I read they went to lighter weight flat tops in the 90-92 motors. With very short skirts on the pistons. verified this as i have mine apart for a rebuild.

 

Mine does have Flat tops with valve relief. I havnt removed them, so I cant say about the skirt size or if they are hypers.

Were did you read this at?

 

On this site and the web. I have my 90 torn down for a rebuild. I read somewhere that gm started using lighter weight, hyper pistons with short skirts. At some time, and just wondered if the 90-92 5.7 had them from the factory.

IM looking for new rings and wondering about gaps and stuff with this build.

 

Factory called them hypereutectic pistons but the silicon content of the alloy is still not nearly as high as a typical aftermarket hyper piston. Consider them regular old cast pistons and you're probably closer to reality than if you call them hypers.

 

Hey Damon....Looking at them compared to an old cast piston i had lying around. The underside is very smooth not as many sharp edges as the old 70s piston i have. Looks like a better quality piston in the 5.7. one of the reasons i asked if its a hyper piston. Shows almost zero wear after 80,000 miles. Very little wear in the bore.

Anybody know what the factory used for ring gaps on these pistons? The reason i ask is im getting a .024 ring gap. on a used 80,000 mile top ring at the top of the bore on the unworn portion. The rings have worn some so maybe that why its such a wide gap. Dont you gap hyper pistons wider then stock cast?

Can Anybody suggest a set of rings and gaps for a stock piston?

 

My 92 L98 had factory flat top hypers.
IIRC the rings were metric.

 

The lack of wear on even a high mile engine probably has as much to do with it being fuel injected (better A/F ratio and spark advance management than a carb) as it does with the design/material of the piston/rings, assuming the engine was kept in good running order through it's life.

Not sure what the factory ring gaps were but I agree with the comment about metric rings.

 

Yeah i wouldnt even have touched the short block. But water got down into one of the bores and caused some pitting. I honed it out and the piston to bore clearance is still well within specs. At least what i can tell with my feeler gages and using the old rings for measurements. I figured what the hell might as well clean it up. throw in some new rings and bearings its down this far.

 

From the '92 FSM for the L98:

Gap:

production top: 0.010" - 0.020"
production 2nd: 0.018" - 0.026"
service limit: 0.035" max

Groove Clearance:

production top: 0.0012" - 0.0032"
production 2nd: 0.0012" - 0.0032"
service limit: 0.0042" max

The piston clearance is tight at:

production: 0.0007" - 0.0021"
service limit: 0.0027"

RBob.

 

Thanks RBob for posting those specs. As always your a big help to me... on the bore i honed out. I can almost force a .003 feeler gauge. Between the piston and bore at the area that tends to wear the most at the top of the bore. Crude i know...The others will be a little less so i should see another 75,000 miles out of it. Give or take a few miles.

This is going in a 56 Belair it will only see car shows and the occasional cruise in. So it should last awhile.

Hey guys whats a good brand of ring to use? Any part numbers? I figure a single moly faced top ring should do it. What size should i buy to get the gaps in spec? I know they have slight over size. I have a new set of rod bearings in standard size from and old project that i can use with plasti-gauge. To get and idea for the rod bearing size. The journals look really good as well. I hate not replace them and the mains while it down this far.

 

I have an 89, are the stock pistons considered a 4 valve relief flat top or dished?

 

Most likely you'll have dished.
I've pulled apart and found:
92 L98 - flat top
91 LO3 - dished
88 LB9 - dished
88 L98 - dished

 

I'll have to check what we have here, pulled the engine on a 92 5.7GTA and it's a very late car, it has all the late additions like the rear hatch cushions and other tids and bits. If I remember correctly the engine had dished pistons, not flat tops. I can take a pic later this week.

 

I think these are dished, aside from the 4 valve reliefe there is what is best described as a ring all around the outside edge of the face of the piston and everything is recessed from there 1/8" maybe? Ill try to get some pics later this week as well. Im guessing a flat top will be just that flat across the top aside from the valve relief's?

 

Yes, that quench ring/squish band and the rest is dished...dished piston. I'm pretty sure the 92GTA had those too not flat tops w/ 4 reliefs

 

Here are a few pics.

Flat top
https://www.thirdgen.org/forums/atta...r-pistons2.jpg


Dish piston
https://www.thirdgen.org/forums/atta...jr-piston3.jpg

 

Thanks for the pics guys I definately have the dished pistons. Whats the benifits/cons of dished vs flat tops other than increased comp ration?

 

My understanding was that 91? 92 L98's had hypereutectic flat tops.
My 92 L98 did have hypereutectic flat tops.

 

Checked today, indeed flat tops...didn't even notice that when tearing down

 

Is 7cc correct, for the stock 91-92 L98 flat top pistons?
Im wondering because I dont get how that would result 9.75 CR, as specified in the service manual.

These are the numbers I use. 64cc heads, 3.48 stroke, 4" bore, compressed head gasket 0.015, head gasket bore 4.1, deck 0.025 and piston volume 7cc (dish).

 

I would like to know this as well. I'm using this compression calculator:

http://www.race-cars.net/calculators...alculator.html

I have read that the 90-92 cars came with a steel shim gasket that is 0.015 inch thick. If we use that head gasket figure and add the 0.025 that the piston is in the bore and plug in 64cc combustion chambers and assume 8cc valve reliefs, we get a compression ratio of 9.9:1. One way to get 9.7:1 would be to make the valve reliefs be 10ccs, but that mean only a 2cc difference between the early pistons and their 12cc dish and the flat top pistons with the valve reliefs? Is there a number we're missing or an error in the math somewhere?

 

You're missing the gasket bore size, if it's 4.1 or thereabouts adding a thicker gasket has much more influence than if you plug in 4" for the gasket bore.

 

Sorry for the resurrection but i just found this while looking for some info on the L98 flat-tops.

Maybe the 3-4 cc you're looking for to get the 9.7:1 CR is in the so-called crevice volume of the piston, which is the volume between piston and cylinder wall from the piston top down to the first ring. 3-4cc is a usual measure for this imho.
So if we figure 7cc for the valve reliefs and add a 3cc for the crevice, there you go - 9.7:1

And one more question ... there's no chance any of you guys would have a complete set of those pistons and rods kicking around, no?

Some more info of a dealer service bulletin i just found:

 

It's interesting that the Corvette got higher compressor than the F or B cars. But same piston.

I wonder how this compares to the ZZ4 crate motor piston??

 

Y & F L98 (and there's also a mention of B cars on this information bulletin!?) do have the same piston but remember that the Y L98 has the 58cc chambers ('113? alum head) where our F variant has a 64cc chambered iron head. Haven't looked up the ZZ4 tho..

One more question: do y'all think that the crank would be balanced differently between say a 87 L98 and a 92 L98 to offset the different weight of pistons?

Btw according to Lingenfelter's book, they also changed to a slightly different rod I think in '89. Same weight but smaller beam width and the weight is offset to the end, which he says is the culprit for some high rpm rod failures with these.

 

Org customer 20k mile 91 formula 350 pistons, broken ringlands are from poped head gaskets from to much boost/ lack of fuel.