Since there has been a lot of discussion on different RYOBI engines, I feel it necessary to share with you some information I have collected over the years. I have studied at length three different engines, and these are the spec's as I measured them.
Engine Bore Stroke Inside Measure on Crankcase Outside measure of Back plate (Reed Plate)
28cc 1.32 1.28 2.2 2.1
31cc 1.38 1.28 2.02 2.02 Early Style, recessed reed valve
31cc 1.38 1.28 2.2 2.1 Newer style
I have short shaft engines that have narrowl keyways on the crank, and others with wide keyways. I also have long shaft engines that have both. I have a back plate from a whip sold under the STIHL name (stamped on the WALBRO Carb Cover) that is white in color that is more square, not smooth like the the newer back plates.
I have four different con rods, form very light weight to one that looks like it is really beefed up, but came in an otherwise stock motor.
I am not going to address the different cylinders because there are so many different port shapes, and I haven't really got into that testing yet. However, if you leave out the gasket for higher compression, you should check the port timing and modify as necessary. You can change the port timiming by up to 5 degrees if you leave the gasket out.!!!
All blocks are interchanglable with some minor mods. The early piston has more metal on the front skirt of the piston that can be ground down, but I don't recommend this because the early engines have a smaller counterweight, and this may be a more desireable engine in its original configuration."Don't cut up something you may regret later) "voice of experience".
The stroke, wrist pin and journal size are all the same in the every engine I have checked.
I will do further testing as the weather warms. Presently, I have engines turning a 20 X 6 close to 7500. I use mostly electronic ignition, but also mags. I am still using the stock Carbs with some variations, but will go bigger on my next experimenting. FLATHEAD 4