A65 Thunderbolt - serious engine oil leak behind the primary drive

[Dan Field]

Hi,
I hope someone can help, I'm helping my friend Kerry - who's been on here talking about his A65 - which he has had running, but sadly it has developed a serious oil leak, this is only apparent when the engine is running and would be enough to empty the oil tank in a few miles. While it is leaking (or rather pumping oil out) there is still a return to the tank. The oil being leaked is definitely engine oil and continues even with an empty primary (the cover is off) so its not coming from the breather/chain oiler.

It's hard to see but the leak appears to be coming from in front of the gearbox sprocket, I'm more of a Norton man (sorry!) so dont know what could be allowing oil to leak so quickly from that side of the engine - any suggestions would be gratefully received?

I should add that the bike wet sumps badly and oil is dripping through from the crank shaft oil seal, but I'm not sure if that is connected - is there a hidden breather behind the clutch that oil would be pumped out of if the sump fills with oil? .

Cheers
Dan

[Dan Field]

Ok Update, Ive just drained 2l of oil out of the sump!!  so I guess it is connected!  but is running the engine with that much oil in the sump likely to have blown out an oil seal and would it leak from somewhere in front of the gearbox sprocket? I would have expected it to come out of a breather somewhere.
Dan

[JulianS]

Breather outlet here. All that oil in the sump will blow straight out of the breather.

[DAVE BRADY]

Hi,

This could be a symptom of a displaced return pipe in the sump.  If this pipe comes out of its location the oil level in the sump will have to reise lot higher for the pump to be able to return it to the tank.
The extra oil gets flung about inside the cases and will exit through the breather as pointed out by Julian.
Did you notice if it was out of place when you drained the sump or was this through a drain plug rather than taking the sump plate off?

Dave.

[Dan Field]

Thanks for the replies - the location of the breather is where the oil came from, so that would be it!  I drained the sump from a new sump plate with magnetic plug, can you see the return pipe through the plate? Although after a few minutes running there was only a couple of egg cups full of oil in the sump, I'm guessing thats pretty normal.

But As there was some oil coming from the crank into the chaincase we're going to change the crank oil seal, I presume its fairly straightforward ? Many thanks for the help so far. I should add that an oil pump inspection is probably on the cards.
Dan

[DAVE BRADY]

Hi,

It is unlikely that you would see the pipe through the drain plug hole so you will probably need to take the sump plate off to clarify.

Dave.

[Dan Field]

Hi Dave
Thanks, thats not difficult, but what should I see? Is there a diagram/photo anywhere?
Cheers
Dan

[DAVE BRADY]

Hi,

I would suggest getting a workshop manual and a parts book to help to get to know the bike.
If you are a BSAOC Member you will probably find them in the 'Members' section of the BSAOC website.

Image attached shows the sump plate removed and the 'oil pump scavenge valve' can be seen.  I call this 'popeye's pipe' and if it has come out you will see why.

Dave.

[JulianS]

An empty 1971 crankcase showing oil pick up.

[Dan Field]

Thanks Julian
That's really clear, I'll have a look tomorrow.
Dan

[Dan Field]

Ok Ive just found some time to look at my mates bike, it has wet sumped as expected but this has shown a leak from the crank into the chain case, however it seems to be coming from between the crank and the "sleeve" rather than the crank seal. How is this supposed to be sealed?
Many thanks
Dan

https://i.ibb.co/WvrgY2v/IMG-0770.jpg

Ps ill post a pic if i can work out how!

[DAVE BRADY]

Good morning Dan,

The sleeve/crank has no specific seal but as the sprocket and stator are fitted and the nut done up to 60lb/ft there should be no space for oil to get through.
Was there an excessive amount of oil in the primary case or could this oil just be residual from draining the primary case?

Dave.

[Dan Field]

Morning Dave!
The initial problem was wet sumping, but when we took the chaincase off it was clearly half full of engine oil, so we drained the sump put the oil back in the oil tank and ran the engine with the cover off, we could see oil coming from behind the engine sprocket (at this point there was probably too much oil in the crank case which wouldn't have helped) - Anyway we parked the bike in my garage while Kerry ordered a crank seal - as that seemed the likely culprit - that was a couple of weeks ago, the bike has wet sumped again and the sump is now nicely full of engine oil! which has revealed this leak, but if I'm understanding you correctly this may be a red herring and is normal if the rotor isn't on the shaft properly torqued down. So I guess we'll change the crank seal and see whether that cures it, unless anyone else has any bright ideas?!
Cheers
Dan

[DAVE BRADY]

Hi,

Fit the new seal as you have it to hand and see how it goes once reassembled.
It may be worth tying to cure the wet sumping. This will likely involve removing the timing side and the oil pump to replace the ball and spring behind the pump.

Dave.