In a thread from last winter, someone asked these same questions about how concerned they should be. I don't know where the thread is, but there was a reply from comnoz that said something like "I wouldn't ride around the block on a bike with an unkown layshaft bearing in it". That did it for me. I was putting in the rear iso anyway and pulling the swing arm. So I tore down the gearbox. It was not difficult and was very rewarding with the peace of mind that new bearings can bring.

tban52 wrote:Well now you've got me worried. My MK-III just passed 15k. Shifts nice and smooth, no movement on the kickstart, no evidence of metal when the gear oil is drained. I don't know if the PO maybe changed the bearing??? Is this problem absolutely gonna happen? Or were some gearboxes assembled with different bearings? Is there any way to tell?
I could make this part of my winter projects list, but there are a lot of potential miles between now and then. Just how concerned should I be? I'm liable to run into something while I'm staring down at the kickstart.......

I would do it now if that is the actual mileage and it has not been done already. You are riding on borrowed time. My bike gave me no warning
signs that I had noticed before it broke. I may be extra cautious here but when my bearing broke at 50 MPH it did major damage to the gearbox and I was lucky to get away with a little roadrash on my hands from the 10 mph highside.
It is not that tough a repair. Chances are you will only need a bearing and a couple gaskets. A good heatgun will make removing it easy. I have removed them by heating the case to around 300 degrees and using a couple dent pulling hooks. Anymore I can do them in a couple hours in the bike but the first one will be a little more trying. The trick when removing the bearing is to keep it straight . If it cocks a bit in the bore it sticks. When that happens just tap it back in, heat it a little more and try again. Do not use the hook to apply pressure to the case behind the bearing. The case is thin there and easy to crack.
I did one at a rally one time with a piece of ready rod with the side relieved near the end to form a hook. I inserted that through the bearing and stuck a nut into the bearing next to the rod so it could not come back out of the hole. Kind of like a monkey with a peanut in a jar. I heated the cases with a torch as best I could and used a large crescent wrench like a slide hammer to pull the old bearing out. Jim

So looking at the old brits website I see they have 3 or 4 different bearings listed. I'd just like to replace with a like ball bearing. I don't care to go with an expensive roller and mess with shimming. So which of their ball bearings is the one to get? It's hard to decipher from their site which would be best.

I just got off the phone with Fred at Old Britts. He told me that in 1975 all MK-IIIs had the upgraded layshaft bearings. The one he would sell me would be the same one already in my gearbox.

1975 all MK-IIIs had the upgraded layshaft bearings.

I don't think so, my 75 Mk3 certainly didn't and I know know of another that hadn't

Cash

Seems like I just saw a few threads ago that there were certain bearings that were better than others. for a few bucks I'd like to make sure I'm putting the better one in place, don't want any worry's for the next 30 years.

tban52 wrote:He told me that in 1975 all MK-IIIs had the upgraded layshaft bearings.

Unfortunately not. From previous discussions we've had on the forum, Fred appears to be wrong about that. The roller bearing part number 06-7710 is listed in the MkIII parts book, however, if and when this change from ball to roller ocurred then it isn't clear when it actually happened, and I know I found a layshaft ball bearing in my July 75 MkIII's gearbox.

My Mk3 is a 75 model. It locked up at 12000 miles. I was told the change in the part did not happen until after production had ended. Ordering the part number above gets you a roller bearing however. Jim

I'm convinced. I just ordered the AMC Gearbox Rebuild Kit from Norvil for 128.28 British Pounds Sterling (I think that's about $238.00 US plus shipping. It includes upgraded bearings, bushes, gaskets, springs, etc. I'll install the kit in my '74 Commando 850 gearbox which is already out of the bike for rebuild. Also having Fair Spares do the seal mods. Mick Hemmings DVD and Old Britts website give ample instructions for the job. Depending on how that all works out, I'll order another kit to rebuild the gearbox in the '73 Commando 750, maybe in the bike - maybe not. The idea of the gearbox locking up without warning is enough motivation to do this work.
Tim

You can order the roller bearing here . Just a click away. Jim

http://www.intechbearing.com/NJ200Serie ... -shop.html

Good link Jim, I see the site states that a brass cage is stronger and for heavier duties, the bearing I purchased is a FBJ and has the brass cage. Incidently the Japanese Co FBJ claim to be the first to ever manufacture bearings FWIW.

Foxy wrote:Good link Jim, I see the site states that a brass cage is stronger and for heavier duties, the bearing I purchased is a FBJ and has the brass cage. Incidently the Japanese Co FBJ claim to be the first to ever manufacture bearings FWIW.

Yes, the brass cage is tougher but more expensive. I haven't seen any problems with the plastic cage yet . It is well below it's max speed ratings so either one should last about forever. Jim

This is probably a stupid question, but since I don't know anything about bearing tolerances and bearing uses why does it say "nylon" cage?

The cage is the piece that separates the rollers. It is made of nylon. Alternatives would be aluminum, bronze, brass, silver, titanium, steel or one of many composites. Jim