.45". I was using a Comp 1.6 Pro Magnum rocker. The three pictures in a series show the valve closed, then mid lift and fully open. There's also a companion picture showing the witness mark on the valve tip. I'm not certain but it may be the 7.45" pushrod that I eventually used and then added the offset trunnion rocker .
The red lines establish the 90 degree angle that is the ultimate target of the mid lift point. The white line connects the rocker arm reference points.



Valve closed



Mid lift



Fully open



Wide sweep across the valve tip

skinny z

With the short pushrod, the rocker is barely past 90 degrees when it's fully open but the longer pushrod clearly show the rocker almost at 90 at mid lift. Much better geometry for sure and the sweep is noticeably narrower. You can also see how the contact has advanced to the exhaust side.




Valve closed




Mid lift



Fully opened




Narrower sweep across the valve tip but approaching the exhaust side.

antman89iroc

Very interesting skinny, that's a good example of what were talking about. Thanks for posting it. I'd say the top set is basically the "stock" example for a shoe rocker and the bottom set is correct for a roller tip rocker.

And Sofa, those are good point regarding cam/lobe lift and it's affect on pushrod requirements.

I believe I have deduced where some of the confusion about geometry and more specifically, how the rocker contacts the valve tip originates. A shoe tip rocker the designed to "walk across" the valve tip as it goes through it's travel. It doesn't scoot so much as roll. That's why it has a curved tip. Sort of how a pinion gear "walks" or "rolls" across the face of the ring gears teeth. It doesn't scoot or scrub the curved surfaces contact each other as the gears rotate past each other. My guess is this difference was incorrectly applied to the roller tip. In a way the roller itself acts like a link between the pivot point and the valve tip face. Whether the roller actually "rolls" or "walks" or "scrubs" across the valve tip is another question. Again, this is just my theory. Thoughts?

Valve Motion

[QUOTE=antman89iroc;6002805] The rocker arm must be at a 90* angle with the valve stem at half lift. It gives a very straight forward method of establishing push rod length. It's the first time this has been described in a way that can be measured rather than "black magic" and trial and error.
QUOTE]

Was explained that same way about 40 years ago.

sofakingdom

iTrader: (1)

IMO that last pic is still well within the "OK" range for where the pattern lands on the tip.

And of course it's MUCH narrower than the first one, clearly demonstrating more nearly correct geometry, with less "scrubbing" motion of the rocker across the stem as well as better translation of the cam profile to actual valve motion.

You're right, that a stock rocker is "designed to" not slide, but rather, sort of roll, on the stem; however, design intent and physical implementation are often 2 entirely different things. As they are in this case.

As mentioned above, I agree the whole "90°" thing gives a reasonable approximation of a starting point for optimizing push rod length, but suffers from 2 defects that limit it as an ideal or "final" determination method: difficulty in accurately measuring it, and oversimplification of the system as a whole.

skinny z

[QUOTE=Valve Motion;6006194]True enough. However, access to that explanation wasn't easy to come by.
As a young automotive tech student, 40 years ago, information was handed down from the professional engine builder. My college instructors, one in particular who was making V4s and in-line 4s out of small block Chevys, never bothered to include setting up proper valve train geometry as we were dealing stock rebuilds at the time. He may have suggested that I put a 1.6" exhaust valve in my old "fuelie" heads but that was about it. The exotic things, (roller cams were the legion of the racing gods at the time) and the tech behind them, were available only to those who moved along in the racing engine business. Timing the intake valve closing event to compression ratio...? Never heard of it. Not until the age of information (and misinformation) came about and that kind of thing became general knowledge.
Now it's much easier to build an engine that at least isn't a pile of incredibly mismatched and improperly assembled parts.

antman89iroc

[QUOTE=Valve Motion;6006194]Yes, I should have said it was the first time I heard it explained that way. When the light bulb above MY head came on.

And even still there is conflicting information and/or alternate theories of what is the best way to set up a valve train, depending on your objectives. For me the objective is a long lasting combination that performs it's function effectively and without a lot of undesirable side effects like noise, excessive wear, broken parts etc. It appears to me the 90*@mid-lift method would meet those requirements.

lt4orbust

I install my own valve guides.......................but i also agree