Just looking at my (1965 B40F) clutch plates, they look to me to be in pretty good condition except I'm concerned about the thickness of the friction material. My Haynes Unit Singles Manual says the standard thickness is .167" and that the plates should be replaced if there is more than .030" of wear. So by my reckoning that's saying if the plates measure less than .137" across the friction material then they should be replaced. I've measured mine in a few places and get thicknesses varying from.122" to .130", which of course is below the limit given by Haynes. Before I reach for my wallet, does anyone have any comments about whether I need to replace them? I'm guessing I ought to. Thanks. Phil.
They're a bit like brake linings - in principle, you can wear them down to the rivets but their effective diminishes rapidly after a certain points. For clutch plates that limit is normally -0.03" after which clutch slip will probably start to develop.
Okay, thanks, I guess I'll replace them. No need to do the plain plates aswell if they look okay I guess? Phil.
The plain plates don't normally wear that much but you have to make sure the tabs that engage in the clutch centre are in reasonable condition; ie badly damaged or mis-shaped.
As I said, my Haynes unit singles manual says the friction plate standard thickness is .167", and that they should be replaced if there is more than .030" of wear. So by my reckoning they should be replaced if less than .137". I measured mine in a few places and got thicknesses of between .122" and .130", so ordered some new ones from Draganfly. They arrived in the post this morning. Thickness of new ones: .118". That's thinner than my old ones! I think Draganfly are shut at weekends so I can't discuss with them until Monday. In the meantime, anyone know what thickness they should be? My bike is a 1965 B40F. Thanks. Phil.
The B40 plates are the same part as B25 - 40 3233 - the B25 workshop manual gives same figures as your Haynes manual.
Non original no doubt made to different dimensions than original.
Thanks Julian. Seems odd that I'm being sold plates which are below the recommended minimum thickness before they're even used. I wonder why. Maybe friction material is better nowadays than it was when the bikes were new, and so they don't need to be as thick? Just trying to think of possible reasons. Phil.
Draganfly may have an answer for you. Please let us know.
Phil. Can you measure the thickness of the metal plates on which the friction material is bonded to work out the thickness of the actual friction material. I'm assuming the thickness of the friction material is much greater on the new plates than the old ones and it is the thickness of the metal plate that is the problem. Clearly, the new plates of not usuable unless they are designed to be used as a set of 5 plates in the clutch rather than the 4 you've presently got.
I've just measured an old one and a new one, in a couple of places each. The old one measured .067"/.070". The new one .061"/.062". So a bit of difference, but the thickness of the friction material does appear to be an issue. As you say, my clutch has four friction plates (and five plain.) Phil.
This means the old worn out friction material is 0.052" thick and the new material is about 0.056". If someone has a new new friction plate to hand, it would be interesting to know what thickness that is.
Unless it is inlaid, but I can't imagine that. See photo.
It's the friction material that projects beyond the metal backing plate that's important and the usable friction material in the new plates looks to be similar to your worn out old plates and therefore provides no advantage. It will be interesting to hear what Dragonfly have to say.
There are a number of different manufacturers of plates for you bike.
Same plates used C15-B50 though different clutch and 1 extra bonded with some other different parts for OIF models (nice upgrade for pre OIF if your clutch slips particularly when trying to start)
Surflex made in Italy
Barnett made in USA
Emgo made in far east
Indian
So it is likely that there will be some variation in thickness.
Most sellers on ebay or internet do not include a thickness, some just include a diagram (from Wassall catalogue) which gives dimensions except thickness. If they are selling Surflex, a premium brand they generally say so.
Barnett make a lot of bike clutch stuff and they list their version the questioned plate as 0.130 inch thick.
http://www.barnettclutches.com/bsa.html
Thanks Julian. So Barnett's also is below the minimum recommended by Haynes.
I spoke with Draganfly today. They don't know the reason why the new friction plates are thinner than the minimum recommended by unit singles manual- maybe a different friction material? - but say they've sold lots of them and not had a problem. I then spoke with Chris at Ripe Motorcycles who advised me there had been a BSA service bulletin about it. I can't find a bulletin about it, so don't know what the minimum thickness should really be, but think I might as well stick with mine for now. Phil.
I'm sure the main issue is not allowing the friction material to get to a point where you then run the risk before the next major service of them wearing down to the metal and the clutch function failing (ie slipping excessively). You've got just over 1mm of friction material left and that should last many thousands of miles. It doesn't to be worthwhile replacing them with new plates with the same amount of wear left!
I would change the clutch springs - tired old ones do allow more clutch slip!
Thanks everyone. I've decided to keep my old plates. I've also got new springs.