The manual for my Sig Skybolt kit makes mention of this. First I've heard of it, but apparently this is some sort of machined fitting to move the prop an inch or so ahead of the issue thrust washer.

The purpose is to be able to move the engine farther back against the firewall while maintaining a certain cowl location; in this case, to get more of the Pitts muffler inside the cowling, out of sight.

Anybody heard of this? Does it work? Does it cause any ancillary problems? Where to get one?

The engine in question is an OS .61 FSR with a SlimLine Pitts muffler. The Sig kit is not a Super SkyBolt and has a narrow cowl. I may just fabricate a Super Stinker cowl and enclose the cockpit. Now that I think about it, that wouldn't be too involved, and with no risk. The whole concept of a "prop extender" sounds a bit shaky...

Hi!
I got a 1/2 " and 3/4"extensions from FOX 20 years ago...they might have them today?!
See picture of my GE BEE R-2 using the 3/4 inch extension.
Regards!
Jan K
Sweden

They still have the extensions. Here is the link: http://www.foxmanufacturing.com/

Type in prop shaft in the search and it will take you right there.

Thanks.

How does the extension lock into the thrust washer? Or does it?

It doesn't "lock on".

The extension goes against the thrust washer on the engine. Then the prop, prop washer and prop nut. When you tighten the prop nut down everything is held in place.

There isn't room on this 61 FSR for all of that; it has a rather short crankshaft.

See if you can find a prop nut similar to what they use on a 4 stroke for a locking nut. The nut extends down through the prop washer. MOST 4 strokes and some larger 2 strokes (OS 160 FX ) have them.

You will need to enlarge the hole in the prop washer and possibly inlet the front of the prop slightly to accomodate the extended part of the nut. As long as you can get AT LEAST 3 full threads into the nut you will be OK.

Hi!
What you do is you use a long prop nut that goes inside the prop.
I made mine from aluminum.
Regards!
Jan K