What would cause middle 2 cylinders to lose compression?

[rpg_dog]

Exactly as the title states. Could timing slip temporarily enough to bend center 2 cylinder's exhaust valves, then slip back in place? I can't figure why this happened to me, and I want to make sure it doesn't happen again when I fix it.

 

[TOONAH]

You probably thought of this: A blown head gasket. They nearly always blow between cylinders. Water cooling and oil passage ways may not be involved yet.
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[Kastigir]

You probably thought of this: A blown head gasket. They nearly always blow between cylinders. Water cooling and oil passage ways may not be involved yet.
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+1 Most likely cause is a blown headgasket between 2 and 3.

 

[->PrOjEcTGS<-]

How low of compression are we talking? Probably the head gasket especially if it's still the stocker.

 

[xioca]

A problem with the timing belt isn't going to kill the center two. Do a leakdown test.

 

[rpg_dog]

Leakdown test shows air from exhaust. The exhaust valves are likely bent. The head is a fresh rebuild and all gaskets and seals and studs were replaced 1500 miles ago. I'm gonna try to get it apart today, but can't seem to find the time, and I'm afraid its going to be something costly. Why would this happen so soon after a rebuild? Should I try to blame the shop who assembled the engine or the shop who rebuilt the head?