Did a compression test today on my 406 SBC

Cylinder 5 had zero compression, cylinder 7 had low compression @ 90, all other cylinders looked very good at 145 psi.

What do you guys think might be causing this?

What would you recommend as next steps to try and diagnose the problem?

Regards
Mike

It sounds like a bad piston or valve and probably a blown head gasket.Time to take the head off and see what you have inside

First remove the rockers and smack the valves in both low cylinders several times with a soft faced mallet, this will knock anything loose if it's in there. Then I would borrow or rent a leakdown tester and check those cylinders or just air them up with air. If they are leaking that much you will hear where they leak. Crankcase means bad piston, intake means bad intake valve, exhaust means bad exhaust valve. If the leakage is still bad after smacking the valves the head will have to come off.

Thanks for the feedback guys.

Next step is to give the valves a smack.

If that doesn't solve the problem I think I'll just pull the head and get a look inside. I'm thinking that it is going to have to come off anyway so why not just get a look inside.

If I pull the head am I missing something by not doing a leakdown test?

Cylinders 5&7 are next to each other,,smacking the valves wont do anything..you probably have a blown head gasket,compression from cylinder 5 is probably feeding cylinder 7 due to the gasket being blown between the cylinders and that's why you have compression in cylinder 7. pull the rockers on cylinder's 5&7 and fill one of the cylinders full of air if air is coming out of one or the other you have a blown head gasket,if its coming out the oil passages then pull the motor because its leaking past the rings...

This motor sat for quite a while, they normally don't blow head gaskets just sitting, and he had a knock noise from it that could have been caused by a valve sticking and letting the pushrod jump loose. Just smack the valves if you are going to pull the rockers anyhow.

AHHHHH gotcha!! rusty valves!!! but then again it's a 406, steam ports..

I'd pull the exh manifold off and have a look in your exhaust ports. Insptech is right with the stuck valve most likely. Usually when one comes in the shop like that it's had a leaky riser gasket, or pin hole and the water runs right into the center two cylinders. If it sits for a while it'll rust the stem and hang the valve up.. Good luck.

I removed the rockers applied some PB blaster and gave the valves a good smack with a dead blow hammer. Viola full compression...however as soon as I replaced the rocker arms and tightened slightly (1/2 turn) I'm back at 0 compression. Could this be a bad lifter? These are hydraulic flat tappet lifters. Any other ideas?

FWIW I can see that all of the valves seem to be moving freely.

could be a bad lifter OR a bent push rod...

ETA: Might want to pull the intake and take a look at the lifter valley. You might find you problem right there.