Oil Leak/Breather Arrangement

Sep 7, 2006
I gues I am the newbie here, I just received a 1971 Fastback that was restored for me in Atlanta.

I seem to have a main seal leak. I had a Norvil belt drive installed, and they cut a slot in the bottom of the primary chaincase cover under the Alternator. After running the bike on the freeway, oil seeps from this slot. Appears to be coming from the main seal area.

I suspected the engine was not venting properly. The problem only crops up when the engine has been running at elevated RPMs on a sustained basis, and does not show up on city stop and go riding. I installed a PVC valve on the breather hose. This valve was inserted at the place the stock bike shows an adapter that reduces the hose size to 3/8 where it connnects to the oil tank.

This appears to have reduced the amount of oil that leaks, but has not eliminated it. I still suspect a breathing problem rather than a main seal issue, but short of tearing into the engine to inspect the seal (it is a new seal) anyone have any ideas?
The seal is not too hard to replace, you need to remove belt & alternator then dig out old seal & press in new one. I had a leak from here due to the seal turning in the housing. I fitted the new one with loctite. I have experimented with breathing and ended up with a 3/4 inch bore pipe straight from the breather stub to the rear numberplate it makes a chuffing noise when engine running! it seems to be working ok
If your bike is 1971 model year then it would be a 20M3S motor with the timed camshaft driven breather. Not a lot you can do to check that without major dismantling.

What do you know about the restorers ? Did they use a suitable cam ? Did they modify the breather system ?

What do you call elevated r.p.m. ? The Atlas-style breather should be capable of dealing with normal road speeds and the main crank seal should be able to cope with the pressures generated.

As Geoff says, it is quite straightforward to remove the seal carefully and because there is a lip behind, there is no danger of pushing the replacement in too far (unlike the mk111 850).

My preference is for the type of garter seal with a steel outer surface, loctited in place. It does not seem possible to obtain a tight enough fit with the fully moulded synthetic rubber type.

Make sure that all surfaces are properly degreased before assembly.

It is worth examining the crank nose while you have everything apart and making sure that the sprocket keyway is undamaged and not interfering with the seal lip.
Thanks for you input. Cam is stock, as is breather (Yes, 20M3S engine). "Elevated RPM" is 4-5K constant, on freeway (re-built engine still has only 500 miles on it or so). I was surprised by the amount of oil that is spit through the breather when engine is idling. Bike is oil tight except for main seal.

I'm gonna let the engine "settle in" a bit more, then determine if necessary to replace seal. Glad to know only alternator needs to be removed in order to replace it.
If your bike has an early engine with a blanking plate where the distributor used to be you could fit an additional breather through this without too much effort.
PCV valve

Hi Spub,
I was wondering what you used for a PCV valve? I have read that automotive types are typically not up to the job. The breather mods discussed on the NOC site recommend using a power brake booster valve. They are small, cheap, and use a light rubber flap inside that supposedly can better keep up with greater bike rpms.

justa thought,
After reading the above article from the Norton Club site and doing some other reading, I replaced the PCV valve that I had placed in the Breather line (this is a 71 with the breather line running from the left front corner of the crankcase, not from the rear of the crankcase as with the later models) with a Motomite/Help Brake booster (check) valve. The PCV valve slowed the main seal leak, and the Motormite valve seems to have slowed the leak down to just above a weep or minor seep....... The valve is made out of a ABS plastic like material. It has a fitting on one side that accepts a 1/2 inch hose, and a fitting on the other side of the valve that accepts a 3/8 inch hose. The fittings are at a 90 angle to each other.

There are 6-8 small holes in a circular pattern on the 1/2 inch side and behind these is a circular rubber flapper with a hole in the middle, and this is located by a plastic rod that runs through the hole. As you suck on the 1/2 inch side, the rubber flapper is drawn over the holes, sealing off the holes. The pressure needed to activate the PCV valve was greater than that needed to activate the Motormite valve, and this is perhaps the reason the Motormite valve appears to do a better job, since it appears the pressures involved are quite small (estimated at 2 psi in the Norton Club article). I am somewhat concered about the longevity of the valve, given its construction and the fact the breather line can carry a fair amount of hot oil at times.....For now, however, it does appears the Motormite valve is somewhat effective at reducing engine crankcase pressure, which in turn can reduce the severity of oil leaks......