Author Topic: BSA C12 Gearbox identification  (Read 305 times)

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Offline Fritzele

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BSA C12 Gearbox identification
« on: February 27, 2019, 07:35:32 am »
Hi everyone,

My name is Fred, I'm 34, I come from France and this is my first post on this forum.

Few months ago, I bought a BSA C12 bike wich has been modified as a classic trial bike.

I manage to restore it but I now face a problem. The gearbox leak oil because the sprocket and oil seal have been damaged by a chain derailment.

So, I looked at the BSA C10-C11-C12 Spare List and ordererd new parts for BSA C12 4-speed gearbox whitout really thinking about it.

I got the parts yesterday and realised the parts did'nt fit at all (the diameter of the axis is way bigger and the sprocket is 19T when the one on my bike is 16T).

Visually, it is indeed a 4 speed lightweight gearbox, so I looked closely the reference of the gearbox to see if it's really an original C12 gearbox.

There is two references on the shells : 29-3572 and 29-3574. The problem is that my gearbox is obviously a 4-speed lightweight gearbox but these reference don't appear at all on the spare parts list for BSA C Model.

I found some photos of the same gearbox with the same references on it on google ( http://www.vintage-motorcycle.com/imageviewer.php?pid=21945 ) but I'm unable to clearly identify the gearbox type and especially the right reference for the sprocket and oil seal.

I have picture of the parts and their dimensions if it can help.

Thank you for your help.

Fred

PS : sorry if my english isn't very good

Offline John M

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Re: BSA C12 Gearbox identification
« Reply #1 on: February 27, 2019, 12:00:43 pm »
Hi Fred,

The gearbox casing numbers are correct. They are listed in the parts list as being complete with fixed fittings hence slightly different numbers - 29-3571 & 29-3573.

Parts list here: https://bsac10c11c12.co.uk/smf/gearbox-parts-lists/gearbox-c10l-c12-(light-4-speed)/

Not sure what you mean by the diameter of the axis .... can you explain or, probably better, post pictures ?

The 19tooth sprocket is standard for the C12 but you say that yours has been modified to a trials bike hence the reduction to 16 tooth. Perhaps the shaft has been modified to suit ?

Offline Fritzele

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Re: BSA C12 Gearbox identification
« Reply #2 on: February 27, 2019, 01:20:53 pm »
Thank you John.

I now understand the difference in numbering for the shells.

What I meant by "the diameter of the axis" is that the center of the sprocket is larger than the gearbox mainshaft (n°29-3612).

I don't know if the shaft have been changed but the new oil seal is also to large and can't even be insered in the shell. 

Here is a photo with the old and new parts.

I ordererd parts n°29-3608 (oil seal) and 29-3609 (19t sprocket) but maybe I didn't received the right parts. I just would be sure, it was really hard to find these parts.

Thank you


Offline John M

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Re: BSA C12 Gearbox identification
« Reply #3 on: February 27, 2019, 04:14:42 pm »
Your old sprocket looks to be the correct bore ( I've just measured an old shaft ). Haven't got access to an old seal - someone else maybe able to check those measurements - but my guess is that the old parts are correct standard dimensions.

Draganfly look to have the seals in stock https://www.draganfly.co.uk/index.php/accessories-a-misc/product/5195-  Perhaps contact them and ask for dimensions ?

Sorry can't help any further, Fred.
« Last Edit: February 27, 2019, 04:22:13 pm by John M »

Offline Fritzele

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Re: BSA C12 Gearbox identification
« Reply #4 on: February 27, 2019, 05:22:54 pm »
It appears that the seller could have made a mistake.

I didn't think about that at first but I sent him an email to clarify.

I will also contact Draganfly. But the problem is that they have the seal in stock but not the sprocket. The dealer where I got the sprocket (GLM in Germany) was the only dealer to have this part in stock.

Does someone else could confirm these measurements ?


Offline ianinglis

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Re: BSA C12 Gearbox identification
« Reply #5 on: February 27, 2019, 07:06:00 pm »
as far as i am aware the C12 did not have the "lightweight" 4 speed fitted - but i have been known to be wrong on the odd occasion ::)
it should be a "heavyweight" 4 speed



ian
BSA C10L 1956 - JFO 588
BSA C10L 1954 - MVE 107
RM6 RALEIGH RUNABOUT
ENFIELD DIESEL KUBOTA OC 95
BSA M21 RIGID RKN 816
CBF 1000
BSA ARIEL 3

Offline Fritzele

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Re: BSA C12 Gearbox identification
« Reply #6 on: February 27, 2019, 07:14:12 pm »
Hi Ian,

I think my C12 has a 4 speed lightwheit like every C12, if these informations are right :
- https://bsac10c11c12.co.uk/smf/technical/c10-gearbox-innershaft/15/
- https://bsac10c11c12.co.uk/smf/gearbox-parts-lists/gearbox-c10l-c12-(light-4-speed)/

Offline cornish pixie

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Re: BSA C12 Gearbox identification
« Reply #7 on: February 27, 2019, 07:14:46 pm »
Evening all

I have just measured my sprocket from my spare heavyweight C11g box, and it is the same as the new parts on the left of this picture.
You have been sold C11g parts instead of lightweight C12 parts.
I thought all C12s had the lightweight box.

Regards Martin.

here in sunny cornwall

Offline ianinglis

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Re: BSA C12 Gearbox identification
« Reply #8 on: February 27, 2019, 07:41:42 pm »
as i said - i have been wrong on the odd occasion :D



ian
BSA C10L 1956 - JFO 588
BSA C10L 1954 - MVE 107
RM6 RALEIGH RUNABOUT
ENFIELD DIESEL KUBOTA OC 95
BSA M21 RIGID RKN 816
CBF 1000
BSA ARIEL 3

Offline Paul S

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Re: BSA C12 Gearbox identification
« Reply #9 on: February 27, 2019, 08:28:55 pm »
All C12s should have the 4 speed lightweight gearbox, easily identifiable as it is the only C gearbox with an internal clutch operating lever. 
Growing old is compulsory, but growing up is optional.
1955 C12
1961 Greeves Scottish
1998 Kawasaki zzr600
1967 Triumph Vitesse 2 ltr convertible
1959 BSA C1000

Offline chaterlea25

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Re: BSA C12 Gearbox identification
« Reply #10 on: February 27, 2019, 09:08:42 pm »
Hi Fred and All,
I agree you got sent parts for a C11G gearbox, This uses the same seal as the heavy weight A7-10 and B31-33
so are widely available
C12 sprockets are hard to find in any size  :(
The 16 tooth sprocket you have has been modified, to keep your present gearing (or other gearing???)
you will need to machine the old one to remove the toothed disc
Then buy a new blank sprocket and have the centre bored out to fit the old sleeve, and have them welded together

John

Offline Fritzele

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Re: BSA C12 Gearbox identification
« Reply #11 on: February 27, 2019, 09:52:57 pm »
Thank you all for your answers.

I don't have any information about the 16T sprocket that is actually on this bike, but it don't seems that it have been modified.

I can't machine the old 16T sprocket because it is out of service. If you look closely the picture I sent, you can see the the chain has derailed and damaged the surface of the sprocket wich is in contact with the seal and the seal don't retain the oil anymore.

Maybe the solution is to buy a 17T C10L sprocket for lightweigt 4 speed gearbox, reference 29-3610, but it seems that the chain size is 1/2 x 3/16 while it is 1/2 x 5/16 for the C12 sprocket. Is it really a problem ?


Offline chaterlea25

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Re: BSA C12 Gearbox identification
« Reply #12 on: February 28, 2019, 12:25:33 am »
Hi Fred,
I think the wider chain on the narrow sprocket could lead to the chain drifting over and damaging either the primary case or gearbox ????
An alternative would be to modify the rear sprocket to narrow it ??

Some years ago a friend ran a tiger cub as everyday transport, doing a fairly high mileage,
Standard chains did not last at all
I found a chain to fit the narrow sprockets that was intended for a junior moto cross bike, this lasted very well
the label for it is still stuck on the wall out in the shed
Hopefully I will think of noting it and replying tomorrow

John

Offline ianinglis

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Re: BSA C12 Gearbox identification
« Reply #13 on: February 28, 2019, 07:14:32 pm »
you could machine all of the sprocket off and just use the boss part from a C10L sprocket??
then weld on a new correct width one


ian
BSA C10L 1956 - JFO 588
BSA C10L 1954 - MVE 107
RM6 RALEIGH RUNABOUT
ENFIELD DIESEL KUBOTA OC 95
BSA M21 RIGID RKN 816
CBF 1000
BSA ARIEL 3

Offline Fritzele

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Re: BSA C12 Gearbox identification
« Reply #14 on: February 28, 2019, 08:10:46 pm »
I don't have the skills to machine a mixed sprocket and I don't really want to buy two sprockets to make a ugly one.

Maybe I shoul just give my original 16T sprocket to a professionnal and ask an all new machined one with 19T.

I have no idee how much it could cost.