Clutch / crank taper

[Andy_C]

Just starting to enjoy the TS when the clutch let go on from the crank taper.

Very lucky in that I lost drive just as I returned home after a ride.

Why it has let go is a mystery as I re assembled everything correctly / to the book, including de greasing the clutch and crank.

Hoping it has not wrecked the end of the crank and I can reclaim it using fine grinding paste - again mentioned in the service manual I believe.

Should there be a spring washer under the rev counter drive nut by the way ?- the service vide online shows one.

Ah well, a job for the comming weeks.

[Andy_C]

Update.

Took it apart this morning.

Crank taper has some very light scoring on it, I would imagine some fine emery paper would sort that out.

The taper in the clutch is shot though, the surface is badly scored / chewed up and cant see any way of recaliming that, so I will have to get a new clutch hub.

[Puffs]

Removed

[Blurredman]

Unlucky Arry.

I tend to actually use another 10lbs ontop of what the book says, for what it's worth i've not had a problem with the clutch coming off the crank for me.
Thankfully, clutch hubs aren't so difficult to find, esspecially with the same item being used for many years for varying 250 models. So it shouldn't be too much of an issue for you to source one. The crank should be better off as that is hardened???

I was never quite sure why a keyway was not employed in this application to prevent this very thing? People have said to me it is incase the piston should seize you're not left with the rear wheel locking up on you. But as I see it, when that sort of thing happens you grab the clutch anyway, and if it was true (I think it's another one of those MZ myths that fly around) then not only have you got a damaged piston, but also a clutch and or crankshaft.... That, as the MZ manual says isn't good use of replacing parts that shouldn't get broken in the first place.

[Gerryman Ts125]

Brutal, at least you were not caning at the time it let go. Over twenty years ago my courier mates Honka VT 500e's starter clutch/alternator let go on the a8 in Port Glasgow. The nut at the end fell off (he though his engine big ends had failed) he pulled over as the ignition pick ups where also on the same side (talk about all your eggs in one basket type of thing) He got a free ride back to his lock up garage via a fellow van courier. Once back at his lock up he took off the side casing, to find the nut holding all the bits on the crank had vibrated off, and was stuck on the stator. The threads were well gone. So he managed to drop some weld onto the end of the crank
Re-tapped it and stuck on a broader nut. But being him, he had already drilled through the stator and crank (after making sure, he had ground in the lot together with paste) And stuck a chain link pin in to the stator/crankshaft and drifted it well home. As it never had one before. Brilliant bit of hillbilly indinuity from him too. Never let go again, he sold the VT to some else. They also sold it on when the diff let go. But it had done about 250.000 miles by then. Hope you manage your repair as easily, but with less steps.

[Puffs]

Removed

[dave47]

I doubt if that explanation is correct. I've had a piston seize at 70 plus on a dry road and the clutch didn't come off. I think the tyre will always let go first if the clutch is on properly.

Andy, Is your bike the supa5? If so, it is better to use the ETZ manual for engine work rather than the TS manuals, where the instructions for fitting the clutch are misleading.

[Puffs]

Removed

[Gerryman Ts125]

Wow! 250k miles! I heart some of the BMW boxers do silly mileage, but I hadn't heart it for a Honda before. Courier work...

They are quite good engines (along with Cx 500`s) as long as you service them regularly, the heads are a bit weak as the ohc on the Vt 500 (like the CB 250 RS) runs in them, as direct drive no bearing, but easily modified) used on many a small kit car, micro car, bubble car conversions. And the occasional boat. Lots of low down torque you see. And inline shaft, watercooled and light and compact enough. Puffs.

[Andy_C]

I doubt if that explanation is correct. I've had a piston seize at 70 plus on a dry road and the clutch didn't come off. I think the tyre will always let go first if the clutch is on properly.

Andy, Is your bike the supa5? If so, it is better to use the ETZ manual for engine work rather than the TS manuals, where the instructions for fitting the clutch are misleading.

Dave - yes it is the 5 speed, and I havd an ETZ "Blue Book" shop manual, I will take a look as you suggest.

Thanks.

[dave47]

Puffs says:
"Anyway, with the design older than most of us, we'll probably never know the real reasons for not having a keyway. But I wouldn't mind hearing other suggestions!"
The simplest explanation is that the designer didn't think it was necessary. It costs more to add a keyway or splines.

[DAVID THOMPSON]

the magnet on a BMW/2 magneto has no key
if someone downshifts too hard and it slips it would make a bigger mess if it was keyed
many times i have had to time some ones BMW mag because he down shifted too hard
done it a couple times back when i drove to fast
dave
that magnet location is very important on a/2

[Puffs]

Removed

[Blurredman]

It is true for one reason that the stator must be keyed for timing. At least, that is if the outter side of the stator is keyed. If the outter side of the stator isn't keyed, than the inner side doesn't need to be either- as you can set up timing appropriately still. For an additional cost saving measure they could have done this? Esspecially as the stator rotor is also pressed on a taper. Maybe they thought it would be too much of an issue people mis-timing their bikes.

But you look at how simple the design of the key is on the stator side.. Just a pin in the crankshaft, and a slot in the stator.. They could have surely done this on the clutch- it wouldn't have been too difficult to do this, surely- presumably it would have to be a bit beefier but it could have been advantageous to reduce/eliminate clutch taper slippage.

[dave47]

MZ seem to have changed from a keyed taper on the TS125 to a keyless taper on the ETZ125, So they were obviously happy with it for whatever reason and didn't think it was a design weakness.