At what point to you consider a complete rebuild? I just finished a compression check on all 6 cylinder in my 61 f100 , 1-90psi, 2-68psi, 3-70psi, 4-80psi, 5-90psi, 6-90psi. I read that the compression should be 150... Not the outcome I was hoping for.

 

Believe the highest and lowest should'n’t vary by more than 15 percent and as best as I can determine anything below 76 PSI is not within the suggested limits. May want to try adding a good squirt of motor oil down the spark plug opening on the suspected cylinders and if the reading is the same, the valves either are worn (and letting pressure escape) or are out of adjustment. If the reading rises dramatically after you insert the oil, you probably need new rings on the piston in that cylinder.

If the engine/truck has been setting for an extended period of time you may try putting a few miles on the engine then recheck the compression? Recently rescued my 65 with the 390 after it had been setting in the elements for past 6 plus years. Towed home, drained tank, changed fluids and filters then ran a compression test and found there were a couple cylinders that were just below allowable limits. The engine does not have many miles since rebuild so decided to put a few miles on the engine then recheck compression and found the compression had improved. Anyhow, food for thought??

 

Dave,
Thanks for the input, the truck has maybe 100 miles on it in the last 10 years. I bought the truck a year or so ago and it was hard to start and I am still having a hard time to get it started. Any input would be appreciated.

 

You may have stuck rings. This worked for me on an old motorcycle - put a couple of spoonfuls of Marvel Mystery Oil in the cylinders and let it sit for several days. Then rotate the engine. Then try your compression test. If you are still low try a leak down test to determine if its rings or valves that is your trouble.

 

Better, replace 1 quart of motor oil in the crankcase with a quart of Marvel's according to instructions. It's a type of penetrating oil and top cylinder lubricant designed for stuck rings and lifters. It's also suggested on the can to run about 4 oz per 10 gallons in the fuel. Sort of attack the problem from both ends. Worth a try.