O.K. I thought I knew everything but alas I'm just another dummy.
Here's the story and question.
Recieved a "broke" O.S. 91 C-Spec heli engine from a friend. (Long story). Rebuilt it from scratch. Test ran it on a bench with a 3-bladed acft prop (12x6). Tuned and it ran great! SO I wedged it into a GP Lancair cowl with a Bisson/Pitts and the fit is right on. After deciding on a couple of 3 bladed props from 14" to 16" dia. the prop shaft is too short!! Add a spinner backplate and the prop shaft threads don't reach out far enough to grab a prop washer or prop nut. What now?? Counterbore the prop hub?

Doofus

You'll have to make some kind of adaptor. There aren't any parts to convert the O.S. .91 SZ-H C-Spec to an aircraft type. The aircraft engine is the .91FX, and almost all of its parts are different from th e.91SZ Heli engine.

I welcome the advice, Thanks.
With the exception of the shaft length I'm having a pretty successful conversion effort. Muffler fits. Mounting and clearances are good...
Did a check yesterday on the depth/strength of the prop hub area. Going to counterbore into the prop enough to drop in a prop washer and a GP 5/16-24 nut/spinner adapter. Still leaves enough meat on the prop hub for strength and balance.

If all else fails you can find a machinist with a lathe and machine away the stubby prop shaft and drill and tap the end of the crank shaft for a long “set screw�. Jett uses this system as well as K&B.

Prop clearance problem is being addressed by our machine shop here at work, BUT NOWWWW, Guess what? I was wrong about the muffler clearance. Cowling narrows too much on the bottom and ends of the muffler interfere. Have to start looking for a smaller (shorter body) muffler or cut the cowling, YECH!

Tru_turn makes short shaft adapter nuts. I call them tube nuts. I have no idea if they make one for your engine thread sizes, but I do know the made them for YS 120's and OS .46 VX/DF's.

OK, Anyone got any advice or opinion on this? I counterbored the hub on a Master Airscrew 3-bladed 14x8. I did this to accomodate reaching in to a short shaft with the prop hub nut. Question is have I weakened the prop hub by cutting down into the "spokes" of the hub?? Here is what it looks like now.

If you modify a propeller, you can compromise its structural integrity. The counterbore can weaken the center because the full width of the blade would no longer be supported. We cannot advise such a modification.

Enlarging the diameter of the hole in the middle, though, is commonly done. With your engine, you may be able to use something like this:

http://www3.towerhobbies.com/cgi-bin...2934&search=Go

Perhaps one can be made.

Tried your suggestion earlier in the process.
1. Couldn't find a 5/16-24 thread nut.
2. After a spinner plate, prop and washer absolutely no crankshaft threads are available for a nut.