ORIGINAL: HWM77
Hi yakfish
if the engine has the std 7mmØ shaft the cheapest way is to buy a super tigre 27 prop drive and collet.Make up a std venturi about 9mm for a 21.DO NOT use any thing but a carbon fibre prop,i use a Sherlock 7x4 prop for test use,my sts 21 turns 30k plus.
I've wanted to do this same thing for a long time to break in my car engines, I think it is probably the best way to break in any engine, but especially ABC engines. The crankshafts on my .20 smallblock car engines only 5mm, and its probably too short to boot. My big block engines (.26 and .28) are probably long enough, but the crank isnt completely threaded so putting a prop nut/spinner on it is out of the question. I think a different crankshaft may be in order to make this work, at least for me anyway..
The prop driver is one thing, that probably would help clear the carb... But getting/keeping the prop tight to the crank is gonna be the tough part, as I see it.
Edit: Here's the crank from the Super Tigre .27 Airboat engine.
Here's the crank from my SH .28, and Traxxas 3.3
SH
TRX 3.3
If I could fit a prop to one of these without having to change crankshafts, I'd break all of my engines in on a test stand instead of in the car.