Quote:
Originally Posted by Don Rotolo
To others reading this post: Note what he says is the distance between cam and crank - one point two-nine inches. This is one very small engine.
Borna, you may get better results (and more informed responses) on a micro machining / engine hobbyist site. Steam engine folks also.
I am curious why you want to put the bigger pulley on the cam - in most 4-stroke engines the cam runs as twice the speed of the crank, not half the speed. Also, you MUST use a cogged ('timing') belt, since slippage will throw the timing off almost instantly. Or a chain.
As for tensioning - a belt tensioner just pushes against the belt to keep it tight, size is almost irrelevant.
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No he is right, the cam runs at exactly 1/2 the speed of the crank. More importantly though unless the model ran a belt, you should run a toothed belt or chain in order to keep the timing exact. Any minor slippage or delta between the exact 2: ratio would cause all sorts of timing issues.
With that small of a center distance, you could get by with 9 and 18 tooth 25 pitch sprockets. This will give just a tiny bit of tooth clearance between the sprockets. Unfortunately going below 12 teeth gets pretty lumpy, it also won't give you a lot of fine tuning adjustment for valve timing.