Question: Just recently rebuilt my 292 y-block that suffered a broken crankshaft. Had the heads reworked and the new crankshaft turned. Engine had new pistons and we installed new rings.  After rebuild on startup we noticed once engine got hot which was running it at idle for about 15 minutes, started seeing smoke come from the left bank or passenger side of car. Shut down the engine and pulled valve covers on both sides to check for oil flow. Had plenty oil on the driver side but none on the passenger side. After removing the rocker arm assembly, noticed a brass plug in the oil orifice hole with a very tiny hole drilled into it. Never had seen this before. Looked at several engines from different people and could not find any that had this insert into the head.  So the mechanic drilled this plug out and we started getting plenty of oil then. Now when the car is idling it smokes blue out the passenger side. Could the head gasket have blown in that short period of time running the engine?

You don’t say if this engine was assembled with steel shim or composition head gaskets which could make a difference.  Never saw an issue with the composition gaskets as long as the correct length upper head bolts are put in their respective holes.  It’s rare for a head gasket to seep oil around the oil passage area at the decks without there also being a water seepage problem.  I’ll suggest a cranking compression test and a leak down test on all the cylinders but paying particular attention to the readings on the passenger side.  You’re performing the tests also on the driver’s side to give you a good feel for what the baseline numbers should be.  Looking at the spark plugs as you pull them for the compression/leak down tests may also point you to the cylinder in question that’s causing the blue smoke.
 
Potential problems to watch out for are damaged valve seals on the heads, too much stem clearance at the guides, poor drain back of oil at the heads, piston compression rings installed upside down, broken piston compression rings, or problems with the oil rings themselves.

Lorena, Texas (South of Waco)