are these stock pistons?

**ggino85** · 08-16-2011, 12:02 PM

misc gianna 2011 066.jpgim working on fixing a blown headgasket on this car a traded for.......98 trans am in sig.........i pulled the heads a few weeks ago but i just noticed this while cleaning stuff, the pistons have H868CP .25mm stamped into the tops, under that is 20106-13-C. i looked at pics online of stock ls1 pistons and they dont have this on them as far as i can find, anyone know what this means exactly? has the bottom end most likely been rebuilt? the previous owner mentioned something about boring the motor but i thought he was full of it......... i really dont want it to be bored, i know/or i've read that ls1's dont handle boring well..............any info is appreciated.

**TA98** · 08-16-2011, 06:21 PM

do not look stock. I believe the stock have a visible "dish" to them and those look like flat tops.

**ggino85** · 08-16-2011, 07:28 PM

yea they are flat tops......im just a little confused now on what to do...should i crack open the bottom end and see whats going on? i dont want to do all this work, swap heads, lifters, oil pump, timing chain ect......and something not have been done rite with the rebuild, it apparanty had. part of me wants to open it up and part of me says not to, it was running fine as far as i know before i pulled the heads, i couldnt drive it long when i test drove it because of the overheating issue. (which was a blown headgasket.) the little time i did drive it, it didnt knock, tick or anything. i wouldnt have been able to tell there was a problem if the previous owner hadnt spoke up about the blown gasket, i woulda noticed when it overheated of course but he could of been long gone by then. so im trying to figure out wtf to do.......

**Mr. Luos** · 08-16-2011, 08:02 PM

Stock....

**ggino85** · 08-16-2011, 10:03 PM

thank you for the pic lous, so they did come stock with flat top pistons. now im confused lol, all the pictures that came up from google had pistons with dishes in them. anyways, do you know if stock pistons have H868CP .25mm stamped on top of them? im still wondering what the .25mm means.

**TA98** · 08-17-2011, 05:21 AM

you could always check the bore with a micrometer. that would tell you instantly if its been bored

**redbird555** · 08-17-2011, 07:25 AM

all ls1 pistons are flat tops. However those are not stock pistons or at least that one isnt. Ls1/ls6 pistons have a textured look to them whereas these are smooth on top. Stock pistons were made by MAHLE for gm and will usually have an M on the inside of the piston skirt. Those pistons are sealed power pistons as sold here by jegs http://www.jegs.com/i/Sealed+Power/844/H868CP/10002/-1 .25mm means that the piston is over sized by .25 mm which converted to inches is .010 overbore (the max overbore for an ls1 block)

**ggino85** · 08-18-2011, 05:30 PM

awesome, thanks for the info redbird. i checked closely and all the pistons are new, the clean piston is the one that had the blown head gasket. so now im stressing because i've been reading that 98 blocks shouldnt be bored more than .05 over. opinions? anyone have a motor with .010 overbore? hows it holding up?

**redbird555** · 08-19-2011, 03:49 AM

I've heard of some people going .030 over on newer block so .010 may not be a huge issue. I'd run it and see what happens even if it is an issue you wont know unless you drive it and if it is you'll be out maybe a hundred bucks for the new gaskets and bolt? But I think it'll be ok especially since its been running a while.