After pushing my luck with the red bike engine, I've decided to build a better motor. Some of the parts will be from the first one, other parts will be new. For now I'm planning on using the same block, Bullfrog piston, ARC rod & flywheel, head (the new one I just installed) & the Black Mamba cam. I've sent for some stainless steel valves from ARC, with new springs, keepers & retainers.
I don't trust the stems on the original valves, as they're pretty thin up top & I've pushed them too hard at this point to trust them. If I break a stem at the top, even at a low speed, the valve will drop into the piston & the motor would be completely destroyed in one turn of the crank. The new valves should take care of this potential problem before it happens.
I had a 60363 motor from a sale last year, so today I stripped it down. One thing I'm planning on doing is replacing the crankshaft in the new build. I had "fixed" the first crank by sanding it down by hand, as it was about .005" out of round. It was a real PITA & took a lot of time & effort to make it right. This worked pretty well, but I'm not going to take any chances with this build. I checked the crank on the new motor & found it's pretty close to round, only out by about .0005" as close as I can measure, so it's going to be a keeper. It will be much easier to just install new bearings & put it back together.
I've been looking into balancing this motor, as the original one had a bad case of the shakes which would go away at higher rpms, but it was pretty uncomfortable when it got into the speed where it vibrated badly. You either wanted to run faster or slower, but not in that rpm range.
I'll be checking the Bullfrog piston for weight as a part of the balance, but I ran the numbers with the stock piston, crank & rod, just to see where this motor comes in from the factory. The piston, rings, wrist pin & clips weighs in at 201.8 grams. The small end of the rod is 32.6 grams. The amount of mass needed to counter-balance the crank & rod for stability is only 59.2 grams. Not a lot of weight really, compared to the piston.
The "normal" balance factor for a single cylinder engine is about 50%. This can vary up or down by a few percent due to cylinder angle & a bunch of other stuff, but the 50% number is typically considered to be what works well. When I ran the numbers for the factory piston, rod & crank, it came out to only a 39% balance factor, so it's not too good from the start. They build this motor way out of balance at the factory. Can't wait to see what it looks like with the Bullfrog piston set up to run. I'm thinking that the Bullfrog piston is heavier than the stock one (not sure about that yet), which would explain the bad vibration problem I had. I'll get back on this as I find out more & start taking down the old motor.