OK I replaced my 240 head with a 300. I have everything torque down right, but when I do a compresion test I get 60. I read in a chilton book that it should be around 130. Now I am not sure if I got a bad compression gauge or something is really wrong. I am really stumped due to the fact I am helping my neighbor do all this work.
When I read the instruction that came with the compression gauge. It says let the truck run for about 10 15 mins and then do it. When my neighbor talk to some mechnics (sry bad speller) in town, that he doesn't have to do that step. The booklet also said to keep in the spark plugs, but lossen one turn. We tried that and it didn't work, still held at 60lbs. Is there a chamber differnce between the 2 head it that why we can't get a good reading. Does everything need to be put back together. like the headers, intake...will that even make a difference. We are so stumped and just plain tired at this moment, that my brain is starting to hurt. Any thoughts or comments are all welcome.

I didn't list anything we did, because frankly I really can't remember what all did.

 

The compression tests that I have done in the past were down by removing all the spark plugs. You can test the motor either warmed up or cold, but the results may very a little. Thread your tester into one of the spark plug holes and turn the motor over about 5 to 6 revolutions. Watch what the numbers are the first few strokes and on the last one. The speed in which the motor turns over can have an effect on the results also. Normally good compression numbers are over 100 to about 150 or so. Need to keep in mind also the higher the compression ratio the higher the resulting number will be. Did you happen to change the timing any while working on the motor? If you are only getting 60 in all cylinders and the numbers a correct I would think you maybe getting some kind of blow by thru your valve cover. I don't know if there is a difference between a 240 and 300 head. The combustion chamber may be a little smaller but I'm not sure on that. I don't think it would drop your numbers down to 60 psi. Are you sure the valve clearance is correct.

Sounds like maybe the valves aren't closeing all the way. Maybe try and loosen the rocker arm adjusting nut a little on one cylinder and try the compression test just on that cylinder to see if it makes a difference.

One other thing they say to try which I haven't ever tried is to put a little bit of engine oil into the spark plug hole and see if that raises the compression. If it helps then the rings could possibly be bad.

 

The compression on my (96,000miles or so) '89 F150 SWB 4WD I-6 is 165psi (+/-3psi: lowest cyl is 163, highest cyl is 168psi). I could never find a reference as to what it really should be.

 

I don't think you'll ever find a reference number for compression. It was explained to me once that there a few variables that can change the compression number. How fast the starter turns the motor over, what the engine compression is (calculated compression 8.0 to 1 or 9.0 to 1) can all make a difference. If your getting 165 on all cylinders then it sound like your motor os in pretty good shape. Once the numbers drop down to 100 or below then start to worry.