Well it sounds like something I might be able to pull off. If for some reason I can't use a Yamaha piston I guess I'll try that. Thanks, I appreciate the information!

You were correct... 997 definitely looks like the size.

And I'm not that fluent on the crazy rod length changes that Yamaha made over the years on the 250/360/400... but if you wanted to go with a Honda piston the 125mm rod would eliminate the need for a spacer. 4mm + 0.030" + 0.030" = ~ 5.5mm total under cylinder.

5mm shorter rod + using stock 0.020" (~0.5mm) gasket should be within a couple thou of the spacer route and look stock.

I already have a crank kit with the longer rod ordered but the seller has kits with shorter rods as well so I guess I could swap kits with him and just be out a little postage. But unless Enduronut tells me there's some reason not to then I think I'll get a first oversize from him and have Bill Bore out my cylinder to fit. I'm pretty sure it'll need bored anyway and if I got those measurements half way close then one oversize should make things just about right. If it turns out it needs bored out even more than that I'll just have to go to 2nd oversize. I think Enduronut said he had those too.

I would go Yamaha myself... Enduronut probably has all Yamaha oversizes... but his supply is probably dwindling. I'm not sure he ever had many standard pistons as their use is "questionable"... not too many bikes... especially this many years later... have a cylinder that legitimately needs or can even use a new standard piston.

Going with Yamaha OEM should be cheaper too.