The first post in this thread cites a post from the NOC forum that I haven't been able to find. But I had an interesting experience yesterday that seems relevant to this topic, and have started a thread on it there. But I'm (mostly) cross-posting my own post here, in case it's of interest, or anyone has any observations to add: 'My Commando has, unfortunately, taken up smoking. Grey-blue smoke, just from the left cylinder. I've tried replacing the oil seals on both inlet valve guides, with generous assistance from [NOC-member] Gordon Robertson, but to no avail – I did both because it seemed sensible. Yesterday I ran the engine with the left exhaust valve cover off, so that I could see if oil was pooling there – maybe the drain there was blocked. No sign of oil pooling, so I'll have to take the head and barrels off to investigate further. But, while the valve cover was off, the smoking virtually disappeared! Of course, it came back when I put the valve cover back on. I've seen people discussing the virtue of a breather outlet for the head alone: from this experience I'm convinced. Is that wrong? By the way I have a one-way reed valve from HNW in the existing breather hose, and yes, I have installed it the right way around! ' Noticing previous mention in this thread of a factory breathing set-up for the cylinder head installed until 1962, I wonder if it's possible to retro-fit this …?