Lay shaft bearing fears

I have a 750 built in November 1972. I thought the poor quality lay shaft bearing covered a certain span of the 850’s, but I occasionally see advice to change it regardless of what the year of manufacture. Do I have to live in fear while riding down the road on a 750? I’d hate to tear down an otherwise good running gearbox.

I was cautioned to change mine (1970) so I did, granted I was going through the entire bike at the time.

The bearing I pulled was in good condition with 14k on the clock. I am happier knowing one potential massive failure has been eliminated.

Ultimately it is up to you. Shouldn't take you more than a day or two to rebuild the box if you've got the tools and the time.

Preventative maintenance is often hard...

Better to know that not to know.

Hi - I've a 750 built Dec 1972, my layshaft bearing failed taking off from a set of traffic lights in a quiet part of Edinburgh, early morning. I'm glad it didn't fail going down Queen Street in the afternoon or <gulp> doing 70 round the bypass . .
So yes, I'd say go easy meantime and get it done as soon as.
On the upside I found the operation (for a novice mechanic) fairly straightforward -try google doov.com norton companion for good info and cross ref with the shop manual.

1973x75 said:

I have a 750 built in November 1972. I thought the poor quality lay shaft bearing covered a certain span of the 850’s, but I occasionally see advice to change it regardless of what the year of manufacture. Do I have to live in fear while riding down the road on a 750? I’d hate to tear down an otherwise good running gearbox.

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Well the 850s have higher gearing and presumably higher torque loading than the 750s . Oddly every bearing in my gearbox was shagged apart from the layshaft !!! I did change it at the time , as eveything was down . What I do remember from old stories in Road holder was that the kickstart begins to flap about when failure is immanent. You may not of course notice , and ifyou think about it when riding and you wantpeace of mind , then change it.. But getting the old bearing out is not a piece ofcake. Its not a simple black and white thing. If you have a 19th sprocket and let the motor rev a bit through the gears , then frankly wouldnt worry to much .. If however you always change up below 3,000rpm and have a22tooth or higher gearbox sprocket then the kickstart might well about to make its presence felt. If so STOP immediately.

“ If however you always change up below 3,000rpm and have a22tooth or higher gearbox sprocket then the kickstart might well about to make its presence felt. If so STOP immediately.”

sorry.

my 850, 22T will always flap the kicker when taking off smartly from a stop. (With my 240 lb fat ass on it) Been that way 36,000 miles.

trans is in fine shape.
It’s been repaired & refreshed plenty.

Interesting.Am surprised . the KS can only 'flap' because the lay shaft is picking up in the bronze bushing.. But if its been ok for 36,000 miles

Also note the GB work can be done in bike to save some effort. I could not get my box out of the engine plates so just did it all insitu.

Stop scaring me. My old gearbox is like silk.... If you stop I promise to do it this next cool season because I don't really get winter here.

I was doing an oil change on a MK2A and discovered chunks of bronze in the bottom...a short job became a long job. I inspected gear by gear all the bronze and discovered that the bushings in the sleeve gear were crumbling. There were 3 bushings and were working their way towards the interior of the GB. Easy enough fix and the rest of the GB was if excellent condition....except for way in the back.....there was the infamous Portuguese layshaft bearing. It required a blind hole bearing puller to remove it. and heat. This box had never been opened up.
Anyway, I finally had a clever idea when assembling the internals. It is an easy job except for the selector forks. So I found an aluminum rod 1/4" and 6" long and shoved it through the gearbox FROM THE LEFT side. It holds the selector forks quite well. Then use the original rod to push out the alu rod and screw it in. It made a sometimes very frustrating job very quick.
No, if someone would invent a quick method of holding the clutch release body in perfect alignment whilst tightening the lockring. I do have a partial method if the clutch release arm ends up slightly trailing the cable hole. Rather than starting all over again I heat the area with MAP gas and give the lockring another shove. Not a lot but when the case cools it really secures the lockring. Can only be done for small movements.

seattle##gs said:

Anyway, I finally had a clever idea when assembling the internals. It is an easy job except for the selector forks. So I found an aluminum rod 1/4" and 6" long and shoved it through the gearbox FROM THE LEFT side. It holds the selector forks quite well. Then use the original rod to push out the alu rod and screw it in. It made a sometimes very frustrating job very quick.

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Mick Hemmings does that in his gearbox rebuild video except he uses a bolt.

Tornado said:

Also note the GB work can be done in bike to save some effort. I could not get my box out of the engine plates so just did it all insitu.

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yes it indeed can, just a thought , but if you get an old tap to match the id of the bearing you could use that to fashion a puller. do use heat do smear casing with soap. heat until it goes brown... becareful you might crack the case between the layshaft and the sleavegear bearing

oldmikew said:

yes it indeed can, just a thought , but if you get an old tap to match the id of the bearing you could use that to fashion a puller. do use heat do smear casing with soap. heat until it goes brown... becareful you might crack the case between the layshaft and the sleavegear bearing

Click to expand...

I followed the Mick Hemmings DVD for this whole process (save for off bike work). He uses a big old propane torch, huge blower flame on his casing and gets it hot enough to boil spittle. Initially I tried an electric heat gun but found my standard propane torch did it better. Just kept moving it around outer/inner sides the casing and the bearing just came out easy with a long blade screwdriver to initiate. My bearing was the dreaded brass cager from Portugal...but was looking fine at 9k miles. Replaced with the MH recommended phenolic cage, high quality bearing...no fussing with shimming as with more popular roller. MH states the rollers can put more load on layshaft as they limit flexing.

I replaced quite a few bronze bushes and some gears when I fitted a 'superblend' roller to the layshaft on my MkIIA as particularly the 2nd gear pair were pretty mangled. I found that once it was all shimmed up there was a bit less kickstart shaft exposed. When I fitted an RGM kicker I couldn't get it to fit due to this, so I went with a Hemmings bearing and tossed the shims. Mick's bearing positively locates the layshaft as per the original, so a much better idea than the superblend. I also found the new 2nd gear pair were knackered again. Clearly the layshaft gets a beating because the mainshaft is whipping around due to the overhung, unsupported clutch. As I understand it he new bearing doesn't do much at all to control this, but is tolerant of the shaft whirling around without the cage exploding.
Definitely the best gearbox ever fitted to an AJS 350

B+Bogus said:

I replaced quite a few bronze bushes and some gears when I fitted a 'superblend' roller to the layshaft on my MkIIA as particularly the 2nd gear pair were pretty mangled. I found that once it was all shimmed up there was a bit less kickstart shaft exposed. When I fitted an RGM kicker I couldn't get it to fit due to this, so I went with a Hemmings bearing and tossed the shims. Mick's bearing positively locates the layshaft as per the original, so a much better idea than the superblend. I also found the new 2nd gear pair were knackered again. Clearly the layshaft gets a beating because the mainshaft is whipping around due to the overhung, unsupported clutch. As I understand it he new bearing doesn't do much at all to control this, but is tolerant of the shaft whirling around without the cage exploding.
Definitely the best gearbox ever fitted to an AJS 350

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There is a local Norton guru, a bit past his prime, that showed me a custom? Layshaft he uses that does not have the 90 deg shoulder but a radius instead to handle the stress. He indicated the shoulder is a known failure point due to stress concentrations. Not sure if that is a custom shaft or from some other gearbox.

Tornado said:

I followed the Mick Hemmings DVD for this whole process (save for off bike work). He uses a big old propane torch, huge blower flame on his casing and gets it hot enough to boil spittle. Initially I tried an electric heat gun but found my standard propane torch did it better. Just kept moving it around outer/inner sides the casing and the bearing just came out easy with a long blade screwdriver to initiate. My bearing was the dreaded brass cager from Portugal...but was looking fine at 9k miles. Replaced with the MH recommended phenolic cage, high quality bearing...no fussing with shimming as with more popular roller. MH states the rollers can put more load on layshaft as they limit flexing.

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There was another dodge which was to pack the bearing with grease after the layshaft had been pulled out and then find a socket or something that was a snug fit on the bearing inner and hit it hard with a hammer the idea beingthe pressure would start to force the bearing outer from its housing making it easier to get pullers behind it ..

Yes I was very sceptical about the roller bearing but then was told by Russels of Falcon road London that originallythe box had had a roller bearing which was changed as a cost cutting exercise..At what point in its ancestry the change overcame I do not know.

As far as I am aware Russels still exist the last of the old time dealers and they have loads of NOS for amc as well as Norton..

jbruney said:

Stop scaring me. My old gearbox is like silk.... If you stop I promise to do it this next cool season because I don't really get winter here.

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If you really want to give yourself a fright , whip off the primary outer start the engine and blip the throttle a couple of times... The mainsshaft whips about so much you would think it was made of bamboo

Been there... Things I can't see are what irk me.

The stories of layshaft bearing failure potentially locking up my rear wheel in fast moving hiway traffic eventually got the best of me, so I ordered the bearing that Matt (CNW) sells as the upgraded replacment bearing and rebuilt my gearbox. Replacement was much easier than I thought it would be. Thankfully, all my bushings were perfect, as was the original layshaft bearing that was removed...

The layshaft bearing is just one of those changes you make for peace of mind. When I rebuilt my engine, I removed the original early model engine main bearings, 1 roller and 1 ball bearing, to replace them both with superblends. Both of those bearings were still good too.

I have a lot of money in the karma bank, replacing all those bearings that were still good..

oldmikew said:

Yes I was very sceptical about the roller bearing but then was told by Russels of Falcon road London that originallythe box had had a roller bearing which was changed as a cost cutting exercise..At what point in its ancestry the change overcame I do not know.

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I'm not sure there ever was a change as only the 040100 (6203) layshaft bearing appears in the part lists for the AMC gearbox.
No layshaft roller seems to have been fitted as standard to any Commando even though the 06-7710 roller bearing part number is listed in the 850 Mk3 parts book.