Full-circle cranks

[steve miller]

..I still think a stock blue printed mcculloch with correct combo.. pipe., clutch .,gearing .,chassis .,and driver ..is hard to beat ..

[Tony Severino]

I agree with Steve. I only use Full Circle Cranks on Motors with Box Mufflers and Max Torque Clutches, then again it doesn’t seem to hurt all the Foriegn Motors.

[David Luciani]

historically full circle cranks were part of the whole "stuffing" the crankcase theory that was popular in the late fifties to early sixties.
the idea was to increase fuel flow to the combustion area by reducing crankcase volume from crevices by filling every cavity you could with epoxy.
the theory was a smoothed/filled crankcase cavity would direct fuel where it was need without any being "trapped" by open spaces in the inside of the block.
this was before fuel flow was well understood by engine builders.

the effect of stuffing does have some benefits with open header engines and performance can be increased .
but when expansion chambers came into prominence the game changed .
the performance gains with slight mods to the e-chamber were alot easier.
stuffing became a little too exotic and time consuming to bother with anymore unless you wanted every available "trick" to be in your bag.

how stuffing was achieved on the crank.
bell cranks were an obvious area that could be fill by making them full circles.
as steve said the extra wt. from the epoxy could cause cause imbalance.
several solutions are suggested in the old hop up articles to counter the imbalance.

adding extra wt. to the counter ba1ance was strongly advised.
carving balsa wood inserts then covering outside with epoxy to reduce added wt. was advised.
manufactured circle crank kits had magnesium bolt on 1/2 circles with extra wt for the counter ba1ance.
i have several of those cranks in my collection.

there was also a manufacturer that produced full circle cranks down texas way.
they were stamped in a full circle then six holes were bored in the circle.
lead was added to the counter ba1ance side and the opposing holes were left empty.
i've run that crank and it doesn't seem to increase vibration .
to me there wasn't any really noticable performance gains but it is a nice crank.

i've always thought if new mcculloch cranks were repo'd this would be an easy machining solution for an assembled circle crank made for mccullochs.

[Simon Murphy]

Thanks to everyne for your responses to my question. 

We now know why most Macs don't have full-circle cranks - I for one had not realised that expansion chambers reduced their value.  My retired karting buddy seems to be all enthused, and wants to see my Macs next time he is down here - perhaps he will want to get them running? :-)