Gearbox blowups please

Hi all,

After I started the thread 'gearbox gear replacement' I got thinking about the horror stories I have heard about boxes locking up and was wondering what the most common cause of g/box failures has been.

Can I ask that if you have had a box failure, can you please post here, what happened and what actually failed.

Condolences, sorry for you gearbox loss.


Cheers,
D

Dean,

It will be interesting to see what folks have to say about road gearbox failures, poor maintenance may contribute?

My 4 speed road box coped with a season of race use with the only issue being the sleeve gear bearing turning in the case, solved at the time with loctite beering fit, but it was retired from racing after that.

As far as I am aware 4 speed road box failures have been mainly due to shaft flex, the longer Commando shaft and heavy clutch flexing the mainshaft, and the bushing wear in the layshaft allowing movement there, with subsequently the teeth shifting out of mesh.....this can lead to a case breakage, total lock up and loads of other fun.

The problem is exacerbated when using a 5 Speed conversion in a road box, but essentially the failures start in the same way.

Plenty has been said about reducing risk of this happening, but that isn't the question you asked anyway.

I think there is more stress in a high gear box than a low gear box ?
Eg. 19 tooth VS 24 tooth ouput sprocket .

Michael

Dean said:

I got thinking about the horror stories I have heard about boxes locking up and was wondering what the most common cause of g/box failures has been.

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The most commonly reported cause is due to the cage of the layshaft drive-side ball bearing breaking up, the balls then bunch together and the layshaft moves out of alignment, a symptom of this is the kickstart lever swinging down under acceleration, however, the bearing can fail with little or no warning.
In extreme cases the balls can fly out of the bearing and jam between the gears!

Although later Commandos have a Portuguese-made brass cage bearing which is more prone to failure, any metal cage ball bearing can potentially fail so it should be replaced as a matter of course with either the NJ203 roller bearing upgrade, or the "Mick Hemmings" FAG 6203 TB phenolic cage ball bearing.

I suggest you search "layshaft bearing" or "layshaft bearing failure".

the absolute worst gearbox blowup I have had was in 72. it broke 6 gears both shafts both shift forks 2 bearings and the mail housing. I have also had several gears have teeth break off but caught it before a major failure. most of if it was trying to drag race it along with a barnett clutch. the old black clutch was a LOT more aggressive than the newer green plate's.

Probably should clarify. The reason I was asking for the feedback about blow ups was cos when I ask the question if you had really had to replace both gears in a set when only 1 was damaged I got a coment that it was better to spend the money than to have your box go. And a very fair comment it is. But after this i tried to think of a time when I heard About a box fail due to a gear breaking. And I couldnt. I know about the layshaft bearing failing and will definatly change that... Still had metal cage one in mine...just lucky it didn't go. And also layshafts breaking.. So will crack test it. But never a gear breaking. Just wondering if there are any out there..

Thanks all.
D

Also thanks LAB, have been all over the layshaft brg threads.

Lay shaft snapped due to early and later size 2nd gear set fitted by po

I've only had one 4-speed blow up (so far), and that was on a race bike. I was at Riverside on the 920 cc MK3 racer I'd just finished, and downshifted at way too high an rpm going into a corner. Teeth sheared off the high gear pair, split the case big time, and locked up the rear wheel. It was pretty ugly. I was used to the close ratio 5-speed in my PR, which didn't require waiting much for the speed to drop before shifting down, and shifted the 4-speed the same way out of habit. I'd built the MK3 because I wanted a Commando race bike with an electric starter. The MK3 was a lot heavier than the PR, and didn't handle nearly as well, so I dropped the idea, and built an external starter for the PR (also a 920 cc by then, and hard to start). Other than that, I've replaced pitted gears in 4-speeds a few times, but never had another one blow up.

Ken

lcrken said:

I've only had one 4-speed blow up (so far), and that was on a race bike. I was at Riverside on the 920 cc MK3 racer I'd just finished, and downshifted at way too high an rpm going into a corner. Teeth sheared off the high gear pair, split the case big time, and locked up the rear wheel. It was pretty ugly. I was used to the close ratio 5-speed in my PR, which didn't require waiting much for the speed to drop before shifting down, and shifted the 4-speed the same way out of habit. I'd built the MK3 because I wanted a Commando race bike with an electric starter. The MK3 was a lot heavier than the PR, and didn't handle nearly as well, so I dropped the idea, and built an external starter for the PR (also a 920 cc by then, and hard to start). Other than that, I've replaced pitted gears in 4-speeds a few times, but never had another one blow up.

Ken

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Hi Ken

When you said you "replaced some pitted gears" did you just replace the pitted gears or did you replace bth gears in the set. Trying to find out if anyone has had a gearbox fail due to running a used gear with a new one?
Thanks

Dean said:

Hi Ken

When you said you "replaced some pitted gears" did you just replace the pitted gears or did you replace bth gears in the set. Trying to find out if anyone has had a gearbox fail due to running a used gear with a new one?
Thanks

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I've never worried about replacing gears as a pair, just replaced any of them that were worn, pitted, cracked, etc.

Ken

Hey Ken remember the ebay Quafe case I sen for repair, the bottom bolt boss had been fractured off? I'd call that a blow up as didn't look like something struck it It fit back in cradle great after much time grinding cradle and shell to clear the boss flats were right on. I sent it to Bradley of gear profile fame to work up some his ideas on. I blew up one of Peel's shell once by hand at frustation the bearing bores were so loose they'd fall out completely cold so gave main shaft bearing a smach to really seat it by 3# sludge and busted ragged hole the bearing passed outside through. I was a bit surprised to see how thin the shell is behind the bearing, a bit over 1/8". I think its shafts twist bowing or worn bushes then misaligning forces on cogs that do AMC's in destructively.

hobot said:

I think its shafts twist bowing or worn bushes then misaligning forces on cogs that do AMC's in destructively.

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Pretty sure that is what I said......sometimes its hard to tell ;-)

Ok Steve I came to agree with you after over tight primary chain event and an over loose drive chain event allowed links to ride on top of teeth instead of valleys and an over powered sports bike drag racing event the Monster had 50 mph head start flying lead to catch before a tigh turn crippled it. Them sports bikes are too dam wheelie limited to me. Norton fame would be even greater if not for the AMC's historic let downs. I wonder if some super duper alloy and Helium ~400' negative F tempering of all componets including the shell might hold the 80'+s hp engines torque. Before I learned that Combats were not a P!! dragster I'd catch rubber in snicks to 2nd entering hwy on uphill direction and thought it was clutch slip for some time till I resolved the sqeal noise and squirrely hesitation in pull coming from rear tire. I don't like the weight of TTI gear boxes nor 15-20 lb of Drouin so thinking a big single carb and AMC transmission may be Peel longest term configuration and still able to spank best sports bikes to the ton by sucking front down a few inches for the straight sprints and not snapping WOT.

Layshaft bearing failed. Replaced with roller (mini superblend?).

Greg