ETZ 250 crancase halves not meeting

[dirtsurfer]

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?

[parrbd]

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.

[dirtsurfer]

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.

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[parrbd]

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

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[parrbd]

"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.

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[dirtsurfer]

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

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[parrbd]

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

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[dirtsurfer]

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.