3100 Piston Help PLEASE!!!!!

[jasonw24]

Hey guys, i am new to the forums and i need some help!! I have a 1995 3100 1st design that was in my wifes car and it threw a couple rod bearings. So i pulled the motor and tore it down. I have about 3 91 and 92 3.1's out of luminas laying around so i tore one down and stole a crank and some rods and pistons. I took the crank to the machine shop to have it checked and all it needed was polished. Great!!!! Now for the problem. The 2 rods that were spun i was just going to exchage with 2 rods from the donor 3.1 and have my 3100 pistons put on them. We honed the cylinders to get a nice cross hatch and was going to rering it standard size. I noticed that 1 of the 3100 pistons had really nasty skirt wear and was kinda grooved even but it did not hurt the cylinder. So i was going to put 1 of the good 3.1 pistons in it with the other 5 being the original 3100. Now i am seeing that apparently there is a difference in the pistons or at least in compression ratios from the 2 motors. Is this due to the head cc's or are the pistons really that different? when i look at autozone for replacement pistons it calls for the same ones in both 3.1 and 3100. Can i mix these or should i just put the 3.1 pistons and rods back in it. I know i will loose about half a point in compression but will this make a big difference in a grocery getter? Whats up i am confused!!! I am on a really tight budget so guys...... PLEASE HELP!!!!!! Thanks for any help in advance

[jasonw24]

Anyone??? i've got to get this thing back together. Advice??

[tractorman]

I can't give you any specific information about WHY those compression ratios are different... but I've put some strange things together in my time.

Just how different do those pistons LOOK? If they look pretty similar and measure the same, I'd loosely install the odd piston and rod without rings and check the height at TDC. If it's pretty close to the other pistons, I wouldn't worry about it.

I put together a ford 2.3L engine in a ford ranger years ago with on odd piston from a later model engine- the bowl was shaped a bit different. I never noticed any problem from it.

[torq455]

as long as the bore is all the same. you will loose some CR. you may notice a slight bump, bump, bump as it idles. if you don't mind that, go for it.

[Rhedalert]

The piston is most certainly different, with regard to the measurement of the "dish" area. The 3.1 will be a little bit bigger. As far as fitment it should work, but how bad was the skirt wear on the 3100 one?

[jasonw24]

Well the skirt wear was bad enough that it actually put a slight groove on both sides of it. I finally called GM and they said that the pistons, crank, rods are identical in the 3.1 and early 3100, up to 1997 that is. The difference in the 3.1 pistons and the 3100 is that the 3100 has a higher compression height which makes it stick ot of the hole and the 3.1 sits below deck and that accounts for the difference in comp. ratios. 8.8:1 versus 9.5:1. They recommended to not mix and match the 2 and they also strongly urged me to use coated skirt pistons.