Why when they only go 30 over on the bore do they need to replace pistins? Can't they just replace rings?

 

Good question - I asked a buddy who usually knows about this stuff and he laughed at me... then he said you actually can just get overbore rings - BUT... (there's always a but...) Here's what I think he said-

IF the pistons are perfect, you could probably just hone or even bore the cylinders - they make overbore rings out as far as .060 over for some engines - but the result will be more stress on the rings and less life for them and cylinders. (The clever mechanic at a somewhat less than reputable used car lot might do this and they'd tell you its a rebuilt engine...) But if the engine needed rebuilt in the first place the pistons are not going to be perfect, they would need trued round just like the cylinders need honed or bored round. That's really the main reason - the pistons are also worn. Another reason is that the larger bore will result in a bit more displacement and all else being equal, a little more power.

Again, if you're gonna rebuild an engine, why balk at a few more bucks to do it right and insure that you get the most out of the motor?

 

Greetings,
In order for the rings to seal, they have to "gapped Properly." if you widen the cylinder, which is what your doing when you go "30 over", and don't change the pistons and rings, the gap enlarges when the rings try to seat on the cylinder walls and oil and pressure will pass through and kill your performance.
If you change pistons, I would purchase new rings even if the cylinder was not enlarged (bored). Why would you do all that work and leave an old set of Rings in to cause problems? Like Stated by Flame, one has to consider why the engine was torn down to begin with.

MV

 

i would not recommend it, your asking for major piston slap!!, if the piston was centered in the bore you would have .015" on each side most piston manufactures want a lot less, not exactly sure but a lot less than .015, and when they bore the block they will need the new over sized pistons to measure and bore each hole to each piston

 

No silly...

I was asking, why change the pistons when you can just change the rings?

 

Greetings,
Ok.....Why do all that work and leave an old set of pistons? Same thing goes, makes not sense, piston slap, all kinds of trouble.

MV

 

0.0035 is a pretty good target to shoot for. It all depends on where you measure your piston-to-wall tolerance at, too. Since the piston is tapered with the "skinny" side at the top, you have to pick a spot (usually about an inch up from the bottom of the skirt) to measure the tolerance from and stick with that. If you had 0.015 on boths sides of the pistons, you'd have a motor that rattled like nobody's business and would quickly destroy a lot of parts.