I am a 1981 LTD rider and now the proud owner of a 1980 Invader 4/6 Ltd ( which is not mobile at the moment ). I am looking for info on the bottom ends of the subject machines, are there any of these cranks interchangeable? If so which models with which? I also have other query on the clutching throw ramps , and on the drives the helix ramp degree' . I am looking for dimension specs so we can fabricate if I can't find replacement parts. All comments and or contacts that can reply I thank you in advance. Regards wildgilly.