Starter drive lockup

[Jack in NB]

The worm drive extended against the spring and jammed against the drive gear threads.

This used to be a frequent problem if the mag was timed too far ahead and the engine kicked back against the starter. Have a half dozen in the parts bin.

But this was not the case this time.

Locked up on me last night as I tried to start her to plow out from under yesterday's foot of snow.

Is there something I'm missing in assembling the units?