smokey

I`ve got the MKIII out of winter hibernation and after checking oil levels etc. it started right up. However it displayed the same symptom as the last time I ran it (of course).. The left cylinder smokes (out of the exhaust) especially when revved up. I hate to take things apart again (rebuild it 2000 miles ago) , but what can be the most obvious cause. Piston ring is my guess......
Any suggestions are welcome.

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What did you do to it!?!?!?

Now that we have the drama out of the way, "..........see if you can catch it in the act. Pull the intake valve cover and see if the intake valve springs are flooding in oil. It could be the drain was blocked by gasket sealant. Not sure if you can see if valve seals on intakes to confirm they are in place.

Check plugs to see if oil is reporting to combustion chamber, if not then look to exhaust guides. Later model Commandos (RH4 heads) are notorious for cracking along the roof of the intake port approaching the valve guide where oil would leak from the valve spring pocket into the port. You may be able to spot something weeping through after you pull the manifold.

Be systematic and check the easy stuff first (ex read plugs, check compression etc). Any chance you overlapped or clipped the expander ring on the oil ring?

My guess is the inlet valve guide seal........I've managed to not fit them properly once (damaged by overcompressing spring) and to have one oversize on the valve bore twice.....Norton parts and annoying but there you go. Result = smokey engine and always the right side on mine. Last time I took the bad seal to AN and found a tight one to replace!