That's the early cup with the half thread, and definately the correct type for your model.
It will screw straight on once cleaned. Make sure you have your tri cup washer fitted first (The washer that covers the 3 Flywheel puller holes) as this is your tension washer. Then rattle it up.
Yes maybe the thread is crossed or dirty.
Correct tap size is 1/2" BSF 16 or if you don't have a tap, then thread file, or if you have a dummy shaft or crap crank with good thread,put a little Valve grinding paste on the threads and slowly run it over back and forth as you go down, just like a tap.
Good luck, should be no Worries unless your threads are stripped.
If it turns to crap, I have a few if you need:)
As for the crank, the simplest test is to put the rope starter pulley on the engine and turn it...If it wobbles and dips/rises then we know its probably out. Occasionaly the pulley can be out to, but generally its the crank, if the bottom shaft is wobbling too.
Both shafts out of alignment is a slipped crank,
A single shaft out of alignment, especially the bottom is usually a bent shaft.
Out of alignment shafts make it really hard to start with a recoil starter.
Anyways, Hope its all Sweet.
