750 engine 1972 - Breather/oil problems

Not open for further replies.
Feb 25, 2007
I have a 1972 commando, which has a problem returning oil to the tank.

On my 1971 commando the breather pipe is to the left high up on the crankcase but on the 1972 engine it was moved to the rear and placed lower down. I believe it was corrected on the 1973 850 engines and is only a problem on the 750cc 1972/3 engines

It is causing the oil to return through the breather pipe to the oil tank and only spits out through the return pipe. With the pressure in the crancase air is forced down into the oil moving it upwards and exiting through the breather so hardly any oil is left to be scavanged from the crankcase throught he return pipe.

This explains why the head gasket was about to blow and oil is trying to exit where it can from the engine.

The engine is in bits due to the prevoius owner bodging a repair to the rear crankcase flanges which have been welded and machined finished as it should be. It has had a new oil pump, which was needed as the old one was worn out though age.

I have searched though the forum and found an answer to solve the problem, so thanks to who put up the link.

This is the link http://www.oldbritts.com/ob_start.html

Look under 'Technical articles' - Then look under engines/oil pressure loss norton 750cc
Pete......my sympathies. I endured the same problem for 3 years on my 72 Combat but worse than that the PO had moved the breather to the timing case and where the old breather returned oil the new one did not. As you read from the Old Britts mod, at high speed (4000+) the oil gets pushed to the back of the cases and leaves the pickup hole sucking AIR.

Mine was so bad that the tank would empty in 10-12 miles of 70+ cruising and did the same as yours, p.....d out the LH barrel/case joint. I only lived with it by keeping the cruising revs under 4k and eventually plucked up the courage to do the work.
The fix worked great but a few months later the engine grenaded after a NEW rod bolt sheared. I'll never be sure if that was the result of running with a full sump and no oil pressure or just a bad bolt. I also think that I had bigends on the loose side, clearance wise and the oil absolutely pours out under that condition. They don't have to be knocking though. Also had a new timing side crank seal split when using 50 oil. Never noticed until the timing cover got pulled to fix the cam seal. It was a thinwall No..il part which they don't sell now.

Do the mod. You won't regret it and assuming you are moving the breather to the timing case (850 style) add a one way valve (I use a 1/2" Norvil part) in the pipe and take the tank breather to a small bottle. You get water and mayo in the bottle but it's a small price to pay for a dry engine.
Not open for further replies.