PCV valve in engine breather

Thanks, Graham Pickering. They should arrive Monday, so will be plumbed in before the tank as you suggest. Ref the blocked off timed breather, I'm pretty sure the extra holes into the crankcase have been drilled, either when it was blanked off by a PO, or when I had the bottom end rebuilt by Norvil; I did specify to check it had been done and if not to drill the holes whilst the engine was apart. The drive side crankcase was replaced at that time, as it had split from a stud hole almost to the centre line, so was unrepairable and since this side contains the breather it would have to have been disassembled.

Grant Tiller, interesting. I see what you are getting at, but if that were the case, I would expect to find a fair bit of oil in my catch tank, but I didn't really. What was in there was water with a slight film of oil on top. The water was a surprise, but I suppose it shouldn't be, an engine produces a gallon of water for every gallon of petrol burnt, or somesuch. The idea to fit a rocker breather wasn't mine, I've seen it on other Norton, usually racers. It is a standard fitment on D Vincents, the enclosed ones where the timed breather was dispensed with and is offered as a mod for earlier bikes (I have a C Rapide with the timed breather blanked off and an "Elephant's Trunk" breather from the timing case and this massively improved the oil tightness of the engine). I know this is a different engine, but it's still an OHV twin which relies on drain down to lubricate the cams and followers. On a D Vincent the breather goes from the valve cap on the rear cylinder, the highest point of the engine, as does my fitment to the Commando. I don't know if this would make any difference. My understanding of the Commando was that the top end is pretty wet anyway, as it has a positive feed to the rockers, straight off the oil pump, as do late Dommis/Atlas, instead of the take off from the return pipe on earlier models.
Graham, I might try that. The parts can be ordered from Fair Spares/Andover Norton. Ref the crankcase mods, there is a clear diagram in the Haynes manual showing you where to drill the holes. But as I explained, the bottom end rebuild was done by Fair Spares, not me, so I don't know for sure, but having discussed it with them at the time I would hope it was done correctly. Les and the team offer a warranty on engine builds, so I'm sure they would be aware of this mod! The cam was replaced at the time (I wish while the crankcases were apart I had had the PW cam fitted, which needed extra fettling of the cases but I opted for the standard SS).

Tony Ripley and Maurice Cloud, I wonder if the "cream" depends on what oil you are using and also the mileage. I did read your article BTW Maurice - very interesting. As noted above, I found water in my catch tank but no oil. Interestingly, when I modified the breathing on the Vincent, which contains a PCV valve, I got a fair amount of cream out of the breather at the rear of the bike, using Castrol GP50. At that time, I hadn't got round to blanking off the timed breather, now I have the cream has gone (I'm also now using Penrite "HD" 25/70 oil). Don't know what that proves. I don't get "cream" in the Nortons at the moment, using the same oil. I always used to get water out of the bottom of the tank when draining them for oil changes. The "cream" as I understand it is emulsified oil and water mixture. You get less of it if you reduce the number of short rides you do, to make sure the engine is up to operating temperature. I've heard that leading a breather to atmosphere near the rear wheel can have a negative effect, as the spinning wheel creates a semi-vacuum, which draws oil out of the bike. You have to ensure the end of the pipe is well clear of the wheel or pointed outwards. I've also seen bikes with the breather nearly dragging on the floor, which, since the breather sucks as well as blows, could lead to foreign matter from the road being ingested - not something you want! Another reason for a one-way valve!

John Crocker, would this work on the big twins? Surely there is an oil seal on the crank, so the air in the crankcase can't get in there (of course you could remove the seal, but would this have ramifications for the main bearing that side?).
My main concerns now are: 1) do I still go ahead with fitting the one way valves; 2) should I blank off the breather hole in the inlet rocker cover and instead reinstate one from the original crankcase breather location, with or without one way valve?
Also, what happened to Grant Tiller's comment?