Number 5 Cylinder Low Compression

[delta31]

Any Ideas? The engine runs pretty good but...
66000 Miles on Odometer.

On our number 5 cylinder the compression is about 30psi. On all others it is about 105 psi.
Adding oil to the cylinder does not increase compression. Opened up the valve galley and
the valve is going up and down and the clearance is about .010.

We had a old spark plug with the porcelain chipped out and threaded to put on an air compressor hose.
We hooked that up and turned the engine by hand to the compressions stroke. We could hear
compressed air thru the exhaust. This was not very scientific though has made me think the leak is with the exhaust valve.
We have run sea foam in the gas and also ran some MMO in the gas. Right not we have some MMO in the cylinder.

Does not appear to be any oil in the antifreeze.

If the valve clearance is good, doesn't that indicate that the valve is not stuck from fully closing?

I have never dealt with a burned valve. Could this be a possibility? What might the fix be?

Thanks

[ashyers]

I'd put my money on a burnt exhaust valve. Depending on your level of enthusiasm you could fix it in place. You'll need to pull the head and find someone with some older valve grinding equipment. It's actually kind of fun on a flathead.

I think the big question is what kind of shape is the rest of the engine in? You don't want to get taken down the road in stages, and these trucks can do that! 105psi is not stellar, but will push the truck around. If you're only going to putter around you could band-aid it and go. If you plan to drive the truck often it may be time to consider rebuilding the engine. You'll have a better idea when you pull the head and get a look at the valves and bores.

[delta31]

Saturday we removed the head. I was expecting to see a burned valve. They looked good from the top so we removed both the intake and exhaust valves for this cylinder.
The seats looked good so we lapped them as I had some compound and a hand crank lapper. The seat and valve lapped nice and uniform. Lost one valve spring keeper in the driveway.

The head gasket looked good with no signs of failure, kind of looks good enough to reuse though we will get a new one once we figure this out.
The gasket is firmly attached to the head and came cleanly off the block. Most of it pretty clean copper. This makes me wonder if the head was taken off before.

The cylinder wall shows no scoring and the ridge is slight. With a small feeler gauge, it appears that the piston is not exactly centered, it is off center a few thousands. The others look similar.

The pan is still on for now.

The only thing I can think of is get a Cylinder Bore gauge and check for wear and out of round. Then remove the pan and piston and check and replace the rings till a full rebuild could be done down the line.

I still don't see how the compression could only been 30 psi.

I check the TM and only found the cylinder diameter to be 3.25 inches. Where can I find data like wear tolerance and ring end gap?

Thanks

[just me]

The engine TM has all the info. It just isn't in one place. You have to read around the document. I'd bet rings at this point, but check the valve guides and springs while you have the valves out. And so rare as to not be on the radar, but a broken piston skirt is a possibility.

[delta31]

Pretty sure the problem is the valve guide. This is something I have not done before. Has anyone here replaced valve guides?
I would greatly appreciate and tips and pointers.
Thanks

[delta31]

I have read in the Canadian Maunal that the guides are removed by driving them down and then taking them out in 2 pieces. I found a You Tube video showing one being driven down, cut in half and then the remaining piece being driven down and out.

Is there a reason why the guide isn't pulled upward with a puller to remove it?

Is there any major differences with the Canadian 251 and the American 230 engine?

Thanks