The long shaft engine I have is 3/8" diameter instead of 5/16" like the short shaft ones. The C-H ignition hub will slide over this shaft. I cut some of mine off.

In theory, it is possible to make a shorter engine with the long shaft engine than than a short shaft one. One the short shaft engine, the adapter must be long enough to engage the threads on the engine shaft as well as the threads for the prop retainer bolt or stud.

Enjoy,

Jim