Author Topic: B31 - gearbox oil leak  (Read 936 times)

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chrisb

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B31 - gearbox oil leak
« on: 01 August, 2020, 09:50:45 »
I was just getting excited about getting the B31 (1954) finally running again, when I noticed an oil leak.  The only thing with oil in it at the moment is the gearbox, and the oil seems to be collecting on the main drive chain before dripping off (slowly).  The leak is on the left side of the bike only.  The right side is nice and dry.  When I replaced the gearbox oil, it had very little left in it, and what was there was really thick and 'green'.   I topped the gearbox up to the recommended level with engine oil when I refilled it.  I'm guessing there might be a leak from oil seal on the drive side of the gearbox?   How big a job is this to change, and what do I need to take apart to get to it?   I'm a little worried it may be beyond my basic mechanical skills.  Is there anything else it might be?   

chrisb

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Re: B31 - gearbox oil leak
« Reply #1 on: 01 August, 2020, 16:31:04 »
just another thought, can I use a thicker oil that 50W in the gearbox (an EP90 for instance) to see if that cures it?

JulianS

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Re: B31 - gearbox oil leak
« Reply #2 on: 01 August, 2020, 18:07:26 »
I would avoid EP oil, the suggestion is that the additives can cause damage to phosphor bushes.

Try a thicker oil or a non EP gear oil such as Morris AG90.

Is your bike swinging arm or plunger?

idie

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Re: B31 - gearbox oil leak
« Reply #3 on: 01 August, 2020, 19:54:50 »
Gear box oil EP 90 is the same thickness as SAE 50

chrisb

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Re: B31 - gearbox oil leak
« Reply #4 on: 01 August, 2020, 20:00:02 »
I would avoid EP oil, the suggestion is that the additives can cause damage to phosphor bushes.

Try a thicker oil or a non EP gear oil such as Morris AG90.

Is your bike swinging arm or plunger?

It's a plunger at the rears.  Many thanks,

jodie89

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Re: B31 - gearbox oil leak
« Reply #5 on: 02 August, 2020, 08:33:35 »
If you look around you can buy EP90 which is "yellow metal friendly". I have used it for many years with no problems.

JulianS

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Re: B31 - gearbox oil leak
« Reply #6 on: 02 August, 2020, 08:56:02 »

Jim S

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Re: B31 - gearbox oil leak
« Reply #7 on: 03 August, 2020, 01:32:42 »
Chrisb
Check out this previous post. Maybe you are putting in too much oil.


https://www.bsaownersclub.co.uk/bsaoc_forum/index.php?topic=8337.0

chrisb

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Re: B31 - gearbox oil leak
« Reply #8 on: 03 August, 2020, 19:21:42 »
many thanks for the link and the advice above.  I will get some AG90 and try 370 ml in the gearbox and see if that helps.  I know the gearbox is definitely the culprit since I've drained all other fluids (and cleaned the oil tank) and the chaincase cover is off at the moment too..    Can I use an even heavier oil, if such a thing exists?  (Edit: just seen they do an AG140 - might be worth a shot!)
« Last Edit: 03 August, 2020, 19:32:27 by chrisb »

chrisb

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B31 - gearbox oil leak - K400 grease?
« Reply #9 on: 04 August, 2020, 15:58:31 »
I wrote to Morris lubricants to ask about their AP90 and they said the viscosity is the same as an SAE50 (as Idie suggested).   I could try a semi-liquid grease (their K400) which has been used in other classics with problems with seals.  Has anyone any experience of this or whether it might work?  I'll try dropping the oil level of the SAE50 first and see how that goes. 

idie

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Re: B31 - gearbox oil leak
« Reply #10 on: 04 August, 2020, 18:42:42 »
You must make sure that the oil level is below the main shaft. My M33 has no seal behind the sprocket much like the WD  gearboxes. It is ok until I leave it on the side stand for to long then it leaks.

Jim S

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Re: B31 - gearbox oil leak
« Reply #11 on: 04 August, 2020, 21:01:46 »
In the link I added above, it indicates that 370-400ml of oil brings the oil level in the gearbox to about 1.0-1.2 inches below the centre line of the main shaft for the older style gearboxes with the round screw on cover on the right side.

This is very easy to check. The bike should be vertical. I use a nylon tie wrap as a dipstick, push it down vertically into the oil, press it against the clutch adjustment bolt (which is at the centre of the main shaft) with my finger tip at the centre of the bolt. Pull out the tie wrap and measure the distance from my finger tip to the oil on the tie wrap.

I use SAE50 in the gearbox.

Pete C

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Re: B31 - gearbox oil leak
« Reply #12 on: 05 August, 2020, 09:24:55 »
Do not use grease. These gearboxes are designed for oil and grease will not get into the small oilways to lubricate the bushes. You might get away with it for a while but it is not a good idea.


Pete C

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Re: B31 - gearbox oil leak
« Reply #13 on: 05 August, 2020, 09:30:47 »
leaks at that end of the gearbox are usually due to oil passing the non-sealed mainshaft bearing and oil seal (if fitted) or the mainshaft sleeve gear bush.

Do you have a plunger/rigid  or swinging arm model?

fitting a sealed bearing and ensuring the sleeve gear bush is not worn shoud fix most problems.

chrisb

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Re: B31 - gearbox oil leak
« Reply #14 on: 05 August, 2020, 20:16:09 »
Mine is the plunger version (1954).  Is it hard to fit a sealed bearing and can I do it with engine and gearbox in situ?   I have basic tools, and only basic knowledge, this being my first classic bike.