The importance of gearbox thrust washers

[roadrocket]

Hello!

My gearbox is clunky after rebuild, and I suspect the reason could be either lack of, or wrong placement of, thrust washers. My parts book show just one: 67-3203 placed between first and second layshaft pinion as seen from the selector/timing side. But a search on this site revealed this quote:

 According to the late John Gardner, Tech Officer of the GSOC UK, "There are four large hardened washers . . . , three on the layshaft and one behind the kickstart ratchet pinion bush . . .Unfortunately, the BSA Spares List illustration [Plate 5, A Group Plunger] does not show clearly the correct assembly.
   Working from the clutch end of the gearbox, the washers should be assembled on the layshaft in the following positions.
   Part number 42-3078 is fitted between the main gearbox shell and the layshaft fixed gear 'B'.
   Part number 67-3203 is fitted between the 'F' and 'H' gears.
   Part number 42-3079 is fitted between the 'H' gear and the middle cover.
   Part number 67-3161 is fitted behind the kickstart ratchet pinion bush."

The "Layshaft Thrust Washer", p/n 67-3203, located between the 'F' and 'H' gears, has the following dimensions;
     .093" (.115 ") thickness*;
     1.093" (1.248") O.D*.;
     3/4" I.D.
*The difference between the two numbers show the practical tolerance that I've experienced. The first number is John Gardner's, the other is what I have in my A-10 and/or Gold Star boxes.

   John goes on to say "Pay particular attention to the washer part number 67-3203 which is also used on the gearboxes fitted with bronze layshaft bushes. The bore of the washer often . . . has a chamfer on one side only. If this is the case, the washer must go onto the layshaft with the chamfer against the spline, to give clearance for the small radius on the shaft at the start of the spline."

Now, can anyone shed more light on this? Should there only be one correct number of washers? I have only one in my box, and have had good clean shifting with this before the rebuild. I might have messed it up, or maybe the closer tolerances have played havoc with old forgiving slack?

[KiwiGF]

All I can say is I have one washer in the box on the (s/a) bike, and one in the spare plunger box it came with. It changes gears ok  albeit a bit crunchy going to third sometimes.

Could your clutch be causing trouble?

[roadrocket]

The clutch was very carefully (to the best of my abilities) assembled, but I will have a look again. I know 90 % bad gearboxes are found in the primary chain case 😊. The box will get stripped, if not anything else, just for peace of mind. It bugs me, since it used to be so sweet.

[muskrat]

G'day Otto.
I have found in the past that the thickness of the gasket between the case and inner cover can make a difference. My plunger box plays up if I use anything thicker than 0.1mm paper. The paper thickness would affect the clearances of the gears on the shafts.
Cheers

[duTch]

 Otto, there are only two in boxes with bushes on the layshaft- the thrust washer is the integral flange, it's RRT2 (close ratio) boxes that have the extra two T.W, as they have needle rollers on the layshaft.

 Also pay particular attention to this;

   John goes on to say "Pay particular attention to the washer part number 67-3203 which is also used on the gearboxes fitted with bronze layshaft bushes. The bore of the washer often . . . has a chamfer on one side only. If this is the case, the washer must go onto the layshaft with the chamfer against the spline, to give clearance for the small radius on the shaft at the start of the spline."

 ...and what Musky said

[roadrocket]

Thanks chaps!

[duTch]

 Just thought I'd add to this, to show what can happen if the

Part number 67-3203 is fitted between the 'F' and 'H' gears.

, is fitted in the wrong place

 This is a Plunger box I picked up at a swapmeet(jumble) a few months ago, just for the outer cover...it felt a bit clunky, so knew it was knackered...the first pic shows it full of grease, and the sheared thrust collar from the bush, and the Thrust washer  67-3203 in the wrong place

 Second pic is fairly self explained, busted casting and four of the eight gears with teeth smashed, and worn dogs on the other four..

 Could've been an ok box if assembled properly , oh apart from broken top mount and two interface-casting, which may have resulted from the impending lockup (not shown)