The carburetor I used on my SS Nelson (Q-500/Q-40) is an OS .60 "7M" Dave Shadel cut it so it would fit the crankcase. It measures .370" with a thin sleeve in the bore. The biggest problem is you cannot keep it from shaking, and coming loose. Dave told me to JB Weld it, but I didn't! Dummy me! It took about a dozen runs before it shook loose. It didn't fall out, but would wiggle. It idled as well or better then any sport engine. Actually, it was a ***** cat! I never tached it at idle, because there was no need to. Greg