We'll say my engine block needs .030" to clean it up. Do I have to get .030" over pistons or just rings? I feel like this is a dumb question, but oh well, no one ever told me and car magazines assume you are born with this knowledge. Please help me unlock the secrets of the universe.

millis

We'll say my engine block needs .030" to clean it up. Do I have to get .030" over pistons or just rings? I feel like this is a dumb question, but oh well, no one ever told me and car magazines assume you are born with this knowledge. Please help me unlock the secrets of the universe.

millis
You will need pistons that match the bore diameter...

Ok, thought so, but wasn't sure. Thanks.

millis

Hey, no problem man. The only dumb question is the one that doesn't get asked...;)

I've torn some down that were .040 over from nothing more than wear - standard bore above the rings and pistons were std. Smoked like Cheech & Chong and had poor compression, I was surprised it would even start and run. Most manufacturers have recommendations for piston-to-bore clearance of a few thousandths. Depending on what you're building, pistons can cost from $75 to $750 for a set. Good luck, this is a good place to ask questions with a lot of experience among members.

Myboty, LOL, you could have put 40 over pistons in and honed and run. That's an amazing amount of wear.

The reason you would want to match the pistons to the bore is that you would get horrendous piston slap with a mismatch like that. Specs usually call for something like .0015" clearance between the bore and the piston skirt and with what you described, you'd have over .030".
Try to picture the piston rocking on the wrist pin at the top and bottom of the stroke. .030" would give the skirt a running start before it hit the bore and you could probably hear it a block away.

hey man, dont worry not a dumb question at all. i know exactly how you feel. im a rookie when it comes to motors(the rest of the car i can handle) and most books or sites just assume you know things like that. that is the glory of this site, the guys on here are real helpful and dont matter what you ask.

That's whats good about this site - guys will never make fun of you for asking a so called "stupid question".

Try that over at LS1tech - you'd get a bunch of nasty replies (no pun intended)

hah, yeah...LS1 tech. Those guys, I would never ask a simple question like that over there. The community is much better on this board, really the best I have seen and I have posted a little on a lot of forums.

Thanks, btw, for the explanations of why not to do it.

I ask because I found a used rotating assembly I want for a good price. It even has forged pistons, but they are standard bore. I may be able to find a good block (GenII LT1), if not I'll try to trade for some forged pistons at whatever overbore.

millis

hah, yeah...LS1 tech. Those guys, I would never ask a simple question like that over there. The community is much better on this board, really the best I have seen and I have posted a little on a lot of forums.

Thanks, btw, for the explanations of why not to do it.

I ask because I found a used rotating assembly I want for a good price. It even has forged pistons, but they are standard bore. I may be able to find a good block (GenII LT1), if not I'll try to trade for some forged pistons at whatever overbore.

millis
If you change the pistons.. be sure to have the rotating assembly re-balanced. ;)

It is funny you asked the initial question.

When I bought my 350 out of the paper, the person claimed it was rebuilt but a cam lobe was wiped out.

When I brought it back I planned on stripping it anyway. After taking the rotating assembly out and taking some measurements, the pistons were standard bore pistons, the bore was 4.030.

Not only that, but after using plastigauge on the bearings, the wrong bearings were installed as the crank had been but 10 / 20...

I of course started from scratch for re-assurance with a trip to the machine shop...

I guess this guy that sold me the engine should have asked your question...