BSA Owners' Club Workshop Discussions

The BSA Workshop => Singles => Topic started by: Stubaker58 on 24 Aug, 2026, 13:23

Title: B40 clutch
Post by: Stubaker58 on 24 Aug, 2026, 13:23
Hi All,

Tomorrow's the big day, DVLA inspection to get the B40 registered.
In getting it ready I looked again at the clutch which was pulling very crooked.
Investigation showed human error! I'd put in a mismatch of 'spring tubes' three long and one short. With four long ones it now, unsurprisingly, pulls square on.
BUT
Which are the correct tubes? The long ones are an inch and an eighth, the short ones seven eighths.
I put in the long ones but the springs went on very easily and then the screw caps bottomed out so I can't think they're correct.
Can anyone point me in the right direction?
TIA
Title: Re: B40 clutch
Post by: limeyrob on 24 Aug, 2026, 13:40
I think it should be 42-3218 which is 7/8" but I'm not sure this is for all years.  Is suspect the key is to match the springs to the cups and the number of plates.  The longer cup is listed as 1 1/16" so there's 3/16" difference, I wonder if that is one pair of plates?
Title: Re: B40 clutch
Post by: rhyatt on 24 Aug, 2026, 15:12

from Draganfly's site
40-3233
Friction clutch plate   4   1961-65  all years
11
40-3220
Plain clutch plate   5   1961-65  all years

and

40-3218
Clutch spring cup   4   1961-65

40-3217
Clutch spring   4   1961-65 (not Star/Sports Star)

40-3219
Clutch spring nut   4   1961-65
Title: Re: B40 clutch
Post by: limeyrob on 24 Aug, 2026, 16:02
That makes sense, so is there a different push rod or does the adjuster make up the difference?
Title: Re: B40 clutch
Post by: rhyatt on 24 Aug, 2026, 16:42
The 3218 is 7/8"
Title: Re: B40 clutch
Post by: Stubaker58 on 24 Aug, 2026, 17:28
Thanks guys.
I'll swap to the shorter ones after the bike has been inspected. Fortunately the bike does not need to be functional to pass.
Title: Re: B40 clutch
Post by: cdsdorset on 24 Aug, 2026, 22:14
If the adjuster bolts are 1 inch its the longer tube. If they are longer its the shorter tubes.
Title: Re: B40 clutch
Post by: Stubaker58 on 27 Aug, 2026, 19:19
Right I've got the clutch exposed again.

Re-reading RR CDS is absolutely correct. Long studs, short cups! Short studs long cups plus the amount the spring nuts extend outwards changes. With the later, shorter cups the nuts go flush with the outer pressure plate.

So the next question is ... can I remove the cush drive unit without taking off the whole clutch (so as to avoid having to remove the alternator etc)?  I need to do this to check whether the centre has long or short studs as the bike is put together from parts and I may have a rogue mixture.

TIA
Title: Re: B40 clutch
Post by: rhyatt on 27 Aug, 2026, 20:44
From memory, you can unscrew the retain screws that hold the cush unit plate to the unit.
That'll expose the rubbers and the X vane paddle that moves back and forth as load is applied/removed.
I think that'll leave the tapered centre body and its nut, lock washer alone.
Apologies if not exactly right, but it's been a while
Title: Re: B40 clutch
Post by: limeyrob on 27 Aug, 2026, 21:53
Can you poke a rod down or a Vernier and subtract the plate thickness to get a measurement?
Title: Re: B40 clutch
Post by: DAVE BRADY on 27 Aug, 2026, 21:58
Hi,

As Above.
No need to strip anything. Take the cups out and measure the bolts with a vernier as a depth gauge adding a bit for the thickness of the plate or measure the depth of the centre and how much the bolts stick out or not.

Dave.
Title: Re: B40 clutch
Post by: limeyrob on 27 Aug, 2026, 22:10
Yes "add" the plate thickness doh!
Title: Re: B40 clutch
Post by: Stubaker58 on 28 Aug, 2026, 07:41
Simple really when you think about it!
Thanks.