Master Cyl Cap Leak (2010)

I've recently resurrected my original master cylinder but am having trouble with the reservoir cap leaking.
I tried fitting a new diaphragm (which is the same as the old one) but still leaking.

Doing a couple of quick measurements the cap isn't compressing the diaphragm against the top edge of the reservoir, its short of making contact by about 2mm.
Am I missing some sort of gasket from inside the cap? Can't see anything in the parts book and I don't remember having any problems last time it was on the bike, but that was about 10 years ago.

Rich_j said:

Am I missing some sort of gasket from inside the cap?

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Yes, there should be a paper sealing washer under the cap, Norton part 60-4328.

New link: https://andover-norton.co.uk/en/shop-details/18172

"APCP2215-101
PAPER WASHER (3853-235) (60.4328)"

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The master cylinder I'm taking off is an AP Racing adjustable racing m/c which shares a lot of parts with the Norton one, it also has a paper gasket which I'm guessing is the same one.
Problem is its very thin and it would need one thicker than 2mm to make any difference.

Strangely, that washer is not actually in the Norton parts lists :?

What is in the parts lists is a change in diaphragm part number from 061946 to 066032, anyone know the difference?

Rich_j said:

Strangely, that washer is not actually in the Norton parts lists

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I think it was actually supplied as part of the cap, although it was available separately as a Triumph part.

Rich_j said:

What is in the parts lists is a change in diaphragm part number from 061946 to 066032, anyone know the difference

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The 061946 part is the old "bellows" diaphragm, which was superseded by the 066032 diaphragm which should be folded back into itself before fitting.

Also note that the original metal '72-'74 reservoir caps are different to the MkIII and subsequent plastic Lockheed master cylinder caps, and the two types have different screw threads.

L.A.B. said:

Rich_j said:

Strangely, that washer is not actually in the Norton parts lists

Click to expand...

Also note that the original metal '72-'74 reservoir caps are different to the MkIII and subsequent plastic Lockheed master cylinder caps, and the two types have different screw threads.

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I think we're getting somewhere now.

Mines a '74 Mk2A but the cap I have is plastic and by look of it the wrong part.

The threads might be different in some way but they are the right pitch and close enough to let the cap fit, it's just too deep and just runs out of thread too soon.
Looks like I'm in the market for a new cap.

Funny thing is it's the cap that came with the bike twenty years ago and I don't remember having leaks back then.

Thanks for the help, think I'm sorted.

Rich_j said:

The threads might be different in some way but they are the right pitch and close enough to let the cap fit, it's just too deep and just runs out of thread too soon.

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Yes, the threads are different, although as you say, it is possible to screw the plastic cap onto the earlier master cylinder unit, but it will run out of thread.
(Plastic cap MkIII master cylinder is the top photo)

To clarify, the MKII MC (lower image of the post above) should have a spacer/washer for the alloy cap? Mine does not and I'm getting some leakage. Cannot find the suggested washer part number at online suppliers. Thinking of crafting one out of plastic/cork etc. Suggestions?