I came across a newer Briggs 3.5 hp to use in place of a Briggs 3 hp. It has a short 3/4 keyed shaft and I could not use with my 3/4 bore clutch on it. So it happens I had designed my build with a rear wheel drive sheave that was smaller originally, about 10 inch diameter. Then it broke and I got a Whizzer Clone Sheave to replace it and it is 16 inch diameter. I was fine with slow speed greater torque as I ride trails. 20:1 ratio works with speeds under 20 mph. With the larger sheave I ended up having to connect two jackshafts together with 1:1 ratio to keep that 20:1 overall ratio. Now that I have a better engine I had to find a way to use it. I also figured out I could also swap back the older engine if I could figure a way not to mess with rearranging stuff permanently. I figured a way to instead of having the 3/4 clutch that would not work on the engines short 3/4 shaft, I am now needing again the two jack shafts connected with smaller to larger gears for reasons you'll see. I got bushing reducer 3/4 to 5/8 adapter to use the 3/4 clutch on the top jackshaft. The rectangular shape key I will make from 12 inch stock I got. The engine 3/4 short shaft works with a 3/4 small gear. A chain instead of belt I had prior now connects engine shaft to the top jackshaft. As soon as the engine is started, the top jackshaft is also moving. I can have 12 tooth to 12 tooth 1:1 ratio so that engine rpm is same as shoes on the clutch rpm. Optionally I can have a 12 tooth to 13 tooth gearing to have higher engine rpm than the top jackshaft which has the clutch on it. Could be I would have more torque at clutch engagement speed the same as rated, but with engine rpm at marginally higher rpm. When the clutch engages the clutch gear connect to a larger gear on the bottom jackshaft by a short chain. Then in line with the rear wheel sheave on the bottom jackshaft I still am using an adjustable diameter pulley with AX Belt connection. 1st picture shows by mistake, but not one that got very far, it was 12 tooth to 28 tooth gear connection. The 28 tooth gear is reserved for use between the clutch top jackshaft and the bottom jackshaft with the 28 tooth gear on it. If it had been assembled with the mistake I don't think I could ever get enough engine rpm to engage the clutch. In some pictures you cannot see, but I am temporarily make shift piecing extra jackshaft to see what it would look like. I eventually slid the parts into position and all I need now is some 3/4 ID washers to shim the bolt at the end of the engine crankshaft. I would also use split collar on it, but there is not enough room. You see the back idler pulley on the adjustable tension brackets in the photos, but it in no way interferes with routing chain instead. In case I need to swap the smaller engine back all its still left to do it.