Well with a bit more spare time, better measuring gear and making up some 'dummy' main bearings to sit square in the crank cases it appears that I have 0.15 mm (0.006") potential end float between the crank and cases. The workshop manual doesn't give a spec but this seems a bit more like what other people would be comfortable with. The smallest shim listed in the parts fiche is 0.3 mm so I guess the verdict is it all goes back together without a shim.

The main problems I found on dismantling (in case anyone else is stripping their motor and wants to know what to look out for) was as follows:

Magneto rotor woodruff key almost sheared. Clutch release axle (the shaft attached to the lever under the gearbox) had a section of case hardening chipped where it contacts the clutch release push rod. Both gearbox ball bearings totally shot. Crank big end side play excessive. The needle rollers in the transmission initally felt rough but after a thorough clean in the parts bath they seem smooth now so I will put it down to dirt.

A tip for anyone splitting the crank cases is that you can make up a cheap crank extractor using a Halford's universal hub puller (

My 1976 ty250 (e) has no shim fitted and its a great runner

Dave