Layshaft bearing question on 1970 750S

I have read all the dire warnngs about layshaft bearing failure to the point where i am starting to be paranoid about my rear wheel locking up and spelling the end of my short lived Norton ownership.There are no symptoms to date of it failing .i.e kickstart falling down . Ive read the gearbox section of the manual several times and its not a project i am at all confident with. Question is>> there are two bearings (one each end) of the shaft. Is one more prone to fail than the other?, I feel comfortable to replace the outside one ( inside cover) but stripping the whole box to get at the other i,m not sure about, i would probably take the box out and give the manual and replacement seals to a gearbox person.
I apoligise for this question in advance and i probably already know the answer but here goes anyway. Am i just delaying the inevitable by replacing just the one in the inside cover? As usual thanks all for your input. This forum makes the job of learning so much easier

A sign that the layshaft bearing has already failed is the kickstart lever swinging down on acceleration. If this is happening, stop! Do not pass go, do not collect $200. If you are beyond the distance you can push the bike home, call a friend with a truck or trailer. It is no fun and down right dangerous to have the rear wheel lock up at any speed, especially in traffic.

The layshaft is supported on one end by a ball (or roller) bearing and the other by a bush in the end of kickstart shaft. The bush is tricky to remove, and I suggest replacing it whenever you replace the layshaft ball/roller bearing, but replacing this alone will not help if the ball/roller has failed. The ball bearing in the inner cover supports the end of the mainshaft and is much less prone to failure.

The layshaft ball bearing is the most likely to fail. This does not mean every transmission has a time bomb waiting to explode. Hard use in the '70's, drag racing and "speed shifting" the Norton box took its toll on gearboxes.

Then the factory installed a Portugese made bearing (in '75 if I recall?) that failed with little provocation.

If you are not experiencing any symptoms, I would not get paranoid. If you are having shifting problems or hear noises in the box, then by all means get someone to rebuild it.

The layshaft bearing on the '73 850 that I owned back in the day failed with no warning whatsoever. The gearbox just locked up while I was riding down the highway at 60 mph. Fortunately there was no traffic behind me and I was able to skid to a stop without getting run over. It was quite a job dragging the bike to the side of the road though.

Teardown revealed the layshaft bearing's cage had broken apart. The ball bearings were all in the bottom of the case and the layshaft had become a free agent on that side. Amazingly there was no other damage.

So I'm in the paranoid camp now. I do have to admit I was a bit lax on gearbox oil changes and I used to ride the bike pretty hard.

Both my 750 and my current 850 had OEM-style ball bearings in there when I checked. So it's not safe to assume they've all been changed out by now.

It's probably ok to ride as is for "a while" if there are no symptoms, but I basically view the gearbox in a new-to-me Commando as a potential timebomb. I don't feel at ease riding them until I've had a chance to go through the gearbox and make sure everything is ok (or make it ok).

Once bitten twice shy...

Debby

Great white sang that song, Rockyrob maybe it could be a winter project, I have a box on the bench now, It's not something to be too afraid to do. You will need to heat the shell to get the bearings out. Check the Old Britts site it has a great piece on how to do it, Give it a try, All the folks here will give you support, Ride safe my friends and have a great day, Chuck.

basically view the gearbox in a new-to-me Commando as a potential timebomb...

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If you don't know the history of the bike, this can be said for both engine and transmission.

I'm with Debbie in the "paranoid camp"!! My layshaft bearing failed with 10 K miles on the bike when I was 8 miles from home. I was lucky as I didn't have any dramatic rear wheel lockup. Just kinda went "clunk" and the kickstart shaft starting doing a dance. Managed to limp home without doing any additional damage. Bearing case had disintegrated and balls were in the bottom of the gearbox case. I also had the famed "Made in Portugal" bearing and my bike is a 74 (serial 318...) so these bearings were in bikes before 75. Rebuilding is pretty straightforward and the Old Britts instructions on the subject are excellent as someone mentioned. I rebuilt mine two years ago and have since done an identical AMC gearbox on a Matchless G12. I would pull the box out of the bike to work on it but some people work on them in the bike. Standard upgrade is to replace the layshaft ball bearing with a roller type. Depending on mileage on the bike, you might also consider replacing the right side layshaft bush and two mainshaft rollers while you are in there. You can do it, we can help!

tpeever said:

Depending on mileage on the bike, you might also consider replacing the right side layshaft bush and two mainshaft rollers while you are in there. You can do it, we can help!

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Sorry, I meant BALL bearings on each end of the mainshaft

If anybody isn't sure exactly which layshaft drive side bearing is fitted, then may I suggest they find out at the earliest convenient opportunity, as I think a couple of members here have experienced bearing failure within the last year or so?

hi the supposedly new layshaft in my 73 850 just snaped. i thought it had jumped out of 3rd,hit 4th carried on for another 50km,go to change down&greeted by $$$noises. the lay shaft bearing is the roller bearing of the correct specs,i will replace with 6203 tb &hope.?? can anyone tell me what the difference between gears with the circle machined on the dogs and the ones with out, as i believe this may be the cause of the failiure.2nd gear was a mismatch, & r now also stuffed.

I have heard the ones with the groove in the dogs have a different profile than the early style, So don't mix them. But check with others also, Chuck.

I concur with RonL. My bearing went south on a machine with under 10,000 miles. I replaced it with a hybrid ceramic ball bearing with the hopes that it will be the last time I see the inside of the gearbox. BTW the 2nd gear with the groove in the dogs is a different profile gear because it has 23 teeth not 24. Don't mix.

Hi there.
About the grooved second gear , they were dedicated to the 850 MK2 model and the mainshaft ones are both 18 teeth but a different OD : 2 inches for the earlier and 2.040 for the later , on the other side the earlier layshaft gear got 24 teeth but the later got 23 teeth. No merit ,just pick up the info in Jim Schmidt racing book! hope this help, and definitly NO MIX...Pierre

Hi, my local bearing supplier suggests the 6203 tb,polymide cage, may resist vibration fatigue alot better,as it appears to be the cage that fails. Although a failed cage is still afailed bearing. As to the mismatched gears I asume excssive gear wear & a broken layshaft after 2000 milesl due to high loading is about right. New layshaft bearing made in Portugal. : :?

splatt said:

Hi, my local bearing supplier suggests the 6203 tb,polymide cage, may resist vibration fatigue alot better,as it appears to be the cage that fails. Although a failed cage is still afailed bearing. As to the mismatched gears I asume excssive gear wear & a broken layshaft after 2000 milesl due to high loading is about right. New layshaft bearing made in Portugal. : :?

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Go for an FAG or SKG bearing carried by all the major Norton suppliers

splatt said:

Hi, my local bearing supplier suggests the 6203 tb,polymide cage, may resist vibration fatigue alot better,as it appears to be the cage that fails.

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Mick Hemmings also recommends the 6203 TB ball bearing over the roller bearing.

Thanks for the replies , . I had to this point assumed it was the original bearing but having just pulled the clutch apart and found 5 friction and 4 plain plates i now wonder what other mods internally have been done as the 750 only had 4 friction and 3 steel according to my reading. If its been upgraded to a 850 clutch maybe the bearing has been done also. Is it possible to see the type of bearing fitted without rmeoiving the shafts and gears?

Cheers