Layshaft bearing failure (2014)

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mschmitz57

mschmitz57

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There I was cruising along into my subdivision when all of a sudden an odd noise accompanied by a mechanical vibration.
It seemed fine after I let off the gas, pulled into my driveway and found the drive chain loose. "Must have been the chain rubbing on the guard" I thought.
I adjusted the chain and took it for a test ride later and I didn't get far from my house when I felt the kick-start lever hit my calf.
Then I could feel a definite mechanical problem in the drive line and a whining noise. I quickly turned around and limped into my driveway.
Very fortunate I wasn't 20 miles from home.

Sounds like classic lay-shaft bearing failure right? If the bike is in neutral, it can only be pushed a short distance before the transmission begins to bind.
Never had an AMC trans apart. I've been watching the John Hudson video tonight getting ready to dive into it.

Any comments welcome.

Mark
 
mschmitz57 said:
Any comments welcome.
Click to expand...

If you elect to remove the shift cam plate it has a hole in it that the end of a Allen key can be placed in, it will then come up against the quadrant when the external bolt is released putting no pressure on the internal teeth.
Warming the case at the layshaft bearing location makes for easy removal of the shaft and bearing, yours might depend on the state of the bearing.
Look for the crack between the two bearing bores in the case.
 
Its the cage that will have failed letting the balls drop out of the bearing, so your choice of replacement is a roller bearing or ball bearing with a non-steel cage material as per the Mick Hemmings supplied ball bearing.
 
kommando said:
Its the cage that will have failed letting the balls drop out of the bearing, so your choice of replacement is a roller bearing or ball bearing with a non-steel cage material as per the Mick Hemmings supplied ball bearing.
Click to expand...

I've used both, but favour the Hemmings ball-race. Shimming the layshaft is a pain and I ultimately replaced the roller with the Hemmings bearing because the kickstart shaft was migrating into the case due to the shims, so I couldn't get my RGM kicker to fit any more.

The gearbox itself isn't too difficult once the clutch release mechanism has been removed - that's the most annoying bit IMHO
 
There is quite a lot of info on the lay shaft and gearbox technicals in threads you can search on this forum
There will no doubt be a crack between the two bearing housings but that's pretty normal (you'll have to look close)
So long as the bearing seats are in good nick and not chewed out the box should be otherwise OK as llong as it's not cracked too bad
Taking the box out to inspect it properly is best

Jed
 
My 750 had no crack but the 850 did.
The counter/output shaft bearing bore on the 750 had little sign of machine tool tracks more like a mottled finish (rivers from space)
The 850 had turning tracks still, I wonder if these jumbo sprockets and folk loading the trans up with high gear low rpms has anything to do with cracked cases (shock loading on racers
 
I don't know that it's high performance bits (which would not help for sure) or just the overall demand
The MK3 engine I restored showed internally that it had not had too hard a life. The rest of the bike was also in pretty good nick
The gearbox however was stuffed
Maybe lack of oil combined with the infamous lay shaft bearing lead to its demise but at that time I was reading a lot of stuff on the crack and found it was more the rule than the exception
The earlier box I put in my bike also had a hairline crack between the two bearings. Extremely hard to see but there none the less. The box had to be modified to take the crossover shaft and neutral switch
The reason I did not reject it is because a new genuine box would likely develop the crack also
I went for a superblend bearing in the layshft as they aren't so ridged and therefore less prone to fail
B+Bogus prefers the roller bearing but he has issues with shimming, I did not need to shim mine

Jed
 
So the trans can be removed from the engine plates huh? Does the primary case need to come off?
I read elsewhere on this forum that the clutch basket and outer trans cover needs to be removed to pull the trans out.
It doesn't look possible. Seems a bit like giving birth but the steel plates wont stretch much.
I wish I'd addressed this last winter when I had her all apart on the bench.
For some reason I thought the `73's had gotten the improved bearings.

Just watched the John Hudson trasmission rebuild video for the first time (bless the old guys heart).
He had me squirming in my seat as he knocked the parts all over the work bench.
Then he used an old 2x4 block of wood as a hammer. Really? Couldn't the NOC afford a leather mallet?
And a brass blow torch from the 40's. I thought the trans shell was going to hit the floor eventually.
Highly enjoyable stuff for a rainy day.
 
Time Warp said:
My 750 had no crack but the 850 did.
The counter/output shaft bearing bore on the 750 had little sign of machine tool tracks more like a mottled finish (rivers from space)
The 850 had turning tracks still, I wonder if these jumbo sprockets and folk loading the trans up with high gear low rpms has anything to do with cracked cases (shock loading on racers
Click to expand...

That's exactly what Norman white reckons... being over geared and over using the low speed torque = gearbox trouble !!
 
mschmitz57 said:
So the trans can be removed from the engine plates huh? Does the primary case need to come off?
Click to expand...
As I remember you will have to loosen the engine and tilt things around to get the GB out (the engine is in the way), unless someone has opened up the cradle. Primary case, I don't think so, but you'll have to remove everything involving the clutch down to the mainshaft which means the outer primary comes off and all the clutch parts.
 
Then if the clutch basket comes off then that means the engine sprocket, rotor and stator as well.
This makes a pretty good case to try to work on the GB in situ on my lift.
 
M sch57

If you,ve had it to bits before then you'll know it's not that big a deal either way. If an old man can do it with a lump of wood then it's going to a piece of cake for you! I'm surprised it's no fixed already.
 
when I did my gbox, I left it in place. Renewed every bearing and bush that needed it. used a super blend on the layshaft. Had to heat the casing with a gas torch to get the bearing outer shells out and fi the new ones. the hardest part to me was getting those bloody selectors in position. a good idea to turn the bike on it side so gravity worked in my favour, but I was stubborn enough to persist, and I got it together ok. Didn't remove the cam plate [ mainly because that bolt was so tight I couldn't loosen it, so as it wasn't broken I didn't fix it. [ o-ring doesn't leak much if anything at all.]
Good luck with you repair.
Dereck

ps mine is a 74 job.
 
Layshaft bearing failure (2014)


Sometimes it just has to come out.
 
concours said:
I did mine in the bike, up on the lift, no trouble.
Click to expand...

Me too.
Bought a cheap electric paint stipping torch to heat the case for removing layshaft with bearing and outer race.
Reverse with new bearing on shaft end. Heat case, freeze shaft with bearing on.
Don't over drive when putting in as if bottomed out can break out case.
 
Yes, proceeding with repairing it in-situ. I've got the bike up on the lift and have begun the dismantling.
Outer cover off. Need to remove the main shaft nut which is pretty tight.

Layshaft bearing failure (2014)
 
mschmitz57 said:
Yes, proceeding with repairing it in-situ. I've got the bike up on the lift and have begun the dismantling.
Outer cover off. Need to remove the main shaft nut which I believe is right hand thread.
Click to expand...

If you want you can leave mainshaft with primary in place. Unless you want to do a full rebuild of the box.
 
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