"As noted previously, particular attention should be paid to the main shaft spline at the sprocket end...If these are badly worn the main shaft will be out of position even when correctly shimmed"

Ian could you clarify the above statement please? Do you mean the sprocket splines? or inside the box?

I've had a complete spare WD box under the bench for some years with the intention of stripping it for inspection one day! Some years ago when I rebuilt the box for my special, some gaskets arrived from somewhere(?) in a fairly thick blue cardboard material. Then when I ordered some parts from Les, he told me that he had some original paper gaskets that are just 7 thou thick, which he sent me. If I had used the cardboard gaskets, I would have had to alter all the shims which were otherwise within limits. Ron

email (option): ronpier@talk21.com

Hi Ron...It's the spline machined onto the main shaft inside the box that the selector dog slides along...The thicker gaskets you describe come from the main UK gasket manufacturer who, bizarrely, also make the thinner ones. It may be they felt the thicker material would provide a better seal, however, if they are fitted it just requires more shimming to get everything right...With the modern sealants the thin gaskets should be quite adequate...Ian

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

Thanks Ian. I'll take a look at it one day......Or bring it down and watch you do it:smile: Ron

email (option): ronpier@talk21.com

Your already fully trained Ron...As the photo from your visit proves...:laughing: ....Ian

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

Hi Ian, Your posts are amazing and has learned me awful lot more of M20's, but really can't follow you on the mainshaft endplay subject.

You keep emphasizing the importance of possible wear on the end of the splines of the mainshaft but in my view it is of no importance at all. The mainshaft and the sleevegear should not be able to touch each other axially as the mainshaft is pulled up to the small bearing. No endplay on the mainshaft so no contact...

Also the end-faces of the splines don't look to be designed to take any force, a very small and splined surface.

In the SA / Norton boxes there is a thick bronze trust-washer between the end of the splines and the sleevegear. this takes up the force when the clutch is pulled. That shaft works with endplay but no need for a trustwasher or contact on the BSA as the small bearing will take up the axial force when the clutch is operated...... a very clever design .....:relaxed:

BR Michiel


BTW.: it is also good practice to clamp up the washers and shims so that they turn with the shaft, otherwise they will wear quickly

The shafts should be assembled within the case with 'minimal clearance' according to the factory worksheets...I've read in other publications that .005"-.010" is the correct setting...I would certainly consider .005" to be a reasonable 'minimum'..

Shimming the shaft achieves that and also serves to position the main shaft within the box, the shimming moving it towards the sleeve gear to take up excessive play..When that has been properly achieved whether the ratchet nuts are tightened sufficiently to take up the end float or not is probably irrelevant in practice I would think...Personally I tighten them only enough to preserve the clearance and that hasn't had any detrimental effects on the shims or splines so far...Either way the clearance is preserved between the sleeve gear and the splines...

However, if the shimming is not done correctly and the shaft is too tight when the inner cover is fully tightened up that will force the spline against the sleeve gear...When the box is in neutral with the engine running the shafts are turning but the sleeve gear isn't so there is running contact between the two parts when there shouldn't be..In that case the spline can wear....Ian

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

Yes, off course..after setting up the mainshaft there should be endplay to prevent locking up.....

Point is that, if the meshing of the gears and dogs is correct, the excess of endplay will be pulled out to the ballbearing.

And what would be the function to set up a small endplay here ??? .... Think no function at all......it is actually very unwanted as makes it possible for the shims, washers and the bronzebush of the kickstartpinion to wear...

This wear increases the endplay and will bring the mainshaft closer to sleevegear when the clutch is operated....and yes....they will probably touch each other at some point and this could be the reason for the wear you find at the end of the splines.... and again, the spline ends are not designed to take that axial pressure like some other boxes..

I only see advantages in tightening up the nut firmly and locking up all the loose parts, just take care not to expand the bronze kickstartpinion bush otherwise the pinion can lock up.


BR Michiel

Pulling up against the bush may in fact be the better method...However, wear of the splines is still possible if the shimming is incorrect making the shaft too tight a fit...

With some end play the splines probably do periodically contact the sleeve gear but there is no pressure exerted on them if the end float is set correctly...I have rebuilt quite a few boxes for my own use and haven't found worn splines or broken shims after substantial mileages...It would be entirely reasonable to argue though that it's not necessary to take that risk if the nuts are tightened sufficiently to remove end float...Ian

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

Why do you think BSA specified the first nut should only be tightened 'finger tight'?....Ian

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

Hi Ian, I don't doubt that it will run OK for many miles when a small endplay is set up.

Just don't see why to bother setting it up in the first place. Never looked if there is wear on the end of the splines, I only check if it free with some play.

What do you do with mainshafts with some wear on the end of the splines, shim it out ?

All of no relevance in my view.....


Don't know why BSA specified the 'finger tight'..., maybe to prevent the risk of kickstartpinion bronze bush from expanding and locking up the pinion. Thight suggests no endplay.

The Standards Book talks about Nil/Free endplay, ballrace controlled.




I just compare it with the crankshaft set up, all parts firmly clamped up to the ballrace, if not, your getting wear and trouble....:slightly_smiling_face:

Michiel, your view of this aspect of the job is more logical than mine so it must be concluded that your method is the correct one to follow...

That could theoretically remove the need to shim the main shaft entirely unless it is shimmed prior to tightening the nuts purely to position it within the box...That appears to be the case as .005" and .010" thick 'main shaft packing washers' are listed in the WD parts list..(Part nos. 66-3072/66-3072)...It seems they would not serve any purpose at all if the 'minimal' end float setting/positioning was not a requirement...

In conclusion and if that is the case, then the advice regarding the main shaft would be to shim it (position it) within the box to achieve minimal end float (approx. .005"), checking that it is totally free to turn when the inner cover is fully tightened. Then take up that float by tightening the ratchet nut finger tight, before fitting the tab washer and locknut...Ian

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

Hi Ian, I shim the mainshaft almost purely with focus on how the dogs of the third gear catch... Maybe that is where the packing washers are for?

I think that is so..

In the event a main shaft was used that had worn splines from previous poor assembly and they were, lets say, .010" shorter, this could have an effect on the positioning between the gear on the shaft and the selector dog as the shaft would be .010" closer to the sleeve gear than it would otherwise be (and there would be .010" more shims at the other end to achieve the .005" end float)...Ian

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

I have to say You are very knowledgeable chaps on this forum .

Lost Me from the first regarding the gearbox ..Mine appears to select fine & Im hoping this continues as I would have no clue how to tackle

issues arising !

Many Thanks to all contributors on this Fantastic website ...Im sure Im not on My Own seeming to struggle working on these machines .

email (option): s.foster11@blueyonder.co.uk

You're not alone Steve...But none of us was born with a spanner in our hand!!..:laughing: ...I'm still learning after 50 years!...Ian

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

Thank you all for the very helpful comments. As I mentioned having bench tested it after rebuild and using parts from a later box, all seemed slick, or the best it could be. Once in the frame, with the clutch and primary/rear chain on I noticed this hesitation to go into third going up and almost a bounce on the selector coming up from fourth. As you do, recheck everything, adjust what you can but still the same. Keeping pressure on the gearchange didn't help either.

So inspection cover off and check again. There seemed play within the selector on the pin at the knuckle of third. In fact considerable more than the other selector. Well spotted Ian.

Strip down in frame and the inspection saw the possible problem. At that point on the knuckle as I call it but the sharp bent in the selector the side towards the outer cover is pointed and the tip worn/broken off giving 40 thou play.

Pretty nifty with welding so using a nickel rod, dabbed a piece on that shaped up with a Dremel. All on the shaft without removing the pin etc. I Left 10 thou play.

Reassembled and smooth as silk with no hesitations. Test run with engine running on stand, again positive and smooth as silk. Up and down the drive, still great. Even better when I put more oil in! Ha

I've taken pics but can't load them. FRUSTRATING. So if any one wants to view them I can forward for someone else to load?

Great results and may help others. Thanks to all

email (option): dwmcrofts@btinternet.com

Hi Dave...Glad to hear you got there in the end..Ian:relaxed:

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