I just rebuilt from the bottom up: new superblends, new shells, new pistons/rings (yes the gaps were in the proper places) crank ground, cylinders honed and still within factory spec. (0 - out of round) heads done with new guides, valves, springs, new cam etc.....started on the first kick, but left hand cylinder smoking like heck....blackened oil seeping from around exhaust pipe and out of joing between pipe and mufler!....I tore down the engine after compression tests of 175-185 psi dry and 180-185 wet...no bleed down test (oops) Lots of oil on left piston after removal of head...good ring gap of 0.012 "...I am at a loss as to what to do next....How do I tell if the shop buggered up my valve guides/seals? What else should I try....?The engine smoked from the start....right cylinder OK, no smoke..... B.Stasuik Alberta Canada (EH!)

I would think guide/stem clearance is a real possibility. Even new unreamed iron guide are a bit sloppy for my taste. I use bronze and make them rather tight and then they don't smoke. You might even have guide to head scoring that allows oil to pass. Close scrutiny can often find machine shops that do way less than acceptable work. Some times you luck out and sometimes you don't. Many of our monthly local club tech sessions revolve around discovering these problems and solutions. Unfortunately bad work runs rampant. May be it's not really all that bad, but the problems always seem to end up on my doorstep..... dave "why pay to screw it up....when you can screw it up for free"

Thanks for you input....I removed the valves and found excessive play in the one guide....It has since been sent back...there is a fellow in Edmonton that deals in Norton, BSA, Triumph parts and service and was the one that had the head done....I am a little disapointed with having to ship and wait longer for their poor workmaship...Thanks again b

Before Taking the cyl. head off you should have blocked the oil feed to the head and run the bike like that for a couple of minutes . That way you would have known for shure if the oil comes in the combustion chamber from up or down . Check for loose valve guides . Is the oil return free ? ( small hole at back of barrel ) Nortons often start to smoke from left pipe when idling on side stand ( oil arround inlet guides flows to left where there is no escape )

when I say loose guides , I mean guide vs head . not guide vs valve .

I just did up my 69 750 Commando and had exactly the same problem. Total rebuild and from the moment it started it was smoking on the left cylinder. I stripped the motor down and rebuilt it and again the same problem. It wasn't till I pulled it down a third time that I discovered that the oil ring had overlapped on itself. It took another 3 goes at sliding the piston into the cylinder before I was sure I'd gotten it right. Put it back together and it runs sweet as.. Hope this helps..