Gearbox main shaft

[Craigy]

Hi guys,

A10 swinging arm this time…..I’ve mentioned before that the drive chain is very close/touching/too close to rear of primary case. I’ve done everything I can on that side of bike, including new ‘original chain’ but still very close.
A guy that had worked on the bike said that the nut in gearbox may need tightening up.
Managed to get it stripped down ok but can’t figure out how to stop shaft turning  when trying to tighten. Bike is in gear.
Also is there a correct torque setting for this?
Would like think this will solve the problem.
Cheers Craig

[limeyrob]

If you have space around the gear or spline you can bend a piece of soft ally over it then use Stillsons.  A technique I sometimes us is to cut some rods to the width of the gear, hold them in the teeth with grease then use an adjustable spanner holding on the rods like a nut.

[berger]

i went to the pub i think i posted somewhere on here about using the quadrant in such a way it locks the shaft, can't remember how i did it or where it was posted

[Swarfcut]

 Usual trick is bike in gear, rear brake on, mainshaft will remain still as the nut is tightened.  Later bikes have a circular spacer between the crankcase and the back face of the primary chain case, this gives a little more more space for the chain, but then we come to the various clutch parts used for different model years, twisted frames, distorted engine plates etc, and the primary chain running close on the inside face. Don't get me on aftermarket clutch adaptors that are not quite right.

 With a box in bits, mainshaft and kickstart ratchet can be assembled as a sub unit along with the inner cover and threaded through  the  mainshaft gears, the method recommended in BSA Service Sheet 311.

 Swarfy.

 

[Craigy]

Bike is in gear and rear wheel on bench so not turning, I can feel crank turning compression over while in gear., when I try to tighten nut.
I’m thinking gripping the gear (covered as Rob said) is the way to go but high risk of damage…..or a rattle gun???

[Topdad]

get someone to help you by applying the rear break as prev mentioned 

[Swarfcut]

  My trick relies on the clutch locking the mainshaft to the gearbox output sleeve gear, in other words relying on the clutch to give a solid connection between the two. From description it sounds as if the clutch is slipping and the mainshaft is turning, driving the primary chain and turning the engine. This should not happen. With no pressure on the pushrod, clutch should grip with no relative movement between the driving and driven plates.

 Clutch locking tools are available, basically a steel stamping with tangs to match the inner and outer fingers of the clutch drum and chainwheel. This is another way of addressing this problem.  Back in the day we made out own, welding a couple of scrap lined and plain plates together.

 A mentioned, two pairs of hands are better. Lots on the Forum about clutches, 4 and 6 spring types were used, both interchange as complete units, so either may be encountered thanks to previous owner mods.

 Swarfy.

[jhg1958]

I locked my chain with a long screwdriver. I see yours is off but if you had an old chain you could probably do the same

John

[Craigy]

Chain is still on. The only thing I’ve taken off is the gearbox outer cover/kickstart/gear lever.

The rear wheel isn’t turning but the crank/pistons are

[chaterlea25]

Hi All,
The nut on the kicstart side must not be over tightened, or it will distort the bush and lock the ratchet.  I seem to remember 25/ft /lbs

John

[limeyrob]

Good point, I'd forgotten, the shaft position is not set by this nut but by the sprocket and nut on the drive side.

[Swarfcut]

 The more I consider this problem the weirder it gets.

 Seems there are two conundrums, loose kickstart ratchet nut and a gearbox sprocket running too close to the rear face of the primary chaincase.

  The sprocket position is determined by the sleeve gear, its bearing along with the oilseal, retaining circlip, the sprocket itself and centre ring nut. So if these parts are the right ones, assembled correctly, that takes care of that. Always a possibility the sprocket is not made to spec, with too much  depth to the rear face into the box. Moving the gearbox sideways or a primary case too close to the gearbox, by whatever means, are other considerations.
 
  Mainshaft sideways location has no effect on this, instead it positions the clutch relative to the rest of the primary drive and chaincase.

 The mainshaft bearing in the gearbox inner cover locates on a shoulder, preventing axial movement towards the clutch end, retained the other way by a circlip. Correct assembly should ensure the mainshaft ends up where it should. The sequence moving to the timing side is mainshaft, bearing, pinion sleeve support washer, sleeve and spring, followed by the kickstart pinion, ratchet, tab washer and nut. As noted this nut needs to be fairly tight but the bush can be distorted by overtightening.

 Why the shaft continues to turn with box in gear, chain on, rear wheel locked is down to something amiss in the clutch, either poor or maladjusted plates, or clutch adaptor loose on the taper, key sheared? but just grabbing enough to move the primary chain and crank.

 As I said, weird!

 Swarfy.

[limeyrob]

It does look like the cluster needs to come out and all the parts checked. It is weird and without seeing it its hard to know, but now is the time while its off the road.

[chaterlea25]

Hi Craigy,
On one A10 I had to make a thinner spacer plate that fits between the crankcase and rear primary in order to get primary chain clearance.
I don't remember what year your A is but the spacers were introduced around 56? When the rear fully enclosed chain guard was fitted,

The rear primary cases are fairly flexible
Have you tried adding washers to the spacers on the rear chaincase mounting?

John

[Craigy]

Hi guys,

My bike is a ‘59.

I’ve just been reading an old post, mainly between Swarfy and Bag’o’nails (who I know from local BSA club).

It’s been our rally today, which was great (BSA South Australia). After a few discussions with members that know far more than me and looking at other bikes, I’ve decided to attack it from another angle.

It seems that ‘normal’ clearance between chain and casing is about 1cm - I can’t get a ruler (2mm?) between mine.

I’ve loosened and retightened the nut on the shaft gearbox side to the recommended 25lb/ft - it seemed that it was about right anyway.

The plan is to go in from the primary case side, pull everything off and see just what I have in there. I’ve had the clutch off before (the basket needed reattached), so I know how to get that far. The Cush drive spring seems like it’ll want to go through the side of the shed with my hand still attached, so I’ll have to have a good look at that to make sure I don’t loose a body part in the process - getting it back on is a worry if it’s under that much tension.

I only learned it was called the Cush drive today….tomorrow I’ll learn what it does 🤔

I’ll strip it down methodically and see if everything is correct, as per the parts book

I did read on another old post that a Citroen ball joint socket fits, so I’ll try to pick one of those up.

I must drive all you wise men mad, but if I don’t ask; do it; mess it up, I’ll never learn 😬