I'm at a loss guys. The motor is a 2000 LS1, bottom end was just rebuilt, used heads with yellow springs. The motor has a misfire, I was hoping it was an ignition problem but after doing a compression test, cylinder 6 makes 0 compression. After removing the rockers (to relieve any pressure on the valve springs), the cylinder made 90 psi compression. I tried swapping rockers and pushrods from a different cylinder, but again it made no compression. Something is preventing the valves from closing entirely, or at least that's my best guess. I purchased the heads with valves and springs already installed by the seller, I suspect the fault may be in his assembly but lack the experience to be certain. The rest of the cylinders made 140-160 psi cold.
Any advice would be greatly appreciated, thanks guys.

 

valve tip height is too tall and this causes the pushrod to bottom out in the lifter plunger, essentially making it a solid and holding the valve open
you can take all the rockers off on that side and use a straight edge to verify this

 

What Tony said and try this
http://www.gregsengine.com/cylinder-...n-testing.html
re putting oil in no 6 clyinder to check leakage around the rings.
Also if you have a depth gauge remove the pushrods and check the lifter height ensuring the cam lobe is on the base circle. Check the pushrod length on cylinder 6 with another cylinder.