A McCulloch with a clutch at the engine (Long shaft) has 3/8-24 threads. One with a short shaft is 5/16-24

It's been a few years but I (For some reason?) remember the back plate bolts as 10-24? All the bolts should be replaced anyway for they are torqx head self tapping screws. The head stips out easy and they break.

When taking an engine apart, the screws are coated with Loc-Tite and will be difficult to remove. A little heat applied with a propane torch seems to soften the Loc-Tite but be sure and not to melt any aluminum.