Pat Hall

iTrader: (31)

Hey guys. From what I've read on posts here, it appears that most aftermarket replacement pistons are designed to sit in the hole about .025 without zero decking the block right? On the motor I'm building, I didn't have the block decked at all, and I'm using Keith Black hypers (KB193). I looked up the specs on the pistons at KB's website, and it lists the crank to deck clearance as being 9.025. What I'm confused about is does this mean that the pistons will be sitting at zero deck height without decking the block, or will they be sitting .025 down in the hole? It also said these pistons will yield a 9.6:1 CR using a 64cc head, so that really makes me wonder if they sit at zero deck height without decking the block. I'm trying to select the right head gasket for proper quench, and I can't figure it out until I know whether the pistons are level with the deck, or .025 down. I'll be removing the heads soon, so I could measure at that point, but I'd like to try and figure this out going by the specs so I can go ahead and order the right head gaskets. I might even take the block in and get it zero decked while it's apart again if it deems necessary. I'm putting AFR 190's on, so I'd like the CR to fall between 9.5-10.0:1. I'd appreciate some help from the experts here. Thanks in advance! P.S. I suppose I could have the heads milled to bump up the CR too. They're 65 cc right now. I don't know which is easier or more cost effective. Milling the heads or decking the block. The heads are off right now, so that might be the better option.

Kurt04

iTrader: (2)

Try and find out what the "Compression Height" is on the pistons. Its the measurement from the centerline of the pin to the top of the piston.

Add that number to you rod length and half of your stroke, then subract it from your deck height and thats how far your pistons are in the hole. Make sense?

Example(my engine) 1.875" (half of stroke) + 6" (rod length) + 1.125" (compression height) = 9". Deck Height is 9.002" so my pistons set .002" in the hole.

If you are going Natural Aspirated you will prolly want a quench around .040" (pistons in the hole + gasket thickness) and then use chamber size to adjust the compression to where you want it.

Pat Hall

iTrader: (31)

Yeah, that makes sense. Guess I need to do a little number crunching. Btw, the compression height is 1.561". Stock 350 deck height is 9.025 right?

sofakingdom

iTrader: (1)

Right; "stock" deck height is 9.025", tolerances approx +.010" -.015". Most are between 9.025" and 9.030". Most are within .010" or so from cylinder to cylinder, but not always; sometimes they're ALOT farther out than that. Especially the mid-70s ones with the super-crappy QC of those days; I've seen them as much as .020" out, from the front cyl on one side to the rear cyl on the other side.

You gotta REALLY REALLY WATCH OUT for that compression height spec. Piston mfrs are REALLY BAD about giving the "nominal" height for the "stock" configuration that their piston is "designed" to "replace"; and NOT the ACTUAL height of THEIR product. For example, nearly all of the TRW/SpeedPro line "says" 1.56" CH, because they're "designed" to "replace" pistons in a motor with that spec; but if you measure one, they're ACTUALLY 1.54". In other words, .020" farther down in the hole than stock. Same for PRACTICALLY ALL "rebuilder" pistons; the Badger, Silv-o-lite, etc. type of things, as well as almost all of the hypereutectics with the possible exception of the KBs. About the only people you can trust even halfway are the people who make racing pistons, like Arias, SRP, maybe Probe, maybe Mahle (or maybe not.... they are an outsource vendor for stock ones too), Manely, people like that.

If in doubt, MEASURE. The stock pin bore is .927" dia, meaning that if you measure from the top of the pin bore to the flat of the piston, it should be 1.096" ± .001" or so, for it to be the stock height.

A stock rotating assembly is 9.000" "tall". In theory, you could deck a block to that exact height, measuring off of the crank journal centers; but in real life, that's tough to do without a CNC setup. Obviously, with a stock height assembly, you'd want the block to be EXACTLY 9.000", for everything to be at its optimum. They almost never are.

Basically, the trueness of the block, that whole aspect of "blueprinting" an engine, is one of the easiest spots where you can get hosed. Accurate and consistent deck height is right up there with whether the cyls are pointed straight at the crank, or if they're tilted to the front or rear; or whether they're centered over the crank journals; or whether the BH flange is actually perpendicular to the crank bore, let alone centered on it; or whether the lifter bores point at the cam exactly like they're supposed to; or whether the cam tunnel is parallel to the crank bore; or where the head dowel pins are; or whether the bolt holes, such as the starter ones, are in the right place; and so on. Lots of stuff that's REALLY hard to measure without fixtures and REALLY hard to fix, but REALLY makes one of those head-scratcher messes when it's off.

brian schuetta

Put it all together and measure your deck height.

Guest

I have some 193's on a 'zero' decked block (9.000" crank cl to deck) and with 5.700" rods, 3.48 crank, they're at the deck. I measured them .002 down.

1989GTATransAm

iTrader: (3)

Bottom line it is best to measure. You cannot trust the specs given to you. I would mock up the short block and measure how far the pistons are down the hole. Depending on what head gasket you use you will then know how much to deck the block. GM makes a good one at .028" and another around .039".

I used the .028" and left my pistons .007" down the hole for a .035" quench. I have the part number for the .028" head gasket and it is 10105117.

I would have the heads cc'd because quite often they are not what is said on the box. Also you might want to consider raising the compression a 1/2 point. I'm running 10.75:1 with the .035" quench here in warm SoCal with no problems and 91 octane gas. By the way it does not cost that much to get the heads shaved.

Guest

True, and there are enough variances in rods and cranks (not to mention the pistons) and tolerances after people messing with those parts that it can throw it off.

Pat Hall

iTrader: (31)

Yeah, I'm definitely going to measure and see where they're sitting for sure. The motor is already assembled, so all I need to do is remove the heads. I'm thinking I could use a straightedge and a 45 degree feeler gauge set to figure it out, unless you guys think that isn't accurate enough. If so, I can have the machine shop measure it again with a more precise tool. As far as the head gasket is concerned, I'm probably going to use the Fel-Pro 1003 (.041 thick) since that's the gasket that AFR recommends for their heads, and have the block decked to "true zero". This should get me about right where I need to be for the proper, recommended quench. I also agree on the point of cc'ing the heads. I bought them secondhand from one of the members on here, and he told me they're 65 cc chambers, but the only way to know for sure is to check. Guess it's time to pick up a graduated burette and a piece of flat plexiglass. I guess I wasn't too worried about the CR originally since I was going to run cast iron L98 heads, but now that I'm bolting on the AFR's, I'd like to be close to 10:1. Also, I was reading a tech article on KB's website, and they recommended 10:1 CR when running aluminum heads and any cam that's over .220 duration @.050. I'm going to be running the Comp 502 grind (218/224 @.050). I've also got the 503 (224/230), and I'm contemplating putting that one in instead. According to KB's CR calculator, if I mill the heads to 60 cc, zero deck the block, and run the .041 head gasket, that should put me almost exactly at 10:1. This motor was going to be in the car before summer ended, but since I decided to run the AFR's, and also since we got a long, cold winter ahead, I figure I may as well take advantage of the downtime to do what's necessary for getting the CR where it needs to be. Between the advice from this post and the online calclulators I found, I think I've pretty much found the answers to my original query. By the time I get this motor assembled for the last time, I should be able to do it in my sleep! As always, a great big to all you guys for your input!

sofakingdom

iTrader: (1)

Or, use one of those little eye-dropper kind of things that you use to give medicine to a baby, or a Nyquil type of cup; and a "free AOL trial" CD (or one of those clear ones they put on top of a spool of blank ones).

Hey, I bet you never had thought of a use for those AOL CDs before, had you?

Sonix

iTrader: (1)

I used a piece of plexi, with a drilled hole, and a bit of grease to seal it to the head. I then used a syringe from the veterinarian across the street. You could grab a 100cc, and a 5cc for accuracy.
Straight edge and a normal feeler gauge is what I used. To be dead sure you're at TDC you can use a dial indicator and base. Set it up to measure where your piston is when it's within 1/16" of the top, then rotate the crank back and forth until you get the highest reading on the dial indicator. Be sure to use your feeler gauges across the piston pin, the piston can rock on the outer edges. Maybe measure a few cylinders and average the number.

My block was decked .015", and I used H345NP pistons (1.54" compression height), and I had .012" deck height...?!?

Damon

I've used KB 193s before. They should be sitting right around .025" in the hole at TDC on an undecked block. No replacement for actually measuring but that's the spec they're built to and they've come very close to it in the engines I've actually built with those pistons.

Pat Hall

iTrader: (31)

Hey guys. Yeah, I got the heads off several days ago, and I measured with a straightedge/feeler guage, then set up a dial indicator/base and measured again. I came up with .024, which is dead on for the compression height of the pistons. I also noticed what you were pointing out Sonix. It was .024 directly above the piston pin, and deeper at the top and bottom parts of the piston. Hey Sofa, that's a great idea for those worthless AOL trial CD's. Lol. It's ridiculous what Summit and Jegs want for a head cc'ing kit. Even their "economy" one is like $38. I'll probably just grab a big syringe and the AOL CD. I was thinking a thin layer of Vaseline should seal it to the chamber good enough. I'm seriously considering changing the pistons to flat tops. It's going to take a lot of machining to accomplish 10:1 with the KB193's. I'm thinking I'll get the block zero decked and switch to the flat tops. I'd hate to have to mill a bunch off the heads. I'll probably have a couple thousandths milled off just to give them a good surface for the head gasket to seal. They do have some really minor pitting, so might as well. I'm also considering getting Speed Pro hypers instead of the KB's. The KB's require a pretty large gap on the top ring, whereas the Speed Pro's gap requirement is fairly close to normal. Plus I like how they have the coated skirts.

Kurt04

iTrader: (2)

I think Speed Pro Hypers would be good.

Guest

I'd consider running the GM .028 gasket if your pistons are .024 in the hole, to get the quench reasonable. Raising compressions via a piston change (if you're at the point you can do that) isnt a bad idea, I have those pistons and heads with 10.0:1 and a slightly larger cam than both you posted and it will run on 87 octane most of the time. I think 10.5:1 would be safe for either of those two cams you posted up.

sofakingdom

iTrader: (1)

The Speed Pro hypers are DEFINITELY among those that are an extra .020" "down in the hole".

Pat Hall

iTrader: (31)

Hey Max, Sofa. Yeah, as far as quench goes, I'd like to get it as close to .040 as I can. I'm thinking if I'm going to tear it down far enough to remove and change the pistons, I'll go ahead and get the block zero decked to the new pistons while I'm at it. Besides, if I do that, I can run the .041 1003 Fel-Pro head gasket that AFR recommends with their heads. With zero deck clearance, that should put the quench right where it needs to be. Hey Sofa, I got a question for ya. You seem to be an expert on engine balancing. I had the rotating assembly balanced with the KB193's, and I'd hate to have to pay for balancing all over again. If I have the new pistons weighed and weight matched when the machine shop presses them on, do you think the balancing will still be pretty close?

sofakingdom

iTrader: (1)

I don't know about all this "expert" stuff, but...

Yes; if the new pistons can be matched to each other and their finished weight matches the old, then re-balancing isn't neccessary. I'd shoot for less than 1 gram of difference, and preferably less than ½ gram if possible; at any more than 2 grams different, I'd get it re-done for sure. Between 1 and 2 grams, IMO it's kind of a judgement call. Shouldn't be too tough if the new pistons start out a bit heavier than the old. If they start out life lighter, then you pretty much have already had the choice made for you.

1989GTATransAm

iTrader: (3)

Hey Pat be bold and go for a -.001" or a -.002 deck on the block. Better quench that way.

Pat Hall

iTrader: (31)

Well, I looked up the info, and the KB 193's I have right now are 528 g's, and the Speed Pro 631's I'd like to replace them with weigh in at 567 g's. I'm not sure how difficult it'd be, or how much material would have to be removed to lighten them up by 39 grams. I guess the machine shop would be able to tell me if it's feasible or not. If it's too much trouble, I'll just bite the bullet and pay for balancing again I suppose. I did find a good deal on the new pistons. Jegs offers them complete with a moly ring set for $220. I paid about $250 just for the KB 193's, then spent another $100 for a set of Speed Pro Plasma Moly, file-fit rings. Hey 89GTA, I'm not one for being too bold. Lol. I'll either zero deck it, or go +.010-.012 if I decide to use the GMPP .028 gasket. Hey Sofa, I merely referred to you as "the balancing expert" due to your myriad of posts where you're always setting guys straight on how to get their motor balanced properly at the machine shop.

sofakingdom

iTrader: (1)

The difference is 39g. That's almost 7% of the total weight of the piston.

To put that into perspective, one ounce = 28.3g.

One penny = ~2.5g. Take about 16 pennies; that's about how much weight you'd have to remove. Since aluminum is about half the density or a little less than whatever it is they're making pennies out of these days (some zinc alloy IIRC, it's sure as hell not copper), imagine removing material equal to roughly double the volume of 16 pennies from a piston.

Probably not a good idea.

The problem now of course, is that your crank counterweights are lightened (by drilling) to counterbalance the lighter pistons; and now you have to make them heavier to balance the heavier ones. It's hard at this point to make them heavier, cheaply.

Bottom line, it's not impossible that you'll be looking for a new crank PLUS balancing, given what you've got right now.

sofakingdom

iTrader: (1)

On second thought, there is one other possibility...

Try finding some lighter-weight wrist pins. I seem to recall that the ones in TRWs are about 150g. You might be able to knock off ALOT of that with some thinner-wall pins, made of better material.

Pat Hall

iTrader: (31)

The KB wrist pins are 145g. I have to agree that it probably wouldn't be feasible to try and remove 39g from the pistons. With that in mind, I went to KB's website again and looked at some more of their pistons. Their KB120 is a flat top with 7 cc valve reliefs (2), and it weighs 535g. It's supposed to yield 10.2:1 with a 64 cc chamber. The KB193's I have now weigh 528g. It probably wouldn't be too hard to match up a 7g difference, so maybe I'll look into that. Besides, my $100 Plasma-Moly rings are already filed to the proper gap for the KB hypers, so I'd be able to use the same rings again. Both KB pistons use the 5/64 compression rings, and the 3/16 oil ring, so that'll match up too. They have a KB106 piston that has all the same specs as the KB120, except that it uses the thinner 1/16 compression rings. I thought I was SOL until I looked up the specs on the KB120. So whaddya think Sofa? I should be able to remove 7g safely right?

sofakingdom

iTrader: (1)

7g is more feasible. That's about the mass of 3 pennies. Still kind of alot, but at least not so outrageous.

As far as wrist pins, TRW themselves used to sell tapered thin-wall tool steel pins, might still for all I know, if I could find a listing of them (I don't have my old paper catalog any more); which was one of my favorite ways to get suposedly "identical" parts lists to go faster when I built it, than it would go when somebody else built the same parts list. Wiseco sells them, and KB sells a few varieties too.

If you're using pressed-pin rods, you have some flexibility in what pin length you can use. Looking through Wiseco's catalog, they offer a .927" pin 2.500" long, .150" tapered walls, that weighs 106g; or about 40g give or take, less than what comes with most pistons. So right there, you can bring the SpeedPro ones back into tolerance. Pins are usually $15-20 apiece, so they're not cheap; but maybe less than a whole other set of pistons.

Something to think about.

Pat Hall

iTrader: (31)

Well, so much for me thinking I had an easy solution by using the KB120's. I researched them a little further, and they only work with floating pins, so I'd have to get my rods bushed for them. I did some more calculations, and if I keep the existing pistons, mill the heads to 62 cc chambers (they're 65 cc at the moment), and use the fel-pro 1094, .015 shim gasket, it'll put me at 9.8:1. My pistons are .024 in the hole, so with the 1094 gasket, my quench will be .039. Every other solution I've tried to come up with seems to be too much of a headache and too much expense. Besides that, the shortblock is all assembled and ready to go. I really don't want to tear it all down again for the decking and piston swap. I'll admit I'm kinda settling to an extent, but 9.8 CR isn't really too far from 10.0 anyways. Who knows, maybe I can even manage to run on mid-grade gas at that CR. Another plus as far as the octane/detonation factor is we're at 5,000 ft. above sea level here. I'm thinking this is the most feasible solution at this point in time. A buddy of mine has an L98 shortblock assembly tore down for a rebuild that he'll probably sell for about $100, so maybe I'll pick that up from him and build another shortblock down the road a bit. If I go that route, I can get the next shortblock zero decked like I should've done with this one, and get it balanced with a nice set of flat tops instead. Then I can slap the AFR's on that shortblock when that time comes. 9.8 may not be ideal with the AFR's, but I'm pretty sure it will suffice for now. Thanks for all the suggestions you threw out there for me Sofa. I'm definitely more knowledgeable on this aspect of engine blueprinting compared to a few weeks ago. For a serious race motor, I like the idea of using the lighter wrist pins. Any extra weight you can shave off the rotating assembly is a nice advantage. Same goes for valve train components if you've got the cash to spend (i.e. titanium retainers, solid rollers, etc.).

Kurt04

iTrader: (2)

Sounds good to me.