Engine manufacturers use close tolerances in the production of their engines and there are times when cylinder bores may have to be machined oversize to stay within production limits. This is entirely normal, but can confuse mechanics who are not aware of this practice.
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All cylinders should be measured for size with a micrometer, or check the ring for size in the cylinder bore before the rings are installed on the pistons.
The ring should be placed in the cylinder BELOW THE RING TRAVEL for accurate checking. Ring gap on cast iron rings should be at least .&#; for each inch of bore size. For instance a 4&#; ring should have a minimum gap of .014&#;. Ring gap of more than .008&#; per inch of cylinder bore indicates an oversize cylinder and oversize rings may want to be considered.
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i rebuild a lot of Tecumseh hs50 engines... probably done at least 50 of them for my bikes. and i always buy .010 or .020 oversize rings on the standard piston, and just adjust the ring end gap to the motor in question. i have never used standard rings. The advertised suggested ring gap should be .006 to .020. If you use standard rings, even on a low hours motor, at best you'll end up at .015 ring gap. heck i've bought brand new short blocks and have measured the ring gap and it's never less than .015, and sometimes .020. If you want that tight ring gap you have to buy oversized rings and adjust them to the desired gap. Personally i like .008 to .010. That's what i shoot for.
But the big problem these days is getting oversized hs50 (or hs40) rings. They are just not available. Also with hs50 motors, there's another complication... in the s they changed the bore size slightly when they went to thin rings. So you have to match that up too. Meaning you can't put a thin ring hs50 piston and rings into a thick ring block (or vice versa.) So it's not just a matter of getting hs50 .010 or .020 over rings... you have to get the right style rings too (be it thin or thick.)
If you are looking for more details, kindly visit Motorcycle Piston Rings Suppliers.