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Tech / General EngineIs your car making a strange sound or won't start? Thinking of adding power with a new combination? Need other technical information or engine specific advice? Don't see another board for your problem? Post it here!
Been doing a lot of searching but haven't found how you go about setting the rocker arm height on stamped roller tip rockers to get a starting point for determining pushrod length. I am building up this engine with a cam lift of .490 so need to measure for new pushrods. I am using roller cam and lifters with stamped roller tip rockers that have a 1.52 ratio. I understand how to do the marking and determining witness mark but how do I set the starting height for the rocker arms? I see a plastic tool listed but its not clear to me if this works for my new rockers or if it is intended for use with full roller rockers and what ratio it works for. Again, just looking to get the starting point on the rockers to set the adjustable pushrod tool. From there use the witness mark approach to fine tune the pushrod length checker.
I'm not a fan of the plastic tool and I prefer determining rocker arm geometry via the mid-lift method. However, folks tend to find that method cumbersome and confusing so the witness mark is the fall back approach. That said, using the witness mark, you're after two things. First and foremost is to achieve the narrowest sweep across the valve stem. That comes before getting it centered. No plastic tool necessary.
Something to consider if you haven't already: The springs you use for running will be too strong to rotate the engine through a cycle without collapsing the hydraulic lifter and giving you false results. Checking springs are used in this case.
Get an adjustable pushrod and set it to the pushrod length that was original to the 5.7 Vortec (L31). That'll be a decent starting point. Work the adjustable pushrod about a tenth at a time and observe what happens to the mark. Adjust accordingly. While checking springs will add an element of error into the actual value, unless this is a maximum effort build, the discrepancy won't add up to much.
If you are interested in the mid-lift approach, I've added the paper by Jim Miller to this post.
Thanks, I have the adjustable Comp Cams steel pushrod checker.. So, I was looking for the starting point so that the rockers are close to where they need to be and then use the witness mark approach to fine tune it... So, I will set it to the stock 5.7L pushrod length and go from there... Will see if I can find some checking springs... Problem is that I already bolted the heads on so changing to checker springs would be difficult... Guessing I could do that with the piston at top dead center so it holds the valve up... Years ago I actually changed valve seals on a 1992 5.7L chev van by putting the piston at top dead center and the pushing some cord in through the spark plug hole to help hold the valves up to remove the springs and keepers... Lessons learned, read first.. One other note, the Comp Cams pushrod checker with the locking nuts doesn't work so well with guide plates as the nuts are too big to go through the guide plates....
Thanks, looks like I bought the wrong adjustable pushrod... I think I can make the one I have work depending upon where it hits and if there is enough room...
That's the one I use too. It worked well for me, I found out the engine I bought had 1/4 inch too long of pushrods. It seems to have made a huge difference in how it runs to have the right ones in there (and that they're not bent.)
Not only is the wrong length hard on valve guides but the full spec of the camshaft lobe isn't transferred to the valve tip. That equals a loss of performance as well.
I used the midlift method recently and it’s ridiculously simple once you understand what’s going on. Kind of unnerved me though because of how much longer the the pushrod ended up being at 7.550” but I backed it down to 7.5” because it was just so close to the edge of the valve tip. Haven’t driven the car yet though since making the change. I still wonder if I’m too close though.
It is simple isn't it? I went so far as to make a small jig that I sandwich between the retainer and spring and this provides a uniform flat surface to measure from.
As for close the edge of the valve tip, do you have any images or can provide a reference? About the only time I've seen this as a problem was with cylinder heads that have longer than stock valves. As an example, the RHS heads I have have a .100" longer valve. If you can visualize how this plays out with the valve stem tip closer to the stud (due to the angles involved) as you increase the length of the pushrod so as to narrow the sweep across the valve tip, it quickly approaches the exhaust side of the valve. The only solution I found was to use a rocker with an offset trunnion. This pulled the rocker tip away from edge of the valve and allowed me to achieve the correct length and a narrow centered sweep.
I learned about the method from this site, an old post I think and those rocker arms were discussed too. I’m hesitant to jump on them it’s yet because they’re $$$ and my Howard’s rockers are dang near unused but I will if I have to. Unfortunately I have no images, it’s been a couple months since I’ve seen them and it’ll be a couple more before I see them again as I’m well off away from home right now. As I remember it though the roller isn’t coming off the edge, just close and I wasn’t sure how close it could be if I plan to zing the engine to 6k rpm somewhat regularly. As I remember it was <.1” away from the edge but I really need to see it again to be sure. I’ve got stock AFR 1038’s on there and they use a 4.9” intake valve and 4.95” exhaust valve. Stock valves are 4.91” I and 4.93” E but height up top depends on how deep the seat is and there I don’t know how the AFR compares to stock.
Last edited by SilverChicken; 06-09-2020 at 10:51 PM.
I learned about the method from this site, an old post I think and those rocker arms were discussed too .
Pretty sure I was a part of that discussion.
What we've discovered, and perhaps not surprisingly so, was a difference in sweep between rockers of the same ratio but different brands. While the Crower .050" backset rocker did the trick on my RHS heads, the same couldn't be said for another SBC although it had the AFR 195's. Further to that, we couldn't get an acceptable measurement with Comps Pro Magnums either. We tried the Crower standard trunnion rocker (all measurements were with 1.6 ratio), and nearly perfect geometry was achieved. Between my racing buddy and myself, we've managed to mangle more than a few sets of valve guides following the misdirection you find on the internet (or the days before there WAS an internet). It wasn't until I came across Miller's paper, that it fell into place. No guessing. Just measurements and a few empirical tests to verify.
Never thought about how the differences in how manufacturers achieve their rocker arm ratio but it makes sense if you’re talking about the width of the sweep after doing a little visualization. If the rocker portion trunnion to valve is somewhat short it would cause a longer sweep pattern vs a longer arm. The ratio is only determined by the distance of trunion axis of rotation to roller tip axis to the distance of the trunion axis to the center of the pushrod. You can have multiple varying measurements as long as the ratio was equal to the advertised ratio and a shorter overall arm would have a wider sweep. Now I need to go look for the measurements of my Howard’s arms if a such thing exists. Although I’m not sure what I’d do with them if I find them because I don’t know what constitutes along or short rocker arm. A longer overall length arm would have a narrower sweep but would push the contact out toward the exhaust side of the valve. Makes me also wonder if a 1.5:1 arm would be a better decision for geometry even if I lose a little lift.
i don’t know if I’m right in any of that thinking, it’s just me trying to make sense of what you said. As I expected I can’t find jack about the Howard’s rockers short of getting a mic and removing a rocker to measure myself but I still wouldn’t have 1.5:1 rocker measurements. I’m nowhere near home either like I said previously.
Last edited by SilverChicken; 06-10-2020 at 09:59 PM.
I used the adjustable pushrods mentioned here earlier, but didn't use checker springs. I had two extra lifters and disassembled them and stacked washers in them to make them 'solid' lifters. I doubled down on the effort and while checking length and geometry I also checked piston to valve clearance (clay method).
Part of the logic was to not buy checker springs... I also didn't want to disassemble the heads. Brand new from AFR with the spring's shimmed to meet my lift and spring pressure requirements.
That's absolutely the way to go. The checking springs will introduce a certain variable and it gets worse as the springs are beefed up to handle whatever cam and RPM might be involved. Seeing that I have a link-bar style lifter, there's no option to build a solid version as the lifters don't come apart due to the link-bar mechanism (the fastening rivet in particular). I've used the edge of the lifter on occasion although last time around the did the checking springs. The truth will come out soon as the engine is being torn down soon and I can examine whether or not my guides have survived. I beat that engine to the edge last year and it didn't like it. I have to examine what went wrong.
The adjustable push rod that I had did not work because the locking nut interfered with the guide plates. It also had to be extended too far so it was rather "wiggly". I got with my machine shop and we decided that they should be close to stock length so I bought them and tested with real push rods. I did use test springs as when I turned it over with the normal springs, it was pushing the lifter down some (Comp Cams says not to prime the lifters). I used air pressure (90 lbs) to hold the valves up with the piston at TDC. I put a bar on the end of the damper (mounting bolts where the pulley goes for leverage) to make sure it didn't spin with air pressure in the cylinder. Note, I have an 80 gallon shop compressor. It didn't even try to spin under pressure even though this is a new build so everything is tight. Test springs went on without a problem, however, even those pushed the lifter piston down inside the lifter cylinder a bit when I rotated the crank to check them. I used the push rod to bring them up to full lift (lifter piston was at the top of the lifter cylinder with the cam at max opening). Witness mark was very light with the test springs. They are just a tad long (a little towards the exhaust valves) but with preload they should come out about right. I have used the cord method before but this was much easier. I have the advantage that the engine is on an engine stand and not installed yet.
Any indication of how wide the witness mark was? That's the real deal here.
For the record, what's the combination of parts you have? Heads, gaskets, lifters (stock?). Has the block been decked?
Block was resurfaced but not decked. It is a 1996 Truck block (880) with Vortec heads (906). Can’t tell you on the gasket what thickness but I think was stock. Rockers are Comp Cams 1.52 ratio with roller tip non-self aligning hence the need for guide plates. It is a Comp Cams roller cam with dog bones so overall lifter height should be stock. Lift on the cam is .488 and .495. Going to guess the width of the witness mark was a little less than 1/16 inch (eyeball). So, except for the extra lift in the cam, pretty much stock.
One thing about the Vortec is that the stock valve guides are cast iron and durability, as you might expect, is very good. Small deviations from the "ideal" valve train geometry don't have as great an impact on the life of the guide. Your result of less than a 1/16", says that there's still room for improvement however it's unlikely, as was my experience, that any harm will come of it.
My first go around with Vortecs was with a .454" lift cam with stock length push rods for the flat tappet. The 2nd build went to a full roller arrangement (cam and rockers) with lifts in excess of .500" and I paid little attention to geometry. After thousands and thousands of miles, I sold those heads. The buyer checked them and determined the guides were still serviceable so that says something. Now that said, I took the whole valvetrain and put in on my aftermarket Vortecs with bronze guides. The result was complete and total valve guide failure within a few thousand miles. My pushrod measurements were, IIRC, short by a few tenths. Lesson learned.
06-07-2020, 12:32 PM
