Clutch plate issue

I looked at a lot of clutch threads and did not find the answer I am looking for.
In trying to identify the cause of heavy clutch pull on my1972 Interstate, it appears that I found that cause with the stack height of the clutch plates. Inside the clutch basket I found: pressure plate .347" thick; 2 old style friction plates, one .140" thick and the other .144" thick; 2 bronze sintered friction plates, one is .123" thick and the other .125" thick; and 3 plain plates that are .080" thick.
So, it appears the the clutch is set up as a 4/3 plate stack, as I think would have been stock in 1972, but 2 of the friction plates have been replaced with plates that are used in the 5/4 plate 850 clutch. As a result, the full stack is about 0.047" smaller (including wear) than the original 1.167" height.


I would buy 4 new .145" friction plates and put it back together as stock, except I will have to wait to get them,

or another option is to replace the two .145ish friction plates with two .125" plates that I have here along with replacing one plain plate with an RGM 4mm plain plate that I also have here. That would make the stack height 1.161", within 0.5% of stock height.

My question is: Would there be any issues with using the thinner friction plates in a 4/3 configuration?
I'm thinking the reason Norton went to 5 friction plates was to give more surface area for the 850 clutch, but I don't see why the thickness should matter for a 4 friction plate stack (other than for total stack height).

Thanks for any help

NickZ said:

Would there be any issues with using the thinner friction plates in a 4/3 configuration?

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Mixing clutch plates doesn't seem to cause any problems as long as it results in approximately the correct stack height.

NickZ said:

So, it appears the the clutch is set up as a 4/3 plate stack, as I think would have been stock in 1972, but 2 of the friction plates have been replaced with plates that are used in the 5/4 plate 850 clutch. As a result, the full stack is about 0.047" smaller (including wear) than the original 1.167" height.

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The drum depth (from backplate wear etc.) can alter the optimum stack height dimension so 1.167" may not be correct for your particular clutch assembly.

A simple way to check stack height is to assemble the clutch stack into the drum, use the clutch spring compressor to set the outer ring of the diaphragm spring 'flat' and hold the spring in position in the drum, then, without releasing the spring compressor, measure the distance between the spring and the circlip groove as shown in the picture below (or fit the circlip and measure the gap between the spring and fitted circlip) as that will be how far (low) the stack height is from the 'flat spring' position.
Using whichever combination of available plates results in the least spring to circlip/groove distance should give the lightest clutch action.

Thanks LAB. I'll make that measurement.

Meanwhile, I read the section in Norman White's book on this subject. On page 119 he says:
"The thick pressure plate (part no.060745) can only be used with the four fibre friction plates, and the thinner pressure plate (part no. 063768) is only compatible with the five sintered bronze plates."
I'll assume this statement is included only to make sure stack height is correct.

But he goes on to say:
"The early clutch centres that matched the fibre friction plates must not be used with the later sintered bronze plates because the material is too soft to resist the bronze plates' steel splines. With the introduction of the bronze plates came a revised clutch centre with a hardened surface (part no. 063979) This centre is compatible with both friction plate materials."

This causes me to think that maybe my idea of using 4 of the bronze plates is not so good. Except, when I look at the AN 06-1339 which is the replacement fiber plate, it now also has metal (probably steel) splines (as do the Barnett Alloy replacement plates) , as opposed to the original fiber plates whos splines were of the fiber material. Wouldn't these new fiber plate replacements cause the same problem with the early clutch center (which I'm assuming I have)?

NickZ said:

"The thick pressure plate (part no.060745) can only be used with the four fibre friction plates, and the thinner pressure plate (part no. 063768) is only compatible with the five sintered bronze plates."
I'll assume this statement is included only to make sure stack height is correct.

Click to expand...

Yes.

NickZ said:

But he goes on to say:
"The early clutch centres that matched the fibre friction plates must not be used with the later sintered bronze plates because the material is too soft to resist the bronze plates' steel splines. With the introduction of the bronze plates came a revised clutch centre with a hardened surface (part no. 063979) This centre is compatible with both friction plate materials."

Click to expand...

Well, the clutch has already had two bronze plates fitted (for how many miles?) so is there any evidence of excessive clutch centre wear caused by the two 'bronze' plates?

NickZ said:

This causes me to think that maybe my idea of using 4 of the bronze plates is not so good.

Click to expand...

Does four 'thin' bronze plates (plus others including the thick pressure plate) give the required stack height?

NickZ said:

Except, when I look at the AN 06-1339 which is the replacement fiber plate, it now also has metal (probably steel) splines (as do the Barnett Alloy replacement plates) , as opposed to the original fiber plates whos splines were of the fiber material. Wouldn't these new fiber plate replacements cause the same problem with the early clutch center (which I'm assuming I have)?

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I expect they probably would. All centres (at least those sold by AN) are hardened but even the hardened centres are not everlasting and often need replacing after an extended mileage.

L.A.B. said:

Well, the clutch has already had two bronze plates fitted (for how many miles?) so is there any evidence of excessive clutch centre wear caused by the two 'bronze' plates?

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I've only put 400 miles on it since I got it back on the road. According to PO, it was parked since some time in the 1990s. I don't know how many miles, if any, were put on the clutch in this configuration prior to that. Currently, the clutch center looks pretty good, but does have slight signs of wear. Each tooth shows 4 darker spots which I believe are from the original fiber plates, and the inner 2 (where the bronze replacements have been) also show a thinner mark and slight wear from the steel tine. I don't know if that could be called excessive, but it appears to be more than that seen in the positions where the fibre plates remained.

L.A.B. said:

Does four 'thin' bronze plates (plus others including the thick pressure plate) give the required stack height?

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I would replace one of the plain plates with a 4mm one from RGM. I believe that will get stack height to within 0.5% of the stock value. I still need to measure this in my clutch basket to be sure that is the height I need.

L.A.B. said:

I expect they probably would. All centres (at least those sold by AN) are hardened but even the hardened centres are not everlasting and often need replacing after an extended mileage.

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Are there any friction plates available that do not have steel tines, for replacements on bikes that still have the softer clutch centers?

NickZ said:

Each tooth shows 4 darker spots which I believe are from the original fiber plates, and the inner 2 (where the bronze replacements have been) also show a thinner mark and slight wear from the steel tine. I don't know if that could be called excessive, but it appears to be more than that seen in the positions where the fibre plates remained.

https://www.accessnorton.com/attachments/img_9086-jpg.80489/

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It still looks in reasonably good condition from what I can see of it.

This is what a worn (hardened) centre looks like (from 5 bronze plates).

NickZ said:

Are there any friction plates available that do not have steel tines, for replacements on bikes that still have the softer clutch centers?

Click to expand...

Barnett alloy?

RGM in the UK.List 3,6mm Fibre clutch plates

geoff.s said:

RGM in the UK.List 3,6mm Fibre clutch plates

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It seems the "Solid Fibre" plate has a metal (steel) plate inside.
https://andover-norton.co.uk/en/shop-details/15610