SBC no compression in #4 cylinder

Had a bad vibration in a reacent build SBC 350. #4 cylinder had no compression so I figured I would take the motor down. Had checked both ajacent cylinders and they both had 150 psi. So it wasn't the head gasket. Had checked both valves operating properly. So I thought I either had a hole in the piston or the rings were broke. Got it down and found nothing wrong with the piston or rings. Has me stumped any ideas....Dan

 

You said the valves are operating properly. Did you check if one is bent and not seating properly?

 

Sounds like that to me too, moose... possibly a bent or stuck valve would be my 1st guess... robber

 

Fill the hole with compressed air (piston down), bet you hear air escaping from the tail pipe or carb...........................if it's a valve!

 

If the head's still together and off the engine, set the head where you can pour gas in the number 4 intake port. Most good valves will seep a little gas, but not much. A valve that's not seating right will let the gas pour through the valve seat. If it's ok, flip the head over and try the exhaust.

 

With the head off the engine, set the head on your bench, shine a flashlight in the intake runner then exhaust runner, look at the valves, looking for light around the seats.

Perhaps dark shop would work better. GM crate engines are notorious for chinese valves that warp. Not sure of the engine year, but a stuck or plugged egr will cause excesive cylinder heat and war valves.

 

That happened to me once. One of my rocker arms was adjusted too tight not letting the valve close completely.

 

Make sure intake cam lobe is not round,you air the cylinder to compress it,been there

 

Well you are starting over from scratch now.
Just like the first time it was rebuilt .
Without a leak down test or at least putting air into the cylinder and listening for the escape of air in intake , exhaust , crankcase or cooling system you really have no idea without obvious evidence .

0 compression, nothing , a completely dead cylinder would would lead me to think initially two ways - an open valve or hole in piston. Since you don't see a hole , check the valves

 

What did you find ?

 

Well so far the speed shop is working on the heads. I did find the blue seals on the guides were not staying down but were riding up and down with the valve on all of them. And they are also weighing all the pistons. So now I wait...