Help! 401 Buick nailhead compression issue!

I have a buick nailhead 401 that ive been puting together for awhile now and the motor has been completelly gone through and everything is new besides the rocker arms and shafts because they looked great when I tore the motor down. My issue is the number 7 cylender has no compression for some odd reason so it wont fire. When i tore the motor down that perticular cylender was a bit scored from the piston being broken apart and the rod rubbing on the side of the wall. So that was abviously sleeved .030 and the all others bored .030. And the head on that side had a hole in it from the piston exploding. So I got a good set of used heads and had them rebuilt new guides, springs, and valves ground etc. etc. So Im at the point of initial start up to break it in. Any advice on how there is no compression in that cylender? Any advice would be much appreciated hambers

 

Over/under adjusted valves? But i dont know anything about nail heads..

Throw a compression tester on it. Add some oil to the cylinder and try again.. might eliminate the rings being an issue.

Next would be leak down tester..

 

Do the rocker arms for #7 cylinder move up and down like the others? Were the rings on #7 cylinder staggered? Does it have the correct length push rods for #7 cylinder?

 

Dont want to sound like a smart ass but you did put vales in the cylinder and you did not lign up the ring gaps?

 

Thanks a bunch Nathan! I will try a leak down tester. I primed the heck out of the motor as far as using my oil pump drill attachment tool so I know everything has plenty of oil. Do you think it could be the old rocker arms on that side being damaged when the old piston shot apart? They looked great when I took it apart.

 

Yes the piston rings are staggered, and yes the push rods are adjusted correctly. And I think one of the rockers is kind of stiff. Would that be the cause?

 

Putting oil into the cylinder has little or nothing to do with lubrication, the oil in the cylinder is a way to fake the rings being broken in...or to seal older rings...if oil added and no rise in compression, then it eliminates the rings/seal as the issue...

 

Thanks Willy301 it maybe has 10lbs of compression in that cylinder. Anyone think it could be a rocker issue or the shaft being a little tweaked being the issue? Id hate to take the head off for no reason.

 

Sounds like a valve issue to me. Can you put that cyl at TDC and air it up and hear where the air is coming out? Like the intake or exhaust? Just a thought. Lippy

 

At TDC on #7, do you have any kind of valve lash.(clearance) LIFTERS??Are they the old ones? If so you may have a stuck one in #7. Try to compress the lifters by CAREFULLY prying up on the rockers to get some clearance. You wiil have resistance, so do it slowly. What you are trying to do is push some oil out of the lifters to compress the lifter piston. Check for clearance. As lippy just mentioned, A valve seating issue. valve lash too tight could cause it, or if somebody screwed up on grinding the valves or seats, you will not be getting a good seal. You can narrow it down by the pressure test on where you hear the air.

 

Well? Lippy

 

Ya what ended up going on there? Lippy's suggestion sure is simple and you will know whats going on right off the get.

 

Could be a valve with not enough clearance. Loosen the rocker arm shaft bolts three turns on left side and recheck the compression. If OK then stem height could be to high from valve recession or seat set to low in head. If you have enough valve above the retainer the tip could be machined for proper stem height. Or, pull the head and replace the seat.
Good luck.

 

Are you running stock push rods or adjustable ones? Otherwise there is no adjustment in the valve lash on a nailhead.

 

Hay, 6ft6,FNG. Did you take all this in or just took up space and our time???