Re: Cylinder Compression Questions

Posted by David Grubbs On 2014/11/14 20:10:06

I ran a compression test on my 39 120 last month, using a screw in type compression gauge. My car runs decently, but hasn't be overhauled since Jimmy Carter was the President. With the engine cold, all spark plugs removed and the throttle held open, I turned the engine over at least six times on each cylinder. Got readings of 40 to 50. One of the problems in getting a good reading is the small 10 mm plugs - if you are using a rubber cone type compression gauge it is even more difficult to get an accurate reading. I found out that the readings tend to be lower when the engine is cold. If the readings are fairly uniform, I wouldn't worry too much just yet. The earlier comments about the electrical system is right on - if you can replace the points and condenser, rotor and cap, and clean and gap the plugs, then get the timing and dwell set, the engine should start. Once you get it running and warm, try the compression test again. I suspect that it will be up in the more acceptable range of 75-80 (which isn't bad on an older engine).

The Marvel Mystery Oil in the cylinders is a good treatment before attempting to start the engine. The old flatheads were prone to rusting around the valves when they weren't run regularly or sat for a while. Removing the side panels and squirting some PB blaster on the valves isn't a bad idea. While the panels are off, have someone crank over the engine while you watch the valves.

Last hint - find the nearest Packard Club and join. You will find people with the same experience and plenty of advice.