Originally Posted by greggles47
If no lathe handy, put your coin in a bench vice and position the back plate slot over the coin and turn the engine anti clockwise. If it's stubborn place the engine in an over and heat at 200c for about 10 minutes.
The old coin in the vice trick is another good method to get the backplate off, but can I caution against heating an engine in an oven to 200 degrees C.
That temperature is perilously close to the critical heat treatment temperature of some Aluminium alloys. I'd stick to under 100 degrees C and only as a last resort, and after stripping anything inflammable off the engine. Boiling on the stove in a pot of water would be much safer.