BSA Owners' Club Forum

The BSA Workshop => Singles => Topic started by: SlowDave on 27 August, 2021, 01:53:07

Title: New owner with B50mx oil tank questions
Post by: SlowDave on 27 August, 2021, 01:53:07
Hello all.  I purchased a 72 B50MX yesterday from a very knowledgeable long time BSA owner and all around nice.  I had been looking for a while as they are rare near me (Tucson Arizona USA).  We had to drive over 800 miles to make this purchase and brought the bike back in the back of a pickup truck firmly strapped down upright.

At the time of purchase the bike was started and rode a very short distance, like up and down a parking lot.  No issues then and no oil leaks. 

This afternoon we couldn't resist any longer so my helper and I fired it up and rode up and down the block twice, less than a quarter mile total fairly level gorund.  After the second trip up the block the bike was spewing oil out of the in frame tank breather.  It seemed like it started spewing once the bike stopped moving but I am not absolutely sure on that.  Anyway we shut it down immediately and found the source was the oil tank vent tube.  Everything I found online points to the in frame oil tank being over full as being the cause of the problem.  However as a former Triumph twin owner I know there is a learning curve with British bikes and I want to make sure I don't do something or ignore something that could cause much bigger problems.

Considering the sellers experince with BSAs, probably over 50 years including professional racing, I find it unlikely he over filled the oil tank.  Yes it is possible, but unlikely.  He did tell me the bike had not been ridden in over 4 years so I assume he did some prep before selling it.  So are there other causes for oil to spew out the in frame oil tank breather?  No oil is coming out of the engine breather.  Could the long journey in the truck contributed to the oil spewing?

The oil tank does not have a dip stick.  The cap appears to have a boss in the underside with with a round hole in it, not all the way through.  I am assuming this is for a dip stick.  I have a machine shop here so making a new dip stick isn't an issue but I would need to know the over all length and the distance from the bottom of the stick or from the cap boss for the full mark.  Can anyne help me with that information?

My goal is to proceed with caution as I climb the learning curve from complete ignorance.

Any help is greatly appreciated.

Title: Re: New owner with B50mx oil tank questions
Post by: berniej on 27 August, 2021, 08:01:11
Hi Dave,

One quick thought.... I'm wondering if the bike has wet-sumped while it's been stored? Oil slowly leaking from the tank, through the pump and filling the sump.
If this happens, and then the tank is topped up to compensate, then after the bike has run for a while the oil in the sump will be scavenged back to the tank resulting in it being over-full and venting through the breather.

I'd suggest draining the sump and then checking that level in the tank is correct as the first step - you may need to drain the tank some if this is the problem.


Title: Re: New owner with B50mx oil tank questions
Post by: SlowDave on 27 August, 2021, 14:23:10

I had thought about wet sumping but from what I have read (I am new at this) I thought wet sumping would cause oil to exit the engine breather and there would be excessive smoke.  The oil that was coming out of the tank breather looked brand new, surprisingly so.  It didn't look like a mix of old and new oil unless the bike had not been run since a prior oil change.  But if I am understanding you correctly, it isn't that it is now wet sumping, it is that everything down stairs worked as it should have and any excessive oil has now pumped up into the tank resulting in an overflowing situation.

This is pretty close to my original thought that it was wet sumping and that the cause of the wet sumping was not prolonged storage but the vibrations subjected to the non running engine during the over 800 mile ride in the back of the pickup truck.  But then it wasn't going out of the engine breather and no smoke.  And no oil was added after the long trip.  By the way, figuring out these little mysteries is part of my attraction to older machinery.

My gut tells me it has something to do with the long drive.  Which could have caused the wet sumping you are speaking of.  I was leaning more toward something like a spring loaded ball valve ball working itself tight in place due to the vibrations for over 12 hours to a non running engine.  So I was going over the diagrams of the lubrication system trying to imagine where something like that could happen.  I have looked through Rupert, Chiltons, and the original Owner's Handbook as well as searching the Internet and have found very little on the causes of oil exiting the tank breather and it has all been related to over filling the tank.

If it is just a simple tank overfill it seems like the easiest soultiion is to let it blow out the tank breather tube until it gets to the proper level.  However being new to this bike I want to take a do no harm approach which is why I am asking the experts here.  All bikes have their quirks and the manuals seem to overlook many of them.  I just don't want to blow the thing up if there is possibly something else going on than a simple overfill so I am asking the experts for enlightenment.

Title: Re: New owner with B50mx oil tank questions
Post by: Mike40M on 27 August, 2021, 20:55:25
If the bike had stood unused for 4 years, it has definitely wetsumped most of the oil in the tank into the crankcase. All dry sump bikes wetsumps more or less. 55 years since I started riding and competing British motorcycles. With age, I every now and then forget simple things like my wife's birthday or draining the crankcase. So it is possible that your crankcase wasn't drained and oil added by the previous owner. The short runs it had was not enough to clear the sump. Another 1/4 mile overfilled the oil tank.
A good thing is that your engine is so tight that it kept it's oil in the crankcase without going out on the floor. But check that you have correct level in the primary. Have a wetsumped Norton that blow a crankcase seal overfilling the primary.
Title: Re: New owner with B50mx oil tank questions
Post by: BEESY on 27 August, 2021, 20:56:29
Personally, I would recommend a full service and oil and filter change on a bike with unknown history. Then you know
It has the correct amount of the correct grade of oil. Plus a good clean so you can see where an oil may be coming from.

Then start it up on the stand for a good few minutes until its up to temperature and look for any signs of where leaks may be coming from.

Afraid I don't know what the breather arrangements are on the Oil In Frame models, but I can't see how a bit of rattling and jolting in a pick up truck would make much difference.

Good luck with it and let us know how you get on.
Title: Re: New owner with B50mx oil tank questions
Post by: SlowDave on 27 August, 2021, 22:15:52
Thank you for the replies.  So if I am understanding correctly it might have wet sumped and the owner added oil thus over filling it and that triggered the spew out the OIF tank.  No spew came out the engine breather and no excessive smoke.  Am I coorect in assuming the wet sumping was resolved by pumping the accumulated oil in the sump to the OIF tank triggering the OIF breather spew?

I am missing the OIF dip stick so it is a bit odf a challenge for an unexperienced person like me to determine if the tank is over OIF tank is over full.  I have read somewhere that the oil level should be about 3" below the top of the threaded opening for the cap.  Does that sound about right?

I have also seen caps with a disc on them to act as a baffle to keep oil from just splashing up and going out the OIF tank breather.  If anyone has the measurements of one of those I could whip it out in no time as we have a machine shop here.


Title: Re: New owner with B50mx oil tank questions
Post by: Mike40M on 28 August, 2021, 07:05:05
One thing you did not mention is what level you had in the oil tank after it had spewed out oil.
If nobody turns up who can measure correct dipstick size, you can get a decent picture here which may help. (
Edit: Google on the part number, you'll find some for sale.
Title: Re: New owner with B50mx oil tank questions
Post by: DAVE BRADY on 28 August, 2021, 08:23:57

Found this if it helps.

Title: Re: New owner with B50mx oil tank questions
Post by: BEESY on 28 August, 2021, 10:23:29
Instead of hunting down or making a dip stick, why not drain out all the oil that's in there and re fill with 3.4 litres (according to Google) of fresh, you'll then know you've got the correct amount in there and you'll be able to see where the correct level is.
Title: Re: New owner with B50mx oil tank questions
Post by: Mike40M on 28 August, 2021, 10:55:16
I believe Google isn't always the best source.
According to the owners manual oil capacity is 2.25 litres or i US pints 4 3/4.
Found on (
Title: Re: New owner with B50mx oil tank questions
Post by: SlowDave on 28 August, 2021, 12:17:31
Thank you all for the replies and the photo.  This morning I have to focus on getting another bike ready for a ride tomorrow but then I will make up the dip stick.  Then I will report back on the oif oil level.