So, I just picked up a set of rings for my YFM225. All rings but the top compression ring fit perfectly. It doesn't seem to fit the groove...is it something I'm doing wrong or were the rings I ordered wrong?

 

It's just a piece of cast iron and a groove. If it does not fit I would vote for #2; you got the wrong rings. If you have the old ring and a mic you can mic it and see the difference.

 

I've installed rings countless times and never had an issue. This is my first rebuild on a small engine and I figured maybe there was something I'm missing. I've also heard there was a small point on some pistons that the gap was to fit between. The rings seem to fit partially but won't seat fully. I just looked on the yamaha site and there is an option for .50mm O/S rings...does this mean they are larger diameter or thickness?

 

normaly you have to get the next size up ring when putting in new rings did you hone the cylinder? use a caliper to measure the cylinder and the new rings. its also a good idea to replace the valve seals