Piston ring sizes in Technical Posted April 5 7 hours ago, Meadowfleet said: Engine Rebuilding 101: -Can someone explain to me the difference between STD rings and .010, .020, etc.? -Are the standard rings the same size as oversized rings, the oversized rings are just a wider diameter? Or are the oversized rings thicker in size? What I mean by thicker is if you lay the ring down and use a caliper to measure the size of the piece of metal, not the diameter, is it thicker/wider than a standard ring? -Also, can you use an oversized ring on a standard piston? -If someone bored an engine would they bore every cylinder or just the ones that were damaged? Bloo has given excellent insite, if I might elaborate a little more though. Consider a ring starts out as a solid piece of round metal, in simple terms if you turned off an end piece on a lathe you would have a solid slice of metal of a given diameter and thickness, if you then drilled /machined out the centre of this slice, you would finish up with a metal ring of given thickness and diameter, cut a slot in it and you now have a ring which you can carefully expand and slide over the piston top and fit into an appropriate width groove in the piston. So pistons and rings are made to measure for given size cylinders, obviously wear and tear makes the precision sized cylinder oversize in due course, so there are now various options to overcome the problem of lost compression and excessive oil consumption due to this wear, a lot will depend on the severity of wear as to repair choices. Oversize rings is one solution, usually coupled with remachining the cylinder bore and oversize pistons to suit. But just focusing on the rings, to answer your questions, consider a bigger diameter piece of round metal ( say 0.10 or 0.20 or 0.30 greater diameter) machining off a slice and going through the same process as before, you now have an oversize ring; generally they will be of the same thickness as before. So now you have an oversize ring and it can do the job of making up for cylinder wear, however it`s not that straightforward and Bloo has done a good job of explaining why.