White Rook,
On my
Q-52 it is 6 13/16" but my shaft extension is 2 5/8" from front of crankcase to prop hub. The mount holes are 2 5/16" on center each way. (Square pattern)
I have my Q-52 with a Pitts muffler and CH electronic ignition, carb intake extension and Cub style aluminum hub, for sale, if anyone is interested.
The engine is not quite broken in as yet but has had about 6 tanks of fuel run through it. It was in my third scale Cub until a friend (?) knocked a big ladder onto the plane and totalled it. No plane, no need for engine. It ran great with the CH ignition and was very easy to start by hand propping it. Get a good prime in the carb, flip it 4-5 times with choke til it fires, open the choke and flip twice. RUNNING!
Pilotdb09