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Hi all.

Looking for a little info regarding clutch plate minimum thickness.

A friend has asked me to stop his clutch slipping on his AJS model 31. I'm no expert on these, but from what I've read I think it has an AMC gearbox, and therefore the steel clutch plates have the outer tabs, and the friction plates slide onto the clutch drum. There are 4 steel plates, 4 friction plates, and one plate under the cover that has steel on one side and friction material on the other.

The steel plates are all warped, some worse than others, so I will replace all those. They are warped enough that they only have small contact areas on each one. I think this is the main reason for the slippage.

The friction plates are all nice and flat, but some have thinner friction material than others.

So my question is: Does anyone know the minimum thickness for these friction plates to still remain within specification?

I have looked through 4 different workshop books but none mention a figure for this. I have also "googled" this to no avail.

Cheers,

Gary.

Last edited by Douglas Clark; 11/10/21 1:00 pm. Reason: Missing words
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Douglas/Gary (!?), good question. Solid info on setting up AMC clutches seems rather elusive.
Just throwing new parts into them until they work OK seems to be a common practice. !
All degreased and flat plates and just the right tweak on the springs - as the book sez.
I'll be curious if anyone has a detailed answer.

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Hi Rohan.

Thanks for your reply.

I did wonder if I was going to get an answer such as yours, but thought I'd give it a punt on here just in case.

I'll leave it a few days before ordering anything in case someone does have a more detailed answer. If not, then I'll probably be winging it a little and simply replace anything that looks a tad worn.

Was only asking as the friend I'm doing this for is on a bit of a tight budget. He asked if I could have a go at straightening the warped steel plates, and I simply said no. The last thing I want is to be taking it all apart again in the near future!

As for my name, when I signed up to britbike quite some time back, I couldn't choose a username with my own name in, which is Gary. It simply wouldn't accept it, so I used some other names from my family to join up.

Thanks again for your reply👍

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Well that explains the names.

I could have added that I've had success mixing-n-matching clutch plates, such that you get a workable stack.
I don't know the science of it though - there seem to be all sorts of thicknesses, and the Parts Books detail
that various models of Nortons came with 3 plate, 4 plate or 5 plate clutches. (cc dependent).
As long as there is some thickness of friction material they seem to work.
Quite how all these stem from the one basic layout I know not ...

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Thanks for the extra info Rohan.

On looking at the plates again last night, I think I will be buying a pair of friction plates. One is getting a bit thin by any standards, and another has what looks like uneven wear. This may well be the plate that was next to the worst of the buckled steel plates. The steel plate actually has a hump in it that could cause this wear.

Thank-you again for your advice👍

Gary.

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Gary,

Does the clutch have the bonded back plate (sprocket plate)?.
You can probably just stack the plates to the point that they don't come out the basket when the clutch is released. Adjust springs so that the cover moves parallel through its travel and only tension them enough that there is no slip ie you can stall the engine by holding the brake when in gear. Start with adjusting the centre screw allowing slight clearance to the thrust rod.

Rob C

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Hi Rob.

Thanks for your reply.

By back plate, I'm assuming you mean the bottom of the basket, and not a removable plate.

The back plate is steel, and the first removable plate is therefore bonded.

When the plates are stacked, there may be just enough room to get another friction plate and steel plate in, but I think one of them would be proud of the top of the basket once the clutch is pulled in. So it's probably full enough in there. Also, the new steel plates will add just shy of 1mm to the stack.

I had a bit of luck yesterday. I found out that a gentleman living just 50 yards from myself works on classic bikes for a living. I approached him and he was happy to give advice on the condition of the plates. He agrees the steel plates are too warped to use again and advised replacement.

He said that the friction plates were slightly worn, but perfectly good to use again, as they are flat and have thousands of miles left in them. He also said the warping of the steel plates, especially the badly warped one, was almost certainly the cause of the slipping due to the minimal contact areas, which is exactly what I thought too.

I will be ordering 4 new steel plates (Surflex) on Monday. I will inspect and measure the springs today, and order new ones if required.

So I think I'm pretty much ready ready to go now🙂

Thanks for everyone's input👍

Gary.


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