Layshaft bearing?

I have a 73 Mk I 850. Just read the post titiled "Upgrades" and the guy had a 73 850 and asking for suggested upgrades. At the top of lots of people's lists were to replace the layshaft bearing. I thought the layshaft bearing was a Mk III weakness. Is it a weakness for earlier models also? What happens when it goes out and how likely is that to happen?

Yellow_Cad said:

I thought the layshaft bearing was a Mk III weakness. Is it a weakness for earlier models also?

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Although many MkIIIs were apparently fitted with a cheap Portuguese layshaft bearing, this problem can affect any Commando that has a steel-caged layshaft ball bearing, and not just the 850 MkIII.

Yellow_Cad said:

What happens when it goes out and how likely is that to happen?

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The gearbox and rear wheel usually locks-up solid without any warning at whatever speed the bearing decides to let go at, and pulling the clutch lever won't free the wheel, so I will leave you to imagine the consequences of that happening at speed on a busy highway! The only symptom of impending failure is often a "self-lowering" kickstart lever under hard acceleration!

It's maybe not a regular occurrence, but it has happened to a few of our members, and a person ought to consider themselves lucky to end up with just a fright and broken gearbox if it happens to them, as the outcome could potentially have been a whole lot worse.

Most Commandos which have been in regular use should have had the ball bearing replaced with the roller bearing or phenolic cage high spec. ball bearing by now, but it's something which shouldn't really be left to chance, if an owner doesn't know exactly which bearing is in the gearbox, then he/she should take the trouble to find out at the earliest convenient opportunity.

My bike is a pretty early Mk I (300427) and I bought a spare box off eBay from a 74. Both had ball bearings (and not the good kind) - one, stamped "Portugal," one not.

My bike came to me with indeterminate prior miles, but on opening up the engine I found 20 thou oversized pistons, so presumably it had some decent miles on it. POs had replaced the pistons, but left the layshaft bearing....

Considering 70s British bikes, is Commando the only bike that has this particular layshaft bearing weakness?

Yellow_Cad said:

Considering 70s British bikes, is Commando the only bike that has this particular layshaft bearing weakness?

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AMC gearbox seems fine with ball bearing on bikes making 30 hp for which the gearbox was designed. Problem seems "Commando-specific" as far as I am aware and seems to be related to exceeding design specs. Anyone else heard about problems with AMC boxes on other bikes?

Yellow_Cad said:

Considering 70s British bikes, is Commando the only bike that has this particular layshaft bearing weakness?

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Apparently it was a poorly spec-ed bearing that was the issue, not as much an issue with the gearbox design.

If I remember correctly when the AMC box was fitted to the Commando for some reason they reduced it's speed The wrong thing I would have thought.

Cash