So You Just Bought a Near Mint Condition Commando?

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I'm not sure about the layshaft bearing in my '74 850. I will get sure soon though. I have been pushing my luck on this issue too far. Thanks for this post Dan.
 
Saber said:
I'm not sure about the layshaft bearing in my '74 850. I will get sure soon though. I have been pushing my luck on this issue too far. Thanks for this post Dan.
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If yours does fail, you might not be as fortunate as I was.
 
Thanks for posting this Dan. It's a wake up call to all of us.
Making me a little nervous about my high mileage 850.

AON, I went through my 72 Combat a bit ago, it had a NTN ball bearing in it.
I assumed this was a replacement. I replaced it with a roller.
Been wondering, could a Japanese NTN been installed by the factory in 72?

Another comment, I was working on a different application and a 6203 needed replacement.
My local NAPA dealer said the 6203 may be the most common bearing there is.
Just thought it interesting.
 
Mine blew in 79 right on 12k miles on the clock, lucky for me it blew outside my house when I put it into gear, replaced the bearing and one bush in one of the gears and have had no problems with the new bearing after 33 + years, have only been back in the gear box for 4 kick start pawls and replaced the KS gear as where the pawl engages was worn, my gear box has over 160k mile on it now and still shift smoothly.
But after all these years you think all the layshaft bearings would have been replaced by now as it was very common for them to explode around 12k miles, a mate of mine his blew around the same time but was 100 miles from home but was able to get 4th gear and ride it all the way home in 4th and yet others have had major lock ups at speed, just luck of the draw.
Once you replace and inspect the rest of your GB you won't have no more problems if you look after it, they are a simple GB to work on, I prefer to have my GB out of the bike and on the work bench but can be done in the bike.
Do you have WW spanners as the GB nuts are WW.

Ashley
 
ashman said:
I prefer to have my GB out of the bike and on the work bench but can be done in the bike.
Do you have WW spanners as the GB nuts are WW.

Ashley
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That is definitely my plan too. To hard to do a thorough repair in the bike. The gearbox will get a complete rebuild.

I will be purchasing Whitworth spanners and sockets as it has been over 48 years since I wrenched on my BSA A65T. I expect that before I hang up my leathers again I will probably have seen every nut bolt bearing and spacer in my MK II.
 
Dan1950 said:
That is definitely my plan too. To hard to do a thorough repair in the bike. The gearbox will get a complete rebuild.

I will be purchasing Whitworth spanners and sockets as it has been over 48 years since I wrenched on my BSA A65T. I expect that before I hang up my leathers again I will probably have seen every nut bolt bearing and spacer in my MK II.
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I rebuilt my box when i tore down my bike, so it was out. If I had to do it again, I would keep in in the bike. Check out the doov link, he did it in the bike.
 
My bike too, was in good shape when I found it.
With that being said there was no way I was heading out on a 47 year old motorcycle without going though it.
Once I started digging into it and getting past the positive cosmetic appearance I was really glad I went the teardown route, including overhauling the gear box. I discovered a lot of iffy to holy $hit conditions of various components and systems.
I would never have been able to ride with any confidence "not knowing" if it was good to go or ready to let go.
90 mph on 47 yr. old motorcycle that has not gone "under the microscope" has too much downside for me to enjoy riding it. My age, riding experience, a mechanical failure on a used 650 Lightning purchased from a private owner who assured me it was good to go when in truth it almost killed me dictate my approach these days.
It is not my intent to criticize the OP or question his judgement, to each their own...but my cautious stance is founded on experience.

Stay safe Dan, I hope the rebuild goes smoothly.
 
Its like anything mechanical and old age things do wear out or fail from long use, not only our old bikes, cars, trucks etc etc, but anything you buy second hand is always good to go through it and a good way to learn all about your bike.
 
I find it remarkable that the factory never identified this as an issue given the potentially fatal consequences.
The taller gearing and less revvy nature of the 850s would seem to aggravate the problem, but I find it hard to believe this wasn't a known issue at the time, given the amount of race retirements due to gearbox failure.
The 850 Mk3 would have been a perfect candidate for an outrigger bearing behind the clutch. They pretty much re-designed everything else and the fix was already on the race bikes... I guess it wouldn't have been 'British' otherwise!

I don't believe the source of the bearing has anything to do with the failure mode; it was simply under-specified.
When I fitted the roller layshaft bearing I replaced a number of gears on my first 850 due to excessive wear - especially the 2nd gear pair. When I subsequently fitted the 'proper' 6203TB bearing to overcome shimming issues, the new gears had worn the same way.
 
gortnipper said:
I rebuilt my box when i tore down my bike, so it was out. If I had to do it again, I would keep in in the bike. Check out the doov link, he did it in the bike.
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I also rebuilt my trans in the bike with no problems.There are some good articles on how to do this so you don’t have to pull the engine,unless that is part of your plan.A lift will save your back!!
 
grandpaul said:
Not hardly...
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Agreed. I pulled open a gearbox on a 74 850 with 16,000 miles last year, and there was indeed the original Portuguese ball bearing still inside. I replaced it, and took a picture of the gearbox with new roller bearing with gearbox number visible in image. The old Portuguese ball bearing and picture proof both went with bike to the new owner.

Cheers,
Gatsby
 
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YING said:
I also rebuilt my trans in the bike with no problems.There are some good articles on how to do this so you don’t have to pull the engine,unless that is part of your plan.A lift will save your back!!
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I have found some threads that instruct how to remove the gearbox without disturbing the engine other than disconnecting the rear mount plates from the case.
 
I know many will have done the job successfully in situ, but to replace bearings I do think you're better off with the box on the bench.
IMHO..
 
EstuaryBoy said:
I know many will have done the job successfully in situ, but to replace bearings I do think you're better off with the box on the bench.
IMHO..
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Yes, once I remove the gearbox from the bike and strip it down on the bench, I would be able to pop the case in the oven for controlled heating to remove the blind race.

I am getting a new gas range to replace the electric range in my kitchen. There is a 50 amp outlet in the garage so that is where the old range will go, right next to the chest freezer.

I cringe when I read about using applied heat using a heat gun, or worse, a torch, with the case still in the bike.
 
I fixed mine in the street, was a student at the time so no garage. Torch on the box and a clout on a piece of hardwood sitting on the outside of the gearbox under the primary and the bearing outer popped out of the housing. Still have the bike and nothing other than new rings, upgrades and a new harness in all those 35+ years.
 
Dan1950 said:
Yes, once I remove the gearbox from the bike and strip it down on the bench, I would be able to pop the case in the oven for controlled heating to remove the blind race.

I am getting a new gas range to replace the electric range in my kitchen. There is a 50 amp outlet in the garage so that is where the old range will go, right next to the chest freezer.

I cringe when I read about using applied heat using a heat gun, or worse, a torch, with the case still in the bike.
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I've done it with the gearbox out the bike and with the gearbox in
Never had a problem with either method also no problem with using a heat gun
 
I stripped mine at something over 20k miles on the clock.

As far as I could tell, the gearbox had never been stripped.

The Portuguese bearing was still in there, and still in good condition!

However, the case had cracked between the bearings!
 
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