Hey Guru's....
I have 60, 40, 20, 40 compression on the driver side 5,6,7,8 and I didn't bother to check the other side (discouraged). My 87 year old grandfather, aka Papaw, said I could use valve grinding compound and a drill to pull the valve down against the seat and turn it to seal leaking valves. I have never worked on a head, but I always try new things, and I'm about to do this! I bought the gasket set, and spring clamp, so I hope to get a reply back soon on this. This 390 in a 72 F 100 is running rough, and has oil on the spark plugs, blows smoke, and takes forever to start. I had added Marvel Mystery Oil to the oil, 1 quart when it was 1 quart low, I think that's when the spark plugs took on the oil. Anyways, I see lots of blowby. Will lapping the valve seats work? I have only a little cash and can't afford to go see a machinist. So what do you think?

 

From what you are describing, you have the perfect screen name.

Second, you need to go through the compression test again with a couple squirts of oil in each cylinder. If you see a marked difference, the rings are not sealing. If there is not much difference, look at the valves.

Has this truck sat for some time? I'm thinking the rings may need time to loosen up if there is anything left of them.

 

I'd say your rings are shot with those readings. If it is the valves, readngs that low indicate valves in more need than mere lapping.

As far as valve lapping goes, it's a substitute for proper valve cutting/grinding. Ever read how you're supposed to keep your valves in order so they go back in the same locations? That's for lapping. If they're cut, they can go back in any location.

I"ve done it before. I put the valve in with lapping compound between the valve or seat face. I then fasten a piece of rubber hose to the valve stem. Next I chuck in a pice of steel rod in a elecgtric hand drill. Fasten the end of teh rod to the other end of the rubber hose. Next, I spin the valve with the dril, pulling slightly back on the drill. I stop when a nice uniform ring of lapping compound forms around the valve circumference.

Your cylinder heads may not have hardened inserts for the valves. Going on the cheap, you may want to consider adding lead additive to your fuel vs having a machine shop put in hardened seats.

Keep in mind, there is the best way to do something, and that can be expensive. There's the "el cheapo" way, and that typically is not acceptable and can cost more in the long run. But in-between there are ways to get a decent job done and save money. Lapping is acceptable and cheap, but you need to start with valves and heads in decent condition.

I suspect that a cheapie engine job for you will be valve lapping PLUS cylinder honing and rings. Bore taper may caused you not to get high mileage out of new rings,but it will work for quite a while before it starts burning oil. I did all this on a 289 with 150,000 miles on it. Total cost was $500 and that included hot tanking the heads and block, new bearings, gaskets, rings, oil pump, timing chain set, lapping the valves and one new pushrod.

 

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"Sorry to say" But the valves can't just go back in any old place.
All valve guide size our not the same with wear, plus you could end up to much or not enough clearance sucking oil or a sticking valve and bending a push rod or worse. Theres the right way and and the wrong way to do things You choose one of them, if wrong then you'll just have to re do it with more $$ out..
my 2cents
orich

 

There is a test for determining if the low compression is valves, rings or a combo of both. Just dump about a teaspoon of engine oil in the spark plug hole and run the compression test again. If the numbers jump way up 35 PSI or more, the rings are bad, if number don't come up more than about 10 PSI then its valves and if you get about a 20 to 25 PSI increase, it's both.
Lapping valves just isn't worth the effort on a properly done valve job.

 

I really don't think valve stem wear and valve guide wear can cause a valve to stick and bend a pushrod. But you make a good point about too much clearance causing oil burning. A 1/2 job, meaning the valves and heads are cut, but the valves and guides are not replaced, may benifit from matching stem/guide wear patterns. Put the valve with the least stem wear in the guide with the most wear and vice versa. For lapping only, keep them in their original locations.