I've been working on a 1988 4.3 Vortec engine for about six months now and did almost a complete rebuild replacing almost every internal part of the engine except for the cam shaft and piston rods. I also had a machine shop hone out the cylinders to .030 over spec, and replaced the pistons and rings accordingly. They also did some other work to ensure no head gasket leakage. The machine shop also rebuilt the cylinder heads and tested it for compression.
Now, after six months of work, three tear-down and rebuilds, and a couple thousand dollars in parts, the #2 and #4 combustion chambers still have oil leaking into them. The problem seems to only occur under a load (when driving, not when parked). After driving for a little while, the truck starts to smoke. Then I remove the #2 and #4 plugs and they are covered in oil. There seems to be a significant amount of oil in the #2 and #4 combustion chambers.
I know of only two ways that oil can enter into the combustion chamber.

1. Oil seepage past the piston rings.
2. Through the valves from the top-end of the heads.

If anyone has any insight into this, please help!

 

As far as I know, those two sources are it. At least they are on the same side.

I can't tell you anything but what you probably know already...

Pull it apart again,

Pull 2 and 4 pistons out...
Check Piston Clearance in cylinders,
Check roundness of Cylinders,
Check End Gap of rings,
Replace/gap as necessary.

I'd do a turnkey on the Valves of both Cyl's...
Replace Valves, Guides and Seals.

What more can you do !

 

Thank you for your reply. In your experience, do pre-gapped piston rings need to be adjusted?

Also, how could I go about checking the roundness of the cylinders?

 

Maybe they can guess just how close whoever got to 30, but, I just wouldn't take the chance. Check every ring and wipers too, every time !

There's a device called an Inside Micrometer, that is used for roundness checks. Check width in two directions and in four places, 90° apart in the upper cylinder and at the bottom. Align first measurement with rotation of crankshaft.

Video shows the use of a Digital Caliper, either will do.

Cylinder Bore - An Explanation of How To Measure - YouTube

 

I believe I discovered the cause of the smoking. The transmission vacuum modulator was bad, allowing transmission fluid to enter the intake manifold directly above the #2 and 4 cylinders.


There is another potential issue. Between a certain RPM range, the engine sounds like it is going to fall apart. Look at this video to see what it sounds like.