MZ/MuZ Riders

Im finally reassembling my ETZ 250. When I got to the stage of fitting the crankcases back together they did not quite meet by about 4mm. There is something in the gearbox section keeping them apart. I had to separate the halves again and as a consequence the crankshaft has shifted on its bearing. So I will have to temove it completely and start the process over again.
I have replaced the mainshaft in the gearbox. Sliding gear on mainshaft & layshaft and all 3 selectors.
Any ideas on what could be keeping the halves from mastching up ? Or ideas on what process I can use to find out what the obstruction is?

Try putting the halves together with no internal parts. If that works try with just the gearbox installed IE without the crankshaft. Make sure gear shafts are fully home in the bearings. I had to use a heat gun and tap the shafts home with a small copper hammer.

I'm pretty sure that I have found the problem : the replacement main shaft is very tight in the replacement bearing and I believe it was not seating properly. I tried fitting it alone without the other parts of the gearbox assembly and it required substantial persusion to seat it properly.

Good that you found it, but such a tight fit is not ideal for the longevity of the bearings on either side of the axle, if ever you manage to assemble it (they'll have an axial load, which they shouldn't have).

If you want to find the culprit, try fitting the old bearing on the new axle, or the old axle in the new bearing, assuming you still have the old stuff of course. Hopefully it's the bearing shell that is tight, you can easily replace that.

Are your bearings C4 clearance? Might be important if inner race is interference on shaft.

Good point, they indeed prescribe high clearance C4's for that shaft (6204 & 6203). Maybe to cater for the difference in thermal expansion between the alu of the case & the steel of the shaft, maybe for some other reason.

Do you think the tight fit of the shell could increase the OD of the inner race so much as to significantly reduce the clearance, say from C4 to C3?

At any rate, too tight a fit on the shaft does not seem very desirable.

"Do you think the tight fit of the shell could increase the OD of the inner race so much as to significantly reduce the clearance, say from C4 to C3? " .
That would be a pretty typical reason to prescribe C4's.

I always thought that thermal expansion was the common reason: inner shaft heating up before the outer shell/casing. Or balls getting warm & expanding. Or indeed axial movement due to different expansion coefficients.

You might be right, but with the bearing alloy being rather hard & resilient, my gut feeling is that it would require more than just a little persuasion to mechanically expand an inner shell significantly. Have you done the sums?

parrbd wrote:Are your bearings C4 clearance? Might be important if inner race is interference on shaft.

Good call Bruce! Thanks
The new bearing I had installed is in fact C3. It was one of a set that I purchased from Ost2rad.com I didn't pay any attention to the clearance when I fitted the bearing as it came in a set I assumed.... but then as they say assumption is the mother of all f*@k ups. Ill contact my local bearing guy tomorrow and see what he's got

Bearing clearance rating is of course independent of how tight the bearing fits on the shaft. Make sure your shaft OD is not too big, as indicated earlier. You don't want to order new C4's to find that they don't fit either.

C3/C4 may not be all that important, as long as they fit properly on the shaft.

I have not done the sums but this might help. https://www.ntnglobal.com/en/products/c ... 2E_a08.pdf

OK, thanks, I see that document indeed mentions an interference fit as a reason for choosing a higher clearance, in addition to thermal causes.

But the prime issue here is of course that the bearing's inner sleeve was so tight on the shaft that the case would not close. This is not how it is supposed to be. While we agree C4's are best used here, I think the bearing's internal clearance rating is not the cause of that problem. I think that the problem is that either the shaft OD is too big, or the bearing's inner sleeve ID is too small. A micrometer could help, but comparing the fit on the old parts is more reliable & easier. Any news on that OP?

Beware, it might be the shaft.

Thanks for the information. I am now a little bit wizer about bearings. I dont think that I really knew what bearing clearance was until yesterday.
If I have time in the next few days before the outback excursion, I will try heating the inner race before fitting the shaft alone. If I get a better result and it seats properly ,I will do the same at my next attempt at reassembly. I think I will still look for a C4 clearance bearing though.