Hello, I own a BSA B31 1955, I have made a complete upgrade of the engine but after i placed a brand new set of valves with guides i ended up having oil leak , any problem solving advices? Thank you!
Where's the oil leak and how does it relate to the new valve guides?
The oil leak is through the exhaust valve, having oil going downwards through the valve and then through the guide
I assume that the correct clearances were achieved when the valves and guides were replaced?
I'm not particularly familiar with the B31 engine but, like the unit single engines, it must also rely on: 1. not too much oil finding its way on to the end of the valve stems; 2. small enough clearances between the valve stems and guides; 3. adequate drainage paths for the oil to return via the push rod tunnels back to the sump.
In the case of the B31 engine, I'm not sure what the drainage paths are between the valve stem pockets and the push rod tunnels. Whatever they are, I would make sure they aren't blocked.
Could be oil coming from combustion chamber to exhaust valve, due to wear in bore or poor ring bedding in or excess oil in sump.
Could be poor guide to head fit or scored guide or scored guide hole in head.
Sometimes new guides and valves have too much clearance due to poor manufacuring tolerences.
Was the problem there before the new parts were fitted?
I believe it's not the fault of valve stems and guides, drainage paths we've also checked, no problem there, same with the clearance.
Motorbike hasn't been used for 40 years. We've changed all the parts of the engine, all brand new.
Andreas. When you say you have an oil leak through the exhaust valve, could you explain what you mean and how it manifests itself? For example, can see oil leaking down past the valve guide with the downpipe removed, or do you mean that the exhaust is blowing blue smoke which you assume is due to oil leaking past the valve guide?
Suggest check combustion chamber for oil. If bike has new bore/rings/piston it will need time form them to properly bed in.
There is enormous leakage of oil and lots of smoke from the exhaust. When I remove the exaust I actually see oil leaking from the valve.
And, yes, it has new bore, new rings, new valve and new piston.
Are you sure the oil is leaking between the valve stem and the guide, rather than between the guide and the head. Either way, there shouldn't be that much oil present to leak past the valve unless something strange is happening with the oil supply to the head or to its drain path. On unit singles which I'm more familiar with, if the return to the oil tank is blocked or partially blocked then, in principle, you could get an enormous flow of oil into the rocker box which might well result in oil being forced past the valves.
I will again check the valves and guides, drainage path, and oil supply to the head. hope to find the problem ..
As said, I'm not that familiar with the B31 engine but from what I can see the rocker box drain is via an external pipe rather via the push rod tunnel? If that is correct, I would definitely check that pathway.
In terms of the oil flow rate into the head, I would have thought that provided that the flow rate back into the oil tank appears to be good then the former should also be ok from a max flow rate point of view. To check the drain, I would be tempted to fill the exhaust valve pocket with engine oil and check it drains away in a reasonable manner.
I have a similar problem with my 1949 B33 and I think I found the root cause. I have found that the seat for the exhaust valve spring is blocking the drain hole.
To address this I have reamed a champher on the hole and I will install a thin 0.75in. washer under the seat. This will allow oil to flow over and under the seat for better access to the drain