I have a Ryobi with the Carr Precision parts on it and it measures 8 - 1/4" to the back of the prop.

It was a short shaft engine and the hub assembles a little longer on them than a long shaft engine.

A Jag mount and hub stands a good chance of assembling at 6 - 7/8" with a long shaft engine.

With the Jag mount, the carb is right up against the firwall and you will need a slide type choke on the carb.

With the Carr mount, you can stick you finger over the venturi.

On mine, the hub is 1 - 1/2" long and the mount is 2 - 1/2" long.

The main body of the engine makes up to be 4 - 1/4".

With a short shaft engine it is possible to make a special prop nut by taking the original nut and threading it 5/16" - 24 all the way through and then turn it around to hold the prop on. The prop will have to be bored out to 1/2"??? so the prop nut can go inside it kind of like Super Tiger style. You will need to mill the fins off the flywheel flush with the counter weight. To clear the coil, you will have to space the prop out 1/4" from the flwheel hub to clear. On the long shaft engine you can mount the prop up close to the flywheel easy by boring the prop out to 3/8" and mounting it up against your 1/4 " spacer. You use a regular 3/8" fine nut ands washer on the front of the prop.

With the Carr mount it would be 6 - 7/8" from firewall to back of spinner. With the Jag mount, it would be 6 - 1/4 ".

Jim