I think all the five series have the long bolt and spacer set up (I've worked on many of the models) , something to do with heat cycles and tension stretch, have a close look down the cylinder fin holes you may find the missing spacer stuck halfway! What I have also found on disassembly is a spacer has a tendancy to leap out and stick to the flywheel magnet.
My Top tip on rebuilding, is to coat the cylinder gasket with a thin layer of dirko before reassembly. Kind regards, Toby