Burman Gearbox - 25-7-3 cup thrust

56G80S · Post by **56G80S** » Fri Jul 04, 2008 11:07 pm

After overtaking the lorry and throttling back from the legal max on the dual carriageway A19 I heard this tinkling sound from the chain case.

Pulled in the clutch.

No resistance and still 9 miles from home. Got home with a combination of fast cornering, careful judgement of speed to arrive at junctions when there was a gap in the traffic, crash changes (ouch) and starting - pushing - jumping and crash changes!

The 25-7-3 cup thrust for spring pressure plate and it's nut had been circulating round the detachable part of the outer chaincase after undoing themselves. Fortunately the G-38-2 pressure plate and thread in it look OK except for lots of scratching round the circumference of the plate (£24 JLS for replacement - phew). But the 25-7-3(thread badly damaged on the one I now have) is not listed on JLS. Only the equivalent part for the AMC clutch.

Why this prejudice?

I might even try cutting the damaged thread off and seeing if I can get away with a shorter cap to keep me on the road (310 miles in the last week mostly to and from work - great).

Any suggestions / comments / abuse? Don't know why it undid itself. It was done up tight. Could it be the ATF being more penetrative than engine oil?

Johnny B
Edited by - 56G80S on 05 Jul 2008 9:39:18 PM

TommoT · Post by **TommoT** » Mon Jul 07, 2008 4:53 pm

Try AMC Classic Spares at:

http://www.amcclassicspares.com/

or even:

http://www.petersclassicbikeparts.nl/index1.html
Edited by - TommoT on 07 Jul 2008 3:58:02 PM

56G80S · Post by **56G80S** » Sun Jul 13, 2008 11:10 pm

This thread not as dead as I thought. Got my new part, which is fine and prompt turnaround, from Andrew Engineering.

But before simply re-fitting the pressure plate I took a closer look. The most outboard friction plate's tangs had worn over on one side and slipped / cut into the inner circumference of the basket slots.

From the outside the slots look smooth, no ridges or anything to hinder the plates from sliding along them.

Then I noticed what looks like hairline cracks at the closed (inboard) end of some of the slots. Only on the leading angle as the clutch rotates.

So perhaps I now know why the 25-7-3 cup thrust for spring pressure plate came undone.

I'm planning to take the basket off, get some weld into the damaged part of the slots on the inner circumference and dress them up, same thing on cracks and carry on using it as the chainwheel's good.

Any suggestions / comments? Like - you're a tightwad? Just put the spare frction plate that I have in and get on with it? (not a starter at all in my view). Get your order in to JSL now and make the most of the so called summer rather than f*rt about? Still need to think about why it happened in the first place?

I might know the answer. It was put back together in a big hurry. It should have 5 friction plates but they won't all fit in the basket. It was the only one I had. As I only ride solo now, I settled for four friction plates and used two plain plates at the outboard side to get the pressure plate right.

I now expect a swingeing PM from Alan.

And worse from some others (ITMA).

Johnny B

bjork · Post by **bjork** » Sun Jul 13, 2008 11:35 pm

You should be able to get the right number of plates in. I had the same problem with mine a few years ago, till I spoke to Ernie at the old club shop and learned that different thickness steel plates are available. The thin ones are about half the thickness of the others, that made all the difference. Also worth checkling the friction plates haven't been relined with extra meat at some time perhaps?

bjork · Post by **bjork** » Sun Jul 13, 2008 11:38 pm

quote:

I now expect a swingeing PM from Alan.

And worse from some others (ITMA).

Johnny B

WOULD IT HELP IF I SHOUTED? I CAN DO OLD SAXON TOO IF IT HELPS EMPHASISE

56G80S · Post by **56G80S** » Sun Jul 13, 2008 11:39 pm

AAaaargh. I now think I know why I couldn't get 5 plates in - I'm using the wrong clutch case for a B52 box, according to the Spares list.

Indeed. it looks completely different.

But at £87.50 for the clutch case alone, ignoring the new plates that I'd need to get, I think I''ll either stick with the clutch I'm using (repaired) or stick the AMC box and clutch back in. If only I could remember which of the AMC boxes is the good one!!!

Some would say none of them are.

Johnny B

itma · Post by **itma** » Sun Jul 13, 2008 11:49 pm

AMC gearbox ?
finest ever fitted to a motorcycle
theres only one, the others are Burman

56G80S · Post by **56G80S** » Mon Jul 14, 2008 1:22 pm

What I meant was, which of the two AMC boxes that I have is the good one.

I have two. One was bought simply because it was cheap but probably needed rebuilding, the other jumps out of first gear but is fine otherwise unless trying to pull away from a standing start on a very steep slope.

Johnny B

Although working off the material I recall seeing on the Search on this forum I should be able to cure the jumping out. - JEdited by - 56G80S on 14 Jul 2008 12:23:32 PM

bjork · Post by **bjork** » Mon Jul 14, 2008 10:39 pm

quote:
What I meant was, which of the two AMC boxes that I have is the good one.

You'll have to weigh them up, the heavier one will be more complete, so use that.

I have two. One was bought simply because it was cheap but probably needed rebuilding, the other jumps out of first gear but is fine otherwise unless trying to pull away from a standing start on a very steep slope.

Face the other way on the hill, also uses less fuel.

Johnny B

Although working off the material I recall seeing on the Search on this forum I should be able to cure the jumping out. - JEdited by - 56G80S on 14 Jul 2008 12:23:32 PM

Oh yes, it'll be either the detent plunger or the gear dogs or the forks or all of them, but with two g/boxes you have probably got all the spares you need already!

56G80S · Post by **56G80S** » Wed Jul 23, 2008 7:36 pm

I've now taken the clutch basket off and it's being welded and I'll dress it. I didn't have a socket of adequate size and so measured diagonally (roughly 1.52") and borrowed from friendly workshop. The best fit I got was 1 5/16" AF!

Heresy I hear you all shout. But it didn't damage the nut at all despite the leverage required to wind off through all the loctite I had on. The corners still sharp on the nut.

I noticed that the clutch case sat proud of the G-35-2 sleeve for mainshaft, in other words not quite fitting fully onto it Didn't measure but I'd guess 1/32".

So does anyone know the right socket size and I'll buy one rather than borrow. Do I need to be concerned about the clutch case fit?

Johnny BEdited by - 56G80S on 23 Jul 2008 9:31:42 PM