OK. something started to bug me,so I got the digital mic out and started to measure everything again.
I have 1 that is differant.The piston is 1.306 in. It came from the small case.It is 2.0615 in.inside diaganal from bolt to bolt.
The other cases measure 2.1990 in. inside case bolt to bolt.

There are differant cranks. Long and short shafts.I was able to compaire them with one I dropped and bent the shaft.That one was the larger case.
by the measurements,there is no way the large case crank could fit in the small case.
I have put some back together,swaping heads to cases.Some do hit at top dead center. Some don't.
I have 5 differant heads also. 4 slots runners, 2slots, 2 slots with comp releifs, 2 slots that is just differant, and then the smaller cylindar.
Now the question.Are they all 31cc ??
The smaller cylindar is maybe 28 cc.