Compression PSI
 

I checked the compression on my 1970 1/2 ton SWB 350-4 bolt today and every cylinder was between 120-130 psi. The manual states it should be 150 psi. My question is, should I rebuild or get a new long block, or leave it as is? The motor is a bit tired, and I'm getting ready to repaint it, so I'm debating if I should change it out before painting.

Any suggestions?

That is not really bad compression numbers. Mkae sure you have a freshly charged battery, no sparkies in the block. Also do a oil squirt in the cylinder test to compare the numbers, it will also tell you if the rings are worn or the valves are shot. A little more work here will pay off in saving you money for work that you did'nt need to do.

yeah I would definetly not re-build if the numbers are that high, of course I have no clue what you are going to use it for, daily driver? street/strip?

Did you test with the throtte wide open...it makes a big difference! Also crank about 3-5 compression strokes...I know you probably know this but I forget sometimes so I am only providing you a reminder here...LOL!

one of the biggest factors in comp(cranking pressure), is that all cyls are within 10 % highest to lowest. it doesnt sound too bad from that standpoint. you could also have a loose timing chain, retarding the cam timing & also causing slightly lower cranking #, or if the eng was not warmed up, & throttle proped open during the test, the # will read a little low....just fruit for thought. Best of luck,crazyL :burnout:

Thanks for the information.

The engine was cold when I did the test, and the throttle was not wide open. Do you have any idea what percentage difference +/- these two things make on average.? I assume that the PSI would go down, but by how much? 5-10% maybe?

Its not a daily driver.....I use it to go to Home Depot, kids baseball practice, etc. I can live with it this way if I can get away with it. Thanks for the help.