I believe the head gasket failed on my engine.

There was a loud bang, then the sound of air puffing at high pressure.  I discovered that air was coming out between the cylinder block and head by the #6 spark plug.    I didn't find any pieces of the head gasket, nor any discernible holes, as the hole would be very thin, but you can definitely hear and feel the air coming out.  I think there was a small amount of oil expelled, probably only a few drops; there was no noticeable spattering of oil on the side of the engine bay or battery.

I was wondering if anyone has anyone ever heard of this happening?  Since it's a sturdy material and compressed there should be n reason for the material to  be forced out from between the two tightly-fitted metal surfaces, but high pressure and heat can do interesting things.  It should not be much of a problem to install a new head gasket, and I would also replace the intake manifold gasket as well;  it weeps a little oil too. 

I'll need to get the model numbers from the cylinder head, block, and intake, then order gaskets.  The engine is a stock L28 with the dished pistons, IIRC, so there's no reason to get any "performance" gasket or is this an opportunity to improve things slightly?  The thing spins easily when cranking.

Could you put a strong light down around the air and get a magnifying glass and look around real good?  I'd think you could see a gap in the gasket if it's broken.  Could be a hairline crack in the head? or it's warped and lifting up.  Re-torque your head bolts from the centers to the outers and see if it still hisses.

If there's really a chunk of gasket got pushed out, that would be curious.

If you do find that to be the case, before you pull the head maybe I'd take a look and make sure your valves are adjusted properly? Thinking that maybe if your exhaust valve isn't opening (or opening enough?) you might be building up more cylinder pressure than normal?

 It's the "loud bang" that bothers me. What in the world caused that? When did it occur? Start up? Engine warm or cold? RPM's when it occurred? Any recent work on the engine?

1 hour ago, Mark Maras said:

 It's the "loud bang" that bothers me. What in the world caused that? When did it occur? Start up? Engine warm or cold? RPM's when it occurred? Any recent work on the engine?

The last time I heard a loud bang there was a hole in my block.  Rod cap created it and destroyed the block.  I thought the starter was locked up. 

 One other thing that kinda made me cringe is that last sentence, post #1(if we had #s). "The thing spins easily when cranking." I hope that's a good thing.

 Probable good thing is, if there is air going in and out of the leak, there's still a piston going up and down in there. I'd start by pulling the valve cover and plugs (how does # 6 differ from the others), turn it over by hand and watch everything move, including the pistons and check the valve lash. Look for pieces of metal. Rarely does a machine make a loud band without leaving evidence.

13 minutes ago, Mark Maras said:

Rarely does a machine make a loud band without leaving evidence.

That's when I reach for this...

You wouldn't have to bother with any repairs if a spark plug pops out.  That was the source of the noise or bang,  It must have worked its way out of the hole.  You screw it back in and tighten it securely, and the engine is back to normal.

55 minutes ago, TomoHawk said:

You wouldn't have to bother with any repairs if a spark plug pops out.  That was the source of the noise or bang,  It must have worked its way out of the hole.  You screw it back in and tighten it securely, and the engine is back to normal.

It was still hanging from the plug wire?  How did you not see it?

Where did the OP mention a spark plug was blown out? I missed that I guess. 

The noise alone should have pinpointed it, but then again we weren't there to witness it. 

There should have been clues that the plug was backing out though. Like hissing as it backed out. Which brings another question, how did the plug back out with the plug wire attached? 

When I was a kid my go cart shot the plug like a toy rocket.