need 401 buick nailhead compression help

[Todd H]

the compression on all piston except for #5 is 200psi. # 5 piston is 45psi. does any one know if its a bad piston, valves or rings... also a lot of air is being pumped into the crank case from whatever this problem is... if its a bad piston can i replace it with the motor in the car????? any info greatly appreciated... thanks Todd<p>[ 12-11-2001: Message edited by: Todd H ]

[ply33]

The old fashioned way to check for valves versus rings was is to do two compression tests: One "dry" and then again after putting some oil into the spark plug opening. If the compression comes up then your rings are bad. If it does not come up then your valves are bad.<P>I guess there are newer and more accurate ways of doing this nowadays, but I guess I am getting to be old fashioned....

[Todd H]

thanks for the reply.. with oil in, the psi went up 5-10 pounds to 50 psi.would a bad valve pump air into the crankcase?<BR> thanks todd

[Guest Hal Davis (MODEL A HAL)]

I can't see a bad valve letting compression gasses into the crankcase. My money would be on a hole in the piston, or at least a broken ring.<P>I suppose you could replace a piston with the engine in the car, but you may find other things wrong while you are in there that cannot be fixed with the engine in the car. For instance, if the cylinder wall is scored, you'll have to have it bored.

[Guest]

A fairly useful tool can be made by welding an air-chuck fitting to a spark plug shell.With the piston EXACTLY on T.D.C.,pressurize the cylinder,then listen at the carburetor,exhaust pipe,and oil filler.Works every time!