I kind of know my destiny with this, but thought maybe someone can give me hope? I bought a 68 400 motor complete that supposedly was rebuilt about 1 years ago, but never run. Afert gettin it home, I cleaned it up, put a drill into the oil pump drive and confirmerd oil pressure, and all seemed ok. Before I put it in, I decidedd to hook up a battery and test compression. Left side....all 150-160 lbs / right side....3 at 150 and one (#) had maybe 10-20 lbs. I put air into the spark plug hole and only when the valves are open, does air come out of the exhaust or intake. When both valves are closed, you can hear the air coming from oil pan area. My question is....is it possible one of the rings is just stuck in some kind of position that allows leakeage? I probably know the answer, that is more likely a broken ring and will require replacement and teardown. But, is there anything I can check for first?????

 

If its not in the car I would pull the heads off and the pan off for a look see...gaskets are cheap compared to the pain of putting it in, starting it up then having to pull it out again...

 

You could try squirting oil in that cylinder thru spark plug hole and see if it helps, could be a stuck ring I suppose but there are 3 rings that help seal in compression....

 

Sounds like a cracked piston. I broke several of the original Bon-O-lite pistons.

 

Sounds like they broke a ring or two when installing the piston. Pull the head and the pan, pull that piston and see what happens.

Gene

 

you would be best to pull it apart to investigate

 

Well, I pulled the intake and head off today. This definetly was never run after rebuild. Everything inside is new, and it all looks good. I did notice that the head gasket might not have seated against the water jacket. I think I see a residue of surface crud between the lower eg=dge of the cylinder and the water jacket. Can I be so lucky????? Can all it need is a new head gasket???? I am going to hope. I am goint to try a new gasket first, and take compression test again. hopefully, that will fix it. If not, out comes the piston. wish me luck...I need it.

By the way, when I took the intake off, it had the valley pan intake gasket. What do you all think about using the pan or regular gaskets. The motor does have the deflecter cover over the cam lobes. What gasket set up is recommended?

 

i use the valey pan gasket. i am not sure what everyone else uses. while the head is off i would pull the pan and piston just to look at it.

 

I used the stock turkey tray intake manifold gasket on my 455 with Edelbrock Performer. I don't have any leaks.

 

I agree...If only for peace of mind...how did the bore look in that cylinder as compared to the others..

 

Well...tried the head gasket.....not solution. So I took the piston out to inspect. Cylinder walls look good, nice hone, no scores, or cracks, that I can see anyway. The piston looks ok too. I am kind of miffed. No aha moment. No obvious problem. I was hoping to see the problem immediately. Broken ring, cracked piston, something. I am going to mic the cylinder and check for roundness, although it seems to be good.

 

you may take the rings off and check the ring gap. maybe they put the wrong rings on one piston. i doubt that is it but at this point you have to check everything. what about a cracked head?

 

Might have found problem...

Just kept looking for something that didn't look right. Inspecting cylinder walls, piston, rings, and then.....while my drop light was on the floor, next to the head, my eye caught light coming through the intake valve. I put the light directly over the valve, and looking into the intake chamber, I can see light around the valve. Not on the other cylinders, only the bad one. It evidently is not seating properly. I thought when I put air into the cylinder, that it only came out the intake when the valve was open, but maybe it closed enough not to really hear air, buy enough to have the compression escape out. I'm going to bring head to machine shop to do a leak test and hopefully, just reseat the valve.