There seems to be some difference between these two clutch centres.

The length of the splines and the length of the external thread.

Am I right in thinking the shorter thread will give me more clutch movement?

Rob

email (option): robmiller11(at)yahoo.co.uk

Hi Rob,

You sadly won't get more movement as the clutchcentre (which holds the inner-clutchplates) has a fixed position on these sleeves from your picture.

So the length of splines has nothing to do with the clutchmovement, it only has to transmit the torque and the threads only has to hold the nut.



Think extra movement of the clutch can only come from the lever on the handlebar (or - in theory - from changing the lever on the gearbox endcover)

Regards, Michiel

I'm not sure that drawing is accurate, I was under the impression that if the external spring nut wasn't tightened to the extent of its thread it wouldn't stay done up?

My theory is that that clutch stops opening when the spring is fully compressed, if it doesn't fully compress for a longer duration of time the clutch has a chance to open further.

I have been suffering from clutch drag and clutch slip, so an extra millimeter of clutch movement could make all the difference.

How long was the single spring clutch in use? I can't find any different part numbers for this part so is one of these a pattern part? or a re design?

Rob

email (option): robmiller11(at)yahoo.co.uk

Wasn't the single spring introduced as part of the 'AA' mods in the mid 50's?

With mine I had to remove a pair of plates to get enough movement otherwise I would also suffer from both drag and slip.

Cheers,
Bernie

email (option): bernie.a.jones@gmail.com

I've done some measuring and my theory isn't holding water

When I've tried leaving a pair of plates out in the past it's cured the drag but made the slip much worse.

Rob

email (option): robmiller11(at)yahoo.co.uk

No Bernie. The single spring was used during the war on all M20/21's.
Rob I've never had that problem in over 20 years of running my M20 (now M21) with two plates removed. No slip, No drag and the lightest of clutch's. In fact you would normally get one or the other to my mind (Drag or slip)



Ron

email (option): ronpier@talk21.com

You can certainly have both slip and drag if you have certain conditions from both lists below 'in play'...

Clutch slip...
Oil contamination of plates
Insufficient free play at the operating arm
Clutch pushrod sticking in mainshaft (rust)
Worn clutch spring reducing pressure
Distorted metal plates(not flat)
Distorted sliding plate or pressure plate (not flat)
Wear in the clutch basket and/or clutch center slots causing plates to 'hang up'
Wear on internal or external plate tangs (effect as above)
Tight (not lubricated) or badly routed clutch cable
Dust contamination from worn clutch friction discs (restricting movement of the clutch plates)

Clutch drag...
Clutch arm incorrectly adjusted (not set correctly for maximum lift)
Bad wear of rear of clutch center
Bad wear of clutch rollers/basket roller track/sleeve roller track
Uneven lift caused by spring wear
Overly thick fibre plates (if a full set of non original plates are fitted)
Distorted plates as detailed under 'clutch slip' above

Basically clutch drag is caused by insufficient separation of the plates for whatever reason...

Clutch slip is caused by a lack of friction or pressure (or both) for whatever reason...

The single spring clutch was a pre war part but both the Army and the AA retained it post war as it was superior to the six spring BSA clutch that replaced it..
The exception was the last three years of (alternator equipped) AA production (1960-63) when the Triumph clutch was fitted to provide a shock absorber in the transmission. The engine shock absorber assembly was removed to accommodate the alternator rotor.....Ian

email (option): ian@wright52.plus.com

Ian, can't a primary chain being too tight also cause slip and drag?

Great info there!

Kevin

Hi Kevin..A tight chain can cause drag...I'm not entirely sure of the reason for that though.
There is a detailed piece on clutch problems in the Technical Section of the site...Unsurprisingly entitled 'Clutch Problems'... ...Ian

Sheesh Bernie, is this your first visit? Single spring clutches is one of the common subjects regarding war models????

This single spring clutch is a pre war item and was actually called "the 8 plate Empire Star clutch" as it was first used on that model BSA. This was basically just the clutch as we use in on our WM20's but it didn't had the thread to pull the centre of the clutch from the gearbox main shaft.

Henk

email (option): ahum@quicknet.nl

I didn't know that about the extractor thread on early examples of this clutch...I should think they rapidly found out it's nearly impossible to get the mainshaft sleeve off without a puller.. ...Ian

Couldn't you use the outer male thread for a puller? Of course the clutch would have to be dismantled first.

Ron

email (option): ronpier@talk21.com

Henk, is correct. The M20 does use a variation of the Empire Star single spring clutch. I have a 1936 model Q8 Empire Star and the WD M20 single spring is "almost" the same. There is a slight difference in the clutch basket and clutch centre, also the spring is slightly different.. A later M20 single spring clutch should in theory fit if it had to be used.

I also have a single spring clutch on my 1935 W35/7 and that is different again to the 1936 Q8.

BSA worked a lot on their clutch's in the 30's to improve the design. I also have a 1938 M20 and that does not use the single spring clutch, it was a short lived change as it also proved to have issues and BSA reverted back to the single spring for the WD M20.

Rgds Dave

Ian,

You are right. If you look in the 1936 part catalogue you can see that they did modify the clutch halfway through the season. That is why there are two engine prefixes.

Lucky me, since I have the early one on my R5 Empire Star which I got off with a big three legged puller on the clutch basket.

Regards,
Leon

email (option): leonhop3@planet.nl