BA gearbox question

Singles, twins and fours.
murray.mckenzie
Posts: 10
Joined: Wed Aug 08, 2018 11:19 am
Contact:

BA gearbox question

Postby murray.mckenzie » Fri Oct 12, 2018 5:08 am

I have just restored a 1937 VB and it is jumping out of 2nd & 3rd gear mostly on drive but occasionally on overrun. I have stripped it down and found the spring box was worn and not rotating the cam selector far enough for the detent to engage into the groove on the selector shaft. I have rebuilt the box and have partially solved that issue as it is now rock solid in 2nd but still pops out of 3rd occasionally on drive. The detent is engaging correctly in both as I can see it down through the grease filler hole.
The previous owner told me the gearbox blew up on him when a circlip failed and he rebuilt it. Judging by the bronze remnants still in the box I wonder how well it was done. When I look at how it works, if everything is selecting properly it should be virtually impossible for it to come out of gear as the selector shaft cam profile has a smooth land for the selector pin to rest against when it is properly selected and any end pressure against it caused by the dog wanting to come out of mesh should not make it rotate.

What I have noticed is that there are steel selectors in it now, as opposed to bronze ones which seem more common and were obviously originally fitted. My question is are the steel ones in fact the correct ones for the BA box? I wondered if in fact the position of the pins that go through the selector into the selector cam are fractionally different for the steel ones for some reason, such as they are not originally intended for the BA's and when the selector shaft is located in the correct position with respect to the detent the selector pin is in fact a bit off the centre of the smooth land so any end pressure allows it to force the selector shaft to rotate and hence come out of gear? It is OK in top which is obviously uses the same selector. The detent spring is still very strong and the pawl profile that engages in the selector groove appears OK as well.

john.whiting
Holder of a Golden Anorak
Holder of a Golden Anorak
Posts: 1461
Joined: Sun May 22, 2011 2:10 pm
Location: Brisbane QLD
Contact:

Re: BA gearbox question

Postby john.whiting » Fri Oct 12, 2018 6:47 am

IMHO,there is only one circlip in a BA box,being the main bearing retainer....now precisely how thats going to fail,I dont know.....but i expect to be informed.......Now when the box "blew up",what exactly happened.....obviously the PO survived ,and is now well enough ......I suspect he means he let the bushings get so loose, one or more of the gears broke teeth.....needless to say,you should check the casing for cracks,and shafts for straight.................Have a good look at the gear teeth where the dogs engage.....they must be parallel..if backed off,or tapered ,gears will jump out of mesh....I have never heard of steel,or CI selectors..but you have them....One thing to look at is endfloat in the selector shaft fit in the case.Zero endfloat would be ideal.Check all endfloats,and minimize.........But having said all that.....backed off/worn dogs are the usual cause of gears jumping out.

bevanclark
Holder of a Nylon Anorak
Holder of a Nylon Anorak
Posts: 157
Joined: Fri Jun 08, 2018 3:58 am
Location: Methven, New Zealand
Contact:

Re: BA gearbox question

Postby bevanclark » Fri Oct 12, 2018 7:29 am

I regret that this may not be that helpful, but I had very similar symptoms on my Panther BAP gearbox, which I think has very similar internals. My box had multiple issues, so I never definitively confirmed a specific fault, but it had: worn selector forks, plus the main drive gear bush had migrated into the box a bit which, I think, was preventing full engagement of the sliding gear, thus it popped out in high gear from time to time. Since I replaced the forks and all bushes and bearings in one hit, I cannot be sure what the problem actually was - I just wanted to get the damn thing working properly !
As John said (who clearly has way, way, more experience than me), endfloat may also be the issue.
Cheers,
Bevan

murray.mckenzie
Posts: 10
Joined: Wed Aug 08, 2018 11:19 am
Contact:

Re: BA gearbox question

Postby murray.mckenzie » Fri Oct 12, 2018 10:01 am

He didn't go into a lot of detail as to exactly what happened apart from saying that a circlip had failed. He gave me the bronze selectors or what was left of them. They were totally chewed to bits. I note that the outer cover over the clutch has a scuff mark in it so I assume the whole mainshaft drifted that way somehow, but to do that the shaft must have come loose at the kickstart ratchet end as well so who knows. I have gone over all the casings and checked all the gears for cracks and run the mainshaft and layshaft in the lathe and they are true. I did wonder about the endfloat and came to the conclusion that ideally it should be pretty minimal. That said the gasket between the main gearbox body and the first case which essentially closes the gear bit of the box should be of minimal thickness. I did fit an 0.5mm gasket here but taking it out completely I can just detect slight endfloat so gasket sealant only maybe??? I am a bit concerned about your never having heard of the steel selectors and do wonder where they came from and whether I should look to find some bronze ones. The dogs aren't wonderful and I have managed to find a better 3rd gear. Would you think the flat in the selector cam profile was designed to hold the selectors stable? Obviously to see this the selector pins need to come out.
Thanks for your help John and Bevan. Regards Murray

john.whiting
Holder of a Golden Anorak
Holder of a Golden Anorak
Posts: 1461
Joined: Sun May 22, 2011 2:10 pm
Location: Brisbane QLD
Contact:

Re: BA gearbox question

Postby john.whiting » Fri Oct 12, 2018 10:37 am

The nut coming loose on the K/S end of the mainshaft might cause problems,but the BA and CP boxes,the gears cant migrate sideways into mesh ,because they are blocked by larger gears on the layshaft.Even if the sprocket nut comes off,the main gear cant come into the box far enough to cause a lockup......Wreckage of the selector forks in any box,bike ,car, truck or tractor is caused by holding the gears in mesh ...In cars and trucks,just resting your hand on the gearknob is enough to damage selectors and gear grooves......The engaging dogs must be straight /parallel,and in full engagement.Its no good pairing a new gear with a backed off one...you just damage the new gear....We used to backgrind car gears with fine splines,but I think Burman splines are just a bit too fine.

john.whiting
Holder of a Golden Anorak
Holder of a Golden Anorak
Posts: 1461
Joined: Sun May 22, 2011 2:10 pm
Location: Brisbane QLD
Contact:

Re: BA gearbox question

Postby john.whiting » Fri Oct 12, 2018 10:46 am

I would be very surprised if your selectors were steel,more likely malleable cast iron......unless they have clearly been welded up from bits.....They should cause no problem,provided the lube is kept up....Plenty of big trucks have iron or steel selector forks without problems....As I mentioned,holding gears in when they want to jump out,wrecks selectors and gear grooves very quickly......incidentally,if the selectors were wrecked,the gear grooves would be very worn and overwidth......if not,then they dont belong to the worn selectors....and the bits have already been replaced.

User avatar
alan.moore
Holder of a Golden Anorak
Holder of a Golden Anorak
Posts: 1209
Joined: Sat Jul 19, 2014 1:29 pm
Location: Leicestershire UK
Contact:

Re: BA gearbox question

Postby alan.moore » Fri Oct 12, 2018 2:26 pm

Murray,
As others have said I have only seen bronze selector forks on the BA. These have had casting marks on them 43 BA and 42 BA (which are the part numbers not the order part number if you know what I mean)

A you have explained the dowels in the selectors and the grooves in the cam provide the means of moving the selectors sideways to move the appropriate gears, they do not play any part in keeping the selector shaft or selectors in the correct position for any gear. This is done by the detent groves machined at the end of the cam part of the shaft into which the pawl locates. Once the shaft has been rotated by the gearlever and the pawl has fully located in the relevant groove (under pressure from its spring) there should be no way thatthe selector can move sideways.

However, if there were any wear in the dowel pins or cam groove this could allow the selector fork to move sideways, although if this was excessive you would expect it to effect all gears. Same goes for excessive end float of the camshaft.

I think I would look at those selector forks. There may be something in this previous post that may help.

viewtopic.php?f=4&t=6282

Cheers
Alan
Last edited by alan.moore on Fri Oct 12, 2018 5:09 pm, edited 1 time in total.
1939 VH Redhunter;1942 RN WNG;1951 Triumph 6T Thunderbird;1970 BSA B175 Bantam;1986 Yamaha SRX600 single
http://cloggymoore.wix.com/triumph-pre-unit-6t

Pete.Silson
Holder of a Nylon Anorak
Holder of a Nylon Anorak
Posts: 113
Joined: Fri Sep 16, 2016 11:31 am
Location: Wanborough, Wiltshire, UK
Contact:

Re: BA gearbox question

Postby Pete.Silson » Fri Oct 12, 2018 3:15 pm

selector.jpg

The selector forks in my 1939 4G BA (above) appear to be cast iron/steel. The rest of the box appears to be correct for gearbox but that doesn't guarantee the selectors are original. They don't appear to have any casting marks like the later bronze ones do.

Pete

User avatar
paul.jameson
Holder of a Golden Anorak
Holder of a Golden Anorak
Posts: 1835
Joined: Wed Oct 12, 2011 3:04 pm
Location: Herefordshire
Contact:

Re: BA gearbox question

Postby paul.jameson » Fri Oct 12, 2018 8:12 pm

Hells teeth. I have never heard of iron selectors for a BA box since all the ones I have seen - including those in Club Gearbox spares - are bronze / brass / whatever specific alloy was used. But I know that Pete Silson's bike is one of the most original you could ever find so iron selectors were undoubtedly used. That being the case, the issue can only be down to end float. I no longer use gaskets in my gearbox rebuilds so as to minimise endfloat but I was caught out recently by a GB box which clearly had not seen the use to develop the end float that most other boxes now have.
Paul Jameson
36 4G, 37 VH, 53 ex ISDT KHA (project), 54 KH(A), Healey 1000/4 (project)
Former Machine Registrar & Archivist, General Secretary and Single Spares Organiser (over a 25 year period).
Now Archivist once more - but not Machine Registrar.

john.whiting
Holder of a Golden Anorak
Holder of a Golden Anorak
Posts: 1461
Joined: Sun May 22, 2011 2:10 pm
Location: Brisbane QLD
Contact:

Re: BA gearbox question

Postby john.whiting » Fri Oct 12, 2018 11:28 pm

Every other bike gearbox has malleable cast iron forks,Burmans were the odd ones ,and it appears they were trying out iron.....if the forks are loose in the grooves,the play is easily eliminated by a spot of bronze on the ends of the forks......You might be interested to know some truck boxes have plastic slip on pieces on the ends of forks,and other used a liquid plastic,like epoxy on the forks.


Return to “Four stroke”

Who is online

Users browsing this forum: No registered users and 14 guests