.... maybe.

The replacement O-ring on my MZ TS250/1 gearchange shaft lasted around 120 miles before it started weeping again. That weep grew into a drip & got to the point that I was having to empty the caught oil from the drip tray back into the gearbox before a ride - clearly not a sustainable solution !

The problem is a ring of corrosion on the shaft (very slight, but there) where the O-ring sits. Changing the gearchange shaft is an engine strip job, as it is trapped between the two crankcase halves, and the engine is otherwise running nicely so I would rather not do that. So what about adding a conventional seal on the outside of the clutch cover - like a "normal" bike ?

I found a very narrow lip seal (14 x 20 x 3) that looked like it might do the job & eventually managed to track down the spare used covers I have, cleaned one up & did a bit of measuring :



The gearchange shaft "boss" inside the cover looked beefy enough to withstand having a 3mm wide by 3mm deep recess cut in the outer face - the case wall is around 3.5mm thick & the boss just above the fillet radius is around 26mm (14 mm shaft, 20 mm seal).

So on to the milling machine, roughly aligning the shaft bore with the spindle by eye using the chuck key :



and the centering "refined" with a dial indicator.



Boring head fitted, adjusted to just touch the bore & then 0.5mm cuts put on using a dial gauge :



Finished bore came out at 19.97mm according to the bore mic & the seal fits snugly :



The original O-ring groove is stil present & the cover will be fitted with a new O-ring on re-assembly. The curved outer edge of the cover has meant that the new seal is not fully engaged in the bore as you can see. It does engage - hopefully enough ! I didn't want to go deeper, though could if required - no signs of breaking through inside - but this would mean loosing the O-ring groove. Maybe insert the new seal with a smear of 3Bond or similar case sealant ?

Anyways, need to find out my buffing wheel to polish up the casing before building in the kickstart mechanism (it is mounted & tensioned in the casing before refitting - don't think the kickstart lever can be fitted with the case fitted to the engine) & then swap the modified casing over for the original - probably causing the death of another expensive gasket in the process !

Nigel B.