Here's one problem mentioned by Mr Bum but it's not the one I'm looking for;
One last bit of information about the cylinders needs to be made here. In the later 1980's, BMW added a sort of "step" machined into the cylinder spigots, and a similar matching step was in the crankcase opening. If you try to fit late cylinders to an early bike, you MUST machine-off the step.
Never memorize something you can look up.