Wrong clutch pressure plate...

I believe that I just discovered that my 850 has a 750 clutch pressure plate! A couple of days ago, on a 140 mile run, I noticed a bit of slippage at WOT in 4th gear. Today I pulled the clutch assembly (which I have done several times for other reasons) and, for the first time actually measured the thickness of the steels, the friction discs and the pressure plate. The steels/friction discs measure correctly BUT, per Old Britts, (https://www.oldbritts.com/ob_clutch_info.html) the 850 pressure plate is supposed to be .102" thick; the 750 PP is supposed to be .225. The pressure plate in my bike is .225.

My measurement is taken at the edge of the pressure plate - where the splines are - not at the thicker center section. Do folks here agree that the PP in my clutch is incorrect for the 850?

Measure the entire thickness. From the peak where the pressure plate contacts to the friction surface.

Read here: http://atlanticgreen.com/clutchpak.htm

And here: http://atlanticgreen.com/ndnsclutch.htm

MexicoMike said:

My measurement is taken at the edge of the pressure plate - where the splines are - not at the thicker center section. Do folks here agree that the PP in my clutch is incorrect for the 850?

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Not if somebody has fitted a four (Barnett?) friction plate clutch.

If it has four 0.145” friction plates (+3 plain) instead of the original five 0.121” bronze plates (+4 plain) then it needs the thicker '750' pressure plate.

It has the standard 850 5/4 friction/steels.

Stack height is: 1.154 measured at the outer (splined) edge of the discs/pressre plate and 1.30 measured at the inner edges which includes the raised section of the pressure plate. Steels (4) and friction discs (5) all measure correct (spec) thickness as shown on the Old Britts site. Based on the stack height if measured using the peak of the pressure plate, it seems that the stack height is pretty close to what it should be. Is that correct?

My concern is that my clutch lever pull is extremely easy (which I always liked) but I was wondering if there is some slippage due to insufficient spring pressure. IOW, I was thinking that maybe there should be a larger gap between the pressure plate and the circlip than there is, which would then apply more pressure to the clutch plates (and require more pull on the clutch lever to disengage). But if the pressure plate IS the correct plate for the 850 and all the discs are spec thickness, there is no way to "adjust" stack height to make it lower.

MexicoMike said:

It has the standard 850 5/4 friction/steels.

Stack height is: 1.154 measured at the outer (splined) edge of the discs/pressre plate and 1.30 measured at the inner edges which includes the raised section of the pressure plate. Steels (4) and friction discs (5) all measure correct (spec) thickness as shown on the Old Britts site. Based on the stack height if measured using the peak of the pressure plate, it seems that the stack height is pretty close to what it should be. Is that correct?

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No, (1.154") is high (= 1.027” at the splined edge according to the OldBritts way of measuring a five plate clutch).

And high (1.30") according to the DD method (total plate height = 1.172").

MexicoMike said:

My concern is that my clutch lever pull is extremely easy (which I always liked) but I was wondering if there is some slippage due to insufficient spring pressure. IOW, I was thinking that maybe there should be a larger gap between the pressure plate and the circlip than there is, which would then apply more pressure to the clutch plates (and require more pull on the clutch lever to disengage). But if the pressure plate IS the correct plate for the 850 and all the discs are spec thickness, there is no way to "adjust" stack height to make it lower.

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The stack appears over height by the approximate difference between the 'thick' and 'thin' pressure plates (0.120") so it does seem to have the wrong (thick) pressure plate and that's almost certainly the reason why the clutch is light but is slipping.

Thanks LAB, really appreciate it! Guess an Old Brits order is coming up! Unfortunately, we are leaving Mexico (where the Norton is located) on Tuesday and won't be returning until December so I won't be able to actually install the correct plate until then!

A good friend here is a retired machinist and has an extensive shop. If the only difference between the 750/850 pressure plates is plate thickness, he says he can machine the 750 plate to 850 thickness. Is there any other difference or is it simply the thickness of the "flat" part of the plate?

Just machine down the flat face. Machining off less than the full 0.120" would probably help to keep some of the lightness of the clutch action.

OK, great!

All this is assuming the clutch drum is approximately the standard depth and that the thicker pressure plate wasn't fitted to compensate for a worn or machined-down (or missing?) backplate?

https://andover-norton.co.uk/en/shop-details/15855

Assuming you are still using the stock sintered bronze plates, another option would be to install a set of Barnett clutch plates and then you would be able to maintain your light pull clutch.

I find it interesting that it all fit in there unless something was changed previously to make it work.

Mike,
FWIW
When completely assembled is your spring slightly concave? If so you are probably OK. I would think that 5 friction plates plus a thick pressure plate would cause the spring to be convex, if you were even able to get it together.
I would do some careful measuring before buying or machining anything. All the info you need should be on Dave's site.
Pete

Sorry to be so acronym dumb, but what do IOW and WOT stand for?

Joe Schlaberdowski said:

Sorry to be so acronym dumb, but what do IOW and WOT stand for?

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IOW usually means: 'In other words'.
WOT When discussing engines is: 'Wide open throttle'.

Although acronyms often have more than one meaning, for instance, WOT: https://www.allacronyms.com/WOT

How long have you been riding your Norton like this, has it only just started to slip or has it been doing it for some time, what oil you run in your primary, also something to look at is your pressure plate as I had to replace mine many years ago as it lost its pressure to hold the plates, I am still running my orginal plates from new and its still a very light clutch can pull lever in with one finger if I wanted to and I run ford tranny oil in my primary.

Ashley