Hi Hacksaw, The swinging arm A7 and A10 engines are interchangeable in the swinging arm frame. As Paul says all these frames started with an A7 number. The only exception to the rule was that a genuine Rocket Gold Star frame started with an A10 number and the S/A engines would also slide into those frames with no problems. The plunger A7 and A10 engines are also interchangeable but only in the plunger frames. Again, all these frames begin with an A7 number. I have seen swinging arm engines in plunger frames and plunger engines in swinging arm frames but it is not a straightforward job to fit them. You would at least need to make your own engine plates for a mix and match project like this. The swinging arm frames were available in 1955 and ran side by side with the plunger framed models until the plunger frames ended in 1957, so there was a 3 year period when both plunger and swinging arm models were available. The plunger gearbox can not be matched up with a swinging arm engine and the swinging arm gearbox can not be mated with the plunger engine. There are too many differences between the two types of gearbox. One other point to mention is that the S/A A7 and A10 engines will also fit into the B31 S/A frames but the B31 and B33 engines will not fit into the S/A A7 frames. Hope that this answers your question. Beezageezauk.