I recently had a TS250/1 and a ETZ250 engines stripped side by side and looked at the differences in the gear boxes. The 3rd gear woes are well known and I have also heard of 5th over heating at sustained high speed. The only significant difference I could see was to what I call the oil drip tray.MZ had obliviously tried to increase the oil dripping on the cluster. I have heard that ETZs suffer more than TSs so it obliviously did not work.
I noticed a lot of casting remains which could restrict oil getting to the drip tray on the TS so I removed them. The oil is fed to the tray by being thrown up by the clutch / primary drive, I wondered if the oil could not replaced in the primary case fast enough to feed the drip tray at high revs which would explain the revier ETZ suffering more. I then checked the hole allowing oil into the primary case and found it was smaller on the ETZ .The ETZ was a parts donor for race TS engine so I used the ETZ drip tray and enlarged the hole for oil to get into the primary I don’t know if it has helped but the gearbox is performing perfectly after a season of racing and parading .We all know long it takes to fill a TS/1 or ETZ gear box due to the oil needing to fill the primary case so hopefully these mods will help. I hope this is helpful to others.
The only difference between the early Supa5 and later model gearboxes is the slot in the input shaft to drive the oil pump. Also the oil drip tray is exactly the same in the Supa5 and Saxon (ETZ301) engines I have worked on. I’ve also never had any of the gearbox problems described, so I reckon that’s just a phase they went through. One of my current Saxons is running with a few gears from a Supa5, but I can’t remember which gears or which engine!