something broke- revisited

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May 6, 2006
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I finally got into the gearbox and woefully found that the layshaft had broken clean off where it fits inside the kickstart shaft!! An obvious rebuild. My question, what could have caused this failure that I could address during my overhaul? This is my first venture into the gearbox and any tips for my rebuild would be appreciated!
 

Ron L

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Is the section in the kickstart shaft free? If not, it sounds like a fun job removing it. Check the bearing recess in the box for cracks. Replace the box if necessary. It could be that the bearing disintegrated and allowed the shaft to flex at that end and sheared at the bushing end where it was held straight.

Certainly replace the layshaft/kickstart shaft bush as well as the roller bearing on the layshaft. Check all the gear bushings while you are in there and replace any that look suspect. Check the gear teeth for damage and replace any chipped or damaged and the matching gear. Check the mainshaft for trueness. Replace if necessary.

I'm sorry to say that to do this right is not a cheap undertaking. Not to do it right is an invitation to throw good money after bad.
 
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I have heard stories, mostly 10 or 15 years ago of layshafts breaking and suspect that someone might have been making bad ones at that time. I thought that the usual break point was the diameter change at the bearing end but I may be mistaken.

Does your shell have a roller bearing fitted ? I have again heard it suggested that the extra rigidy of a roller can stress the shaft and lead to breakages. Replacing a layshaft bearing without replacing the kickstart bush wouldn't help this. I have not experienced this problem (touch wood !) and have used rollers for 20 years plus and have thrashed the nuts off it in that time.

Doesn't one of the dealers (is it Mick Hemmings ?) recommend an upgraded ball race rather than the NJ203C3 ?
 

L.A.B.

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The layshaft breaking like that was unlucky I guess, although not unheard of, as 79x100 has said.


79x100 said:
Doesn't one of the dealers (is it Mick Hemmings ?) recommend an upgraded ball race rather than the NJ203C3 ?

Yes Mick Hemmings recommends the use of a high spec FAG 6203TB ball bearing which apparently does not support the drive end of the layshaft as rigidly as the NJ203 bearing.

Norvil also sells the upgrade 6203 "special racing ball type" bearing. Norvil part 069366 http://www.norvilmotorcycle.co.uk/gearbox.htm
 
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