1980 911 SC, smoke from tail pipe

[PHB911]

I have a 1980 911 with approx 170k miles. The car has recently begun to produce smoke from the tail pipe. When I first start the car all is well, it is only after it is running idle for about 5 minutes that smoke starts to come out of the tail pipe and it gets pretty bad. Does this mean I need to rebuild my engine? I haven't driven the car since it started smoking last week. I drive it rarely to begin with. It had sat idle for almost 2 months before I noticed the smoke last week. Every time I start it now, it runs OK for several minutes then starts smoking again. Just took off the Cat and replaced it with a bypass pipe which was able to use the existing O2 sensor. Any chance this is causing the problem??

[john walker's workshop]

any oil that gets blown into the exhaust system will take time to heat up enough to start smoking, then it will eventually burn off. did you drive it for an hour to see if that's the case? was it overfilled? oil will burp into the intake through the hose between the filler neck and the intake air boot, and end up in the exhaust system. oil also seeps past the valve guide seals and past the rings when the car sits for extended periods and when started it gets blown into the exhaust system. if it always smokes, then worn guides, rings or a damaged piston/cylinder would be the cause.

[Gunter]

Like John says.
Did you perhaps overfill the oil? Make sure the engine is hot when checking and fill only to midway between markers.
These engines don't like to sit idle for long; go on a spirited drive. Bypass pipe will not have an effect on smoke; you can even leave the O2 sensor disconnected.
If in doubt, have a compression/leak-down test performed.