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Discussion Starter · #1 ·
When building an engine and cylinder boring is necessary ... say the cylinders must be bored .060" over ... and you buy .060" pistons and rings ... which one is really .060" over? It can't be both. I know it is the norm to take cylinders and pistons to a machine shop and have the cylinders bored to be just right for the pistons. But what is it, though? Are pistons usually right at .060" and do the cylinders have to be bored a hair bigger for proper fit? Or are the pistons usually just a hair smaller to cause a near exact .060" overbore?
 

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Pistons are designed so that they'll fit a hole sized to .XX" oversize stock bore. You still measure and size to fit the individual piston because of manufacturing tolerance. Say the bore is 3.5" and needs +.040 to clean up based on condition and inspection. You buy a +.040" piston set. The paperwork that comes with them says .0035" clearance. You measure the pistons and size the bore accordingly. Most quality pistons are very close from one to the next but that's not always the case so you must measure every one and proceed from there using the info that comes with the set.

Better?
 

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Discussion Starter · #4 ·
Thanks for your replies. The reason for my inquiry is I'm tearing down an engine and I found .060" over pistons and no cylinder wear. I've checked closely with a fingernail-slowly-up-and-down-the-cylinder test and I can't feel any wear. I was hoping to be able to buy some new .060" pistons and rings and drop 'em in, but I don't think that will produce the precision I want.
 

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Measure or have it done. If it's within spec,straight and true there's no reason a ball hone and new parts won't be fine. Don't give up until the numbers say to.
 

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Just what geezer said, except I might add it can be hard to measure a bore without excellent tools and some practice. I can measure diameter real good with micrometers, but have learned to let my machine shop cat measure the bore.
 

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Discussion Starter · #9 ·
Is it the case that the engine has .60 pistons in it already, and that they are fine and the bore is fine?
If that is the case You could put it back together untouched. Or get a hone done and just replace the rings.
The cylinders look good. The pistons show wear. I asked my original question because I was curious to know if I could just drop in new pistons. But I think it is almost guaranteed that new pistons with my used-but-without-wear-ridge cylinders would not fit well together.
 

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"Thanks for your replies. The reason for my inquiry is I'm tearing down an engine and I found .060" over pistons and no cylinder wear. I've checked closely with a fingernail-slowly-up-and-down-the-cylinder test and I can't feel any wear. I was hoping to be able to buy some new .060" pistons and rings and drop 'em in, but I don't think that will produce the precision I want."
why replace the pistons? are they scored? how could the bore be good if the pistons have a prob? they came from these bores,so, if you know what piston came from what bore, them hone/rering and youll be good! 060 is pretty big.....not recommended to go any farther over....but it HAS been done!
 

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Discussion Starter · #12 ·
I'm not an expert but I think since the pistons are aluminum and the cylinders are a much harder metal then that is why the pistons in my case are slightly marked here and there and the cylinder walls still look real nice. I'm sure there are plenty of folks out there who would just hone and re-ring and go. I want to be a little more precise than that. You know, get some real nice clearances.
 

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Im working on some cylinder work as well right now. Something to consider might be forged pistons? The forged pistons expand more and if you've got the clearance in your existing bore (after a fresh hone), you can get a good tight fit from a warm engine. Just throwing another option out there. Ultimately, the best plan would depend n what you are trying to do with the motor once done. Are you racing it, selling it, or just trying to freshen it up and keep running it?
 

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"Thanks for your replies. The reason for my inquiry is I'm tearing down an engine and I found .060" over pistons and no cylinder wear. I've checked closely with a fingernail-slowly-up-and-down-the-cylinder test and I can't feel any wear. I was hoping to be able to buy some new .060" pistons and rings and drop 'em in, but I don't think that will produce the precision I want."
why replace the pistons? are they scored? how could the bore be good if the pistons have a prob? they came from these bores,so, if you know what piston came from what bore, them hone/rering and youll be good! 060 is pretty big.....not recommended to go any farther over....but it HAS been done!
I am with kllrjo here, if you are talking about a couple little scuffs, I think I would take it all to your trusted machine shop with a guy you know and get his opinion. I would assume the pistons would be fine, with a quick hone and new rings and you are all set. As far as clearances, if the original pistons where measured and fitted right, you will not be changing anything except price if you change the pistons. All the wear is on the rings and cylinder since they are the only things that should be touching. You may get a piston scuff here and there from the skirts, which is why I would get it honed, but they should still be fine if there is no major damage to the top or near the grooves.

Note,,, if they look like geezers avatar, forget what I said.
 

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In a perfect world, you would take the cylinder bore diameter specs (which includes piston clearance) from the shop manual, add the oversize to it, and that would be the finished bore size. In reality, any machinist is going to want to measure the piston diameter to verify finished bore size. Replacement pistons often have clearance requirements that can vary from OEM specs and that has to be allowed for.

Bob
 

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Discussion Starter · #19 ·
I'm not a machinist, and this was my first time really using a dial caliper, but I recorded these dimensions the day before when I submitted this post. The more I stare at them the more I think I can hone and install some new .060" (same size as old ones) pistons and rings. I know it may look like chicken scratch but I was careful to take accurate measurements. What do you think?
 

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