1974 Mk2a crankcase breather

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I have an 850 Mk 2A, first registered on 1/3/74. I am addressing the tacho drive that has just started leaking after some years trouble free running. I have bought the garter seal from Andover Norton as it seemed a good idea at the time, but maybe not as regards tacho drive shaft lubrication? (Must remember to pack the housing with grease!)
Whilst doing the research I came across a lot of writing about excessive crankcase pressure that forces oil through the tacho housing, which makes sense - but I thought the breathing problems were all to do with earlier engines? I have found no specific reference to breathers to fit to a 1973/74 engine, or am I just not searching hard enough?
I have used the bike a lot this Spring, including the inevitable motorway mileage, and wonder whether I should be looking further than a new O ring on the tacho drive?

Don't over-grease the drive…

Don't over-grease the drive cable as grease can work it's way up into the tacho itself.

850 mk2A and mk3 crankcase breathing is fine but can be improved with a one-way valve, aka positive crankcase ventilation (PCV) valve. Lot's of discussion if you search the forum. But that mostly addresses too much oil going up the breather pipe. The oil seal tacho drive housing should fix your immediate issue.

PCV's

A while back I rang AN for advice on PCV's because they don't include them on their parts list and they said Commandos don't need them, also, they don't need oil pressure warning lights or gauges. Go figure?

Thank you very much for your…

Thank you very much for your prompt response which has put my mind at ease. It never ceases to amaze me how far up cables - and even wires, that oil can travel when you'd rather it didn't.

high temp

You're correct to be concerned about tacho drive lubrication.
I had it machined and fitted a lip seal, then a few years later a horrible squealing noise was heard from the tacho drive area.

Removed it and found it to be partially seized, since then I pack it with high temp grease annually.

BTW
Holland Norton Works do a nice PCV valve which is unobtrusive and seems to work ok for me.

Scroll down to item #H14
https://www.hollandnortonworks.eu/?lang=EN&page=10

Which is actually an EGR…

Which is actually an EGR valve off a Chinese bike that you can pick up for a tenner.

EGR Valve?

Which Chinese bike? Are we talking bike breakers here?

Yes

Not necessarily! Available…

Not necessarily! Available new:
httpww.cmpoparts.com/part/EGR006

As you can see on this parts diagram, one end is flange mounted, into which a tube can be pressed to create a hose connection. There is a similar type with two hose connections as standard, but I haven't been able to find a source of these new.
https://www.cmpoparts.com/microfiche/7969/2562

EGR006

Are these actually a reed valve? If they are then I shall put my hand up at once.

Yes they are

Order placed. I shall report…

Order placed. I shall report how I get on with it - and whether it solves the problem!

Thanks to all who have commented.

EGR = exhaust gas…

EGR = exhaust gas recirculation, a technique on diesel engines where some of the exhaust gases are directed into the inlet manifold to reduce NOx emissions. Not related to crankcase breather.

PCV back pressure

The PCV was delivered promptly and looks a nice tidy piece of kit. It poses the question however of the backpressure this valve will create within the crankcase? You can open the reed valve simply by blowing into the appropriate end, and it isn't difficult, but it's not the free uninterrupted flow that previous crankcase pressure enjoyed. I put my finger over the breather pipe - admittedly only with the engine at tickover and I doubt it was enough pressure to lift the reed. With the engine spinning at road speed I have no doubt the reed would be opened, but not at tickover, unless even at low speed the crankcase pressure would build sufficiently to operate the reed - but it would still retain a level of crankcase pressure that was not there with just the plain tube breather fitted?
I will go ahead and install this unit, but wondered if others had noticed any backpressure effects from such valves. I shall keep my eye on the tacho drive for wayward oil!

Surely the effect is the opposite …

… to the one you fear?

Assume that before starting the engine the crankcase interior is at atmospheric pressure and the pistons are at or near tdc (which is where one would expect them to come to rest after previously switching off).

Then on the first downward stroke of the pistons pressure in the crankcase will rise above atmospheric and thus the PCV will open and vent to the oil tank, slightly raising the air pressure in the latter.

When the pistons reverse direction at bdc the pressure in the crankcase will begin to fall, and once it falls below that downstream from the PCV (which will be at or slightly above atmospheric) the valve will shut, preventing gas flow into the crankcase.

Thus by the time the pistons return to tdc the pressure in the crankcase will be significantly lower than atmospheric. Given that pressure around bdc will always be close to atmospheric it follows that the average pressure over each revolution of the crank must be below atmospheric.

This is the basis of the view that PCVs are helpful in combatting oil leaks.

If you fit one of these…

If you fit one of these valves make sure you avoid short journeys as they take some time for the nasty gasses in the engine to escape properly. We find a those engines fitted with them contain corrosion. It takes a good dozen miles to warm the engine and the oil through completely. My opposite number here has used Commando's with all three types breather fitted and has not had an issue with any of them, but then he does do a minimum ride of over 40 miles at a time.

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