Layshaft roller bearing - shimming needed?

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Apr 7, 2008
I'm about to rebuild my gearbox and have ordered an overhaul kit from RGM. This kit contains a roller bearing for the layshaft (the failed was a ball bearing).
I have read about the need of shimming when roller bearings are used but I'm still a bit confused about how it's supposed to be done.

What would the prefered end float be?
Should I use shims to adjust it?
What type of shims and where should they be fitted?

Any guidance on this subject would be appreciated.

Best regards,
old brits web site has a "how to" or you can install an upgraded ball bearing and eliminate the shimming stuff.
So the Old Britts procedure is the way to do it. Thanks!

I've been thinking a bit more about the difference between using a ball or a roller bearing for the layshaft. I do understand the mechanical differences between the ball (takes both radial and axial load) and the roller bearings (only radial load) but why shouldn't you shim the end float also with a ball bearing?

My 2 cents: The layshaft has a float fit in the bearing and the ball bearing can only take axial load "inwards" the gearbox housing not in the opposite direction so the layshaft will still move axially if you have an end float.
I can also see a risk with shimming the layshaft, you might put axial load on the roller bearing and that's not what it's designed for. Maybe that's why Norton never added any shim procedure changing to rollers???

I feel I'm on thin ice here and maybe I'm just missing a bit of the puzzle :?

Cheers, Per

I'll add a few comments after being in touch with a very experienced British Norton guy.
His 2 cents (or since he's a Brit maybe 2 p) talking about using shims for the layshaft,
"Its not particular to the roller bearing, it applies to all gearboxes, if they have excessive end float on layshaft first gear, this can cause jumping out of gear, depending on amount of end float, dog condition etc"

His recommended end float shim procedure is easier then the OldBritts and should give the same results:
"I would adjust to just touch when inner cover fitted without a gasket, then use gasket to give clearance"

" if they have excessive end float on layshaft first gear, this can cause jumping out of gear, depending on amount of end float, dog condition etc"

I can't imagine this even being a possibility :roll:
With a roller bearing the layshaft can move but surely not enough to cause dropping out of gear. If you follow the Old Britts recommendations that can do no harm.
If like me you install a needle race on the kickstart to layshaft you need to control end float with shims on the k/s end of layshaft because you lose the shoulder of original k/s bush.
The only issue I had with the roller bearing (I now use a racing ballrace from Hemmings) was the dogs of 2nd gear extended too far through lay first gear and contacted the end face of k/s shaft hard enough to move the k/s backwards in first gear (same symptom as collapsed layshaft bearing). Shimming corrected that. That may have been nothing to do with the roller bearing except it started doing it when I fitted the roller.
I think the conclusion is that using shims for the layshaft is good pratice.

Thanks for all your input :D

Hi Per,

I have rebuilt two of these gearboxes now, both with the layshaft roller bearing conversion. I shimmed one to get a very slight amount of endfloat. I do not have a micrometer to measure endfloat so I was guessing it was about right. This box works perfectly. The other box I rebuilt seemed to have about the same amount of endfloat when I put it back together and didn't require any shims at all. That box works pretty well too but it seems a bit tight and can be hard to get into neutral after it warms up. I guess what I am trying to say that even if you measure endfloat and shim it as precisely as you can, there still seems to be individual variation in these AMC gearboxes. I think getting them to be silky smooth yet not juming out of gear etc. takes some trial and error with each box. Fortunately, this is fairly straightforward to do even with the gearbox in the frame.
Thanks guys, I think this forum is GREAT :D Tons of knowledge sharing and it seems different opinions are accepted too.

I have ordered a set of mixed rear Isolastic shims to use. Will be travelling for the next couple of days but hope to have received my parts when back home late next week.

I think it was mentioned on the Old Britts page that the end float is increased when the gearbox warms up. I was thinking that it was the opposite way but maybe the case(s) is growing more then the internals.

Best regards,
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