B31 'snatching' in gear

[Calum]

Hi all

I've been out today on the B31, bit of a poor start as the nipple pulled off the clutch cable as I pulled out of the filling station! Left the girlfriend with and ice cream and the bike and rode her Ariel home to solder it back on!
Back on the move again I began to notice my B31 'snatching' in third gear (it has done it on previous rides, so not a new phenomenon!). If I keep the gearchange pedal held whilst releasing the clutch this lessens the likelihood of it happening (good practice anyway?) but doesn't rule it out altogether. It's a plunger model, so has an M series box (I think??) with screw filler/clutch rod cover.
Sometimes letting the clutch out I feel a 'kick' as it pulls into mesh, but it kicks/snatches a bit too sometimes, mainly on the overrun.

My suspicion is the cam that moves the dog clutch for third gear is worn on the 'peak' so not letting the dogs engage fully, and the snatching feeling is the resultant dropping out and back in mesh. Should I be able to see if this is indeed the problem through the cover plate at the rear of the gearbox? Looks a bit tight for clear vision so judicious use of a mirror or endoscope might be needed...

Any pointers gratefully received!

Cheers
Calum

[Jim S]

Hi Calum
I'm not sure you will be able to see much through the inspection cover. To really determine what is going on you will probably need to disassemble the gearbox. The gearbox doesn't need to be removed from the frame, but the clutch centre needs to be removed which is a complete disassembly of the clutch.

I suspect that you have a worn 3rd/4th dogged wheel and shifter fork. Other people in this forum (including me) have had a similar issue. Do a search on b33 gearbox for more information.

[Ian C]

Not that it helps Callum, but my WM20 is now starting to do the very same with 3rd gear. I need to remove the box and inspect too.

[Calum]

Well I'm glad I'm not alone!

Worn fork and/or dog clutch sounds likely. Sounds as if it is a common problem then? I'm not adverse to taking the box out and sorting it. The bike is new to me (got it last year) so often I find any excuse to take something apart very welcome as it slowly removes any unknowns or hiding horrors which may try and surprise me later on down the line... although I do want to be out riding it whilst the weather is nice!

[Ian C]

Well I'm glad I'm not alone!

Worn fork and/or dog clutch sounds likely. Sounds as if it is a common problem then? I'm not adverse to taking the box out and sorting it. The bike is new to me (got it last year) so often I find any excuse to take something apart very welcome as it slowly removes any unknowns or hiding horrors which may try and surprise me later on down the line... although I do want to be out riding it whilst the weather is nice!

I've heard of excess layshaft endfloat or that the 'peak' on the selector fork slot (that the selector shaft pin runs in) is worn.. Anyones guess until inspected I suppose.. I'd better get the fettling irons out of hibernation!! 

[Pete C]

Hi Calum

which gearbox do you have?  Swinging arm or rigid/plunger?
I have a file with some 30+pages of info on setting up for the rigid/plunger variety which I can send you if you want.

cheers
Pete

[Calum]

Hi Pete

It's a plunger (1953), so has the M-series box (unless I am mistaken) with screw in level plug/clutch adjuster cover.
I have a set of service sheets but any info is always welcome!

I've heard of excess layshaft endfloat or that the 'peak' on the selector fork slot (that the selector shaft pin runs in) is worn.. Anyones guess until inspected I suppose.. I'd better get the fettling irons out of hibernation!! 

Yes those were my initial thoughts, but I called the peg and groove a cam. As you say though pure speculation without stripping it down!

Cheers
Calum

[Pete C]

Hi
no idea how to send you the info except by  email,
mail me at petercomley@web.de and I'll send you the file.

I'm working on one of these boxes at the moment and getting the end float on the various shafts is critical for good gear selection. The BSA service sheets don't go into much detail unfortunately.

cheers
Pete

[Calum]

Hi Pete

Email sent. I can see how excess endfloat would cause selection problems, but I would expect this to affect selection of all the gears? I don't have any problem with any of the other gears, just third.

[Calum]

I've been super busy with work so only just getting round to pulling the box out. I had the cover off and had a peek inside. Looks nice in there in general, but the problem seems obvious. There is a large amount of end float on the gear selector shaft which is causing the 3rd gear dogs (much more than any other for some reason) to move out of mesh. The forks in the grooves look okay, and the pegs in the forks don't seem too bad either to be honest. I'm sure I'll find out more when I strip it fully.

[Rog1]

I have the same gearbox and sometimes it displays the same third gear snatching symptoms. There's another symptom; sometimes it will not go into first at the lights, which can be a bit embarrassing! I'm following this thread with interest.

[Calum]

I stripped the gearbox this evening. Generally good condition, with the usual wear on the pinion bushes (I think I will be rebushing them, and the pinion sleeve too). All three shafts mic up correctly and there's no obvious damage to them.
The 3rd/4th selector dog is worn/galled in the groove where the fork runs, and the fork is worn too (around 15-20 thou of wear on the fork at the worst point) The peg in the track of the selector fork is good.
The end float of the gear control shaft I measured at 60 thou with a DTI once I had removed the outer cover.
The end float on the layshaft was excessive too, but I have not measured it (the mention of drilling a hole in the outer bush to measure end float is a good idea, I may well do that)

Someone has definitely been in there before me though. The oil flinger washer which should go between the 1st speed mainshaft fixed pinion and the spacer was found on the outside of the mainshaft bearing, between it and the kickstart ratchet mechanism. It has heavily dished and rubbed against the circlip/snap ring which holds the bearing in the inner cover. As a result the mainshaft has been sitting too far towards the right (by the thickness of said flinger washer) and as such the 3rd speed pinion on the mainshaft was even further from the selector dog. Coupled with the wear on the fork and the excess endfloat on the control shaft it meant that the selector dogs could slide out of mesh as found on my initial peek through the inspection window!

I'm currently waiting on some spares to be delivered, and fingers crossed the selector forks and dogs in the spare box will be a little better than mine. Even if they are not, I think correctly shimming the control shaft, and correctly assembling the mainshaft will solve the problems I have been having.

[Jim S]

Hi Calum
Looks like you have found the problem. When you do the rebuild, make sure the shifter forks are not contacting the dogged wheel after any gear or neutral is selected. You may need to do several iterations of partial rebuild.
As for gear selector shaft end float, I rebuilt mine with 0.015in of end float and shifting is very good.

[Calum]

Thanks for the heads up. 15 thou is certainly a lot less than what I have now! The 3rd/4th fork really had been rubbing before, potentially due to the mainshaft being too far over to one side. There are also witness marks where the final drive chain has fouled on the back of the layshaft bush where kt pokes through the casting from being too far to the right (even if it was only by 35 thou or so)

My spare gearbox bits arrived today and have provided good forks and selector dogs to build back into my box amongst other bits. I'll get an order in for new bushes and locktabs etc, but given the price I will make new layshaft bushes myself (current clearance in those is around 6 thou which isn't horrendous but whilst I am in there I might as well do them).

I also noticed I have two 3rd gear constant gears fitted rather than a 2nd and a 3rd (ie the 2nd speed constant gear on the layshaft has oil holes drilled through when it doesn't need to have as the bush is fed via the cross drilled layshaft). Can't see it being a problem to be honest but I've got a plain 2nd gear I will fit instead.

[Calum]

So I found the time this afternoon to get the new bushes in the gears and pinion sleeve, ream them out and sort out the end float on the shafts. Very happy with the selection now, although in neutral the 2nd gear dogs were closer together than I would like. This was assembled just with the gear control shafts and detent in place - no ratchet or change pedal mechanism fitted yet. I took it apart again and measured the height of the dog teeth from the fork grooves - a selective fit with the 'shortest' dog teeth adjacent to 2nd gear seemed to gain me around 20 odd thou extra clearance which made things a lot more comfortable, yet still allowed full engagement in all gears.
Tomorrow I'll do a final assembly and hopefully get it refitted into the bike. The primary chaincase is being painted whilst it is off also so it'll be a week or so before that's ready to go back on (especially as I am away this coming week).

Hopefully it'll stop raining by the time it's back together!