I've been having some problems with compression, and wanted to check that everything else is ok before I get the cylinders bored.
I started off with a little rust in one of the cylinders (previous post) (https://www.2strokeworld.net/forum/index.php?topic=6755). I ended up honing the cylinders and replacing the pistons with new Pro X pistons. The hone did seem to clean up the cylinder walls nicely, and there was no indication of rust or pitting. When I test it now, I get slightly higher compression at about 100 (up from about 95 with the stock pistons/rings), but still not great.
When I measured the ring gap on the new piston rings, they were a bit larger than spec at Top: 0.43mm and Bottom: 0.48mm. I believe it's supposed to be .2 to .4mm. Would these being a bit over spec cause the low compression? Could it be anything else I should look for first? The top copper gasket is new and in good condition.
If it is the ring gap causing the low compression, what are the options? Could I put the next size rings on it (64.25), and file the ends to fit, or is it strictly a rebore?
imho time for a rebore
The new rings having excessive end gap means the cylinder bore is worn. It may be out of round too. That will cause low compression. You really need to check the cylinder with a bore gauge and it will tell what's going on.
^ yup.
I agree, that is what it seems like.
I measured it prior to honing, and got pretty consistently 63.95-63.98. I remember measuring after honing it and getting similar results (I didn't write them down, just compared). I just went through and re measured the bore, and again consistently got 63.96-63.98 in both directions.
Maybe there is some error in my measuring? I don't have an actual dial bore gage, so I'm using a cheap set of telescoping gages, and an ok set of digital calipers.
You're missing part of the equation. what you need to know is the piston to cyl wall clearance, that tells you if the bore is worn. The nominal size of the piston just tells you what bore you're on.
Although not Yamaha 2 stroke rings, I have taken a lot of 4 stroke rings out of the box which were at or outside the max gap even when the piston to cylinder clearance was in spec. Hondas were the worst. I always bought the next O/S set of rings and would file as necessary.
I agree that the piston to bore is the key measurement. But depending on the end use, a wide (within reason) piston to cylinder with an O/S set of rings filed to spec can work. At least in the short term. Whether or not it would work depends on a number of factors.
The rings are the seal, the piston keeps them square. The looser the piston is, the more rocking there will be and the more it allows the rings to wear the faces curved rather than square. Then there's the issue of cylinder taper..................
:twocents:
I agree they'll make useable power way beyond the wear spec, especially for street riding.
I put a 64mm Pro x piston in it. The piston measured 63.58 at the top and 63.92 at the bottom. Pro X says it needs .06-.07mm clearance. Is it the skirt diameter that needs to be within clearance or another number?
I guess I figured it was still pretty close to stock bore so the a new piston should fit. I guess next time I need to be more careful with measurements.