using oversize rings on stock pistons

You young punks don't know nothin' about cheap engine building. Used to be sold what was called a '10 up' ring. This was back in the olden days, if the cylinder bores were worn less than .010 from the standard bore you 'knurlized' the standard pistons to 'swell' the skirt, then used a ring set which was good up to .010 over stock (thus '10 up').

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But yeah, if you are having the block bored you need OS pistons and matching OS rings. It concerns me that you can't seem to get a straight answer on the required oversize. Any competent machine shop will check the bores for ovality and taper, then recommend an overbore to clean up the wear and of course the piston must match the bore diameter. Ex: if the stock bore diameter is 4.00" and the measured wear is .020" then the cylinder should be bored to 4.030" to clear all the worn spots out and the new pistons/rings will need to be sized to fit the 4.030" bore.

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Originally Posted by

That's right on the limit, rule of thumb, up to 0.010 inch overbore with oversized rings on standard pistons. You're @ 0." (0.25mm). You certainly must bore cylinders to remove ridge & create the cross hatch pattern for the new rings to seat. You should really bore, then measure to determine if 0.010" oversize is enough to get the bores round & straight.

As stated, make no compromises when assembling a motor that you expect to push hard. Your old pistons might be marginal as well, they should be measured to see if the ring grooves, diameter & skirt diameter are within spec. Once bored, you might very well be greater than 0.25mm over

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