Author Topic: B31 - gearbox oil leak  (Read 937 times)

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JulianS

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Re: B31 - gearbox oil leak
« Reply #15 on: 05 August, 2020, 21:39:34 »
Regret you need to strip the box to change the mainshaft/sleeve gear bearing.

Dennis49

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Re: B31 - gearbox oil leak
« Reply #16 on: 06 August, 2020, 08:38:41 »
I recently stripped and rebuilt my B31 gearbox due to selection problems. It is not a job for the fainthearted. Luckily mine was fitted with a sealed bearing.
The oil leak is due to overfilling the gearbox and as it's not easy to haul a B31 onto its main stand it runs down the bush if left on the side stand.
My suggestion would be to use a block of wood. Roll the back wheel onto the wood then put the centre stand down and pull the bike onto the stand. Alternatively put the wood under the side stand, ensuring it's safe and won't topple over.

Pete C

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Re: B31 - gearbox oil leak
« Reply #17 on: 06 August, 2020, 10:38:18 »
It is a BSA, a few oil drips are part of the charm.... Don't overfill it and use the mainstand rather than sidestand.  -  and I carry a bit of cardboard to place underneath if I am parking somewhere that might not appreciate a spot of oil.

A stripdown of the gearbox can be done in-situ but the better option, and easier in the long run, is to take the box out. If you do decide on that route let me know and I can send you a file of all the info I have collected on working on these gearboxes, instructions, tolerances etc.

chrisb

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Re: B31 - gearbox oil leak
« Reply #18 on: 06 August, 2020, 20:32:44 »
Many thanks Pete C for your kind offer.  I will take you up on it if I decide to bite the bullet and take the 'box out.

How much is an 'acceptable' leak for a BSA from that seal when standing cold (and not been run)?  I'm going to measure the amount coming out over 24 hours.....

I've refilled to 350ml and still some came out last night and left a puddle on the cardboard again!

 

Pete C

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Re: B31 - gearbox oil leak
« Reply #19 on: 07 August, 2020, 08:35:42 »
If it is on the mainstand then the leak should be minimal. The problem is often when on the side stand the oil all heads toward the main bearing and freedom.

I don't know where the oil level is inside the casings when the correct amount is filled into it but I suspect it would be just below the bearing (which is situated towards the top of the casing) otherwise oil passing the bearing would be an inbuilt problem.

I would tend towards using the mainstand and when it stops dripping check that you can see oil from the filler cap. If there is plenty of oil still visible  in the box then it is probably enough for lubrication purposes.

chrisb

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Re: B31 - gearbox oil leak
« Reply #20 on: 07 August, 2020, 17:47:05 »
Drained everything out and put about 320 ml in the gearbox a couple nights ago and hey presto, no leaking! (on centre stand).  With a torch, I can see oil in the box looking down from the filler cap.  I think stripping a gearbox is beyond my abilities at present (not even confident I could get the 'box out), so will reassemble the bike and see how much oil comes out when it is running.   My worry is oil leaking and coating the back wheel but I guess there will be tell tale signs of that when I run the bike for the first time.  Thanks everyone for your advice. 
« Last Edit: 07 August, 2020, 17:50:13 by chrisb »

Jim S

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Re: B31 - gearbox oil leak
« Reply #21 on: 07 August, 2020, 21:51:49 »
With 320ml in the gearbox, it would be interesting to know what the measurement is between the centre of the main shaft down to the oil level, and compare this with the radius of the bearing.

Pete C

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Re: B31 - gearbox oil leak
« Reply #22 on: 08 August, 2020, 10:20:49 »
I thought the oil capacity of the gearbox was 1 pint or 570ml.

In any case I have always assumed that, unless you are stripping the box, there is always some residual left after draining so adding a full pint would probably overfill.

I have always just added oil until it almost reaches the level of the filler cap and not had any problems. Turn over the box a few times while filling to get the oil flowing from the outer cover to the main part of the box - or wait . I have just done a quick measurement on a gearbox, on the shelf, and I reckon that the outer race of the main bearing is about a centimeter (at most) below the level of the filler plug.
I have fitted sealed bearings to my gearboxes so it isn't so critical for me.


martinh10

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Re: B31 - gearbox oil leak
« Reply #23 on: 05 October, 2020, 18:08:44 »
I had a similar problem on a plunger B31 I purchased. I established that the oil was coming from between the g/box mainshaft and the pinion sleeve (which carries the g/box sprocket). I eventually resorted to removing the primary chaincase and sliding a 20mm I/D x 2.5mm Viton 'O'Ring onto the mainshaft into the annular vee at the end of the pinion sleeve. The cork and felt washers between the primary chaincase and gearbox firmly press the 'O' ring against the end of the pinion sleeve. I've done a few hundred miles since with no sign of a leak.