Compression readings

[John Hofstetter <hofs@domain.elided>]

>Date: Tue, 08 Dec 1998 18:14:16 -0600
>From: Scott Machalk <smachalk@domain.elided>
>Subject: Compression
>
>I ran a compression check a few weeks ago and I would like the opinion of
>the more knowlegeable members of the digest.

Don't claim to be one of these. One of the most opinionated, yes.
>
>1  150
>2  155
>3  95
>4  150
>5  125
>6  125
>7  140
>8  150

Scott, 
As you've figured out, you have a particular problem with #3, and a 
couple of the others are borderline. Rule of thumb, all cylinders should 
be within 10% of each others. On #3, you need to take out the spark plug, 
squirt some motor oil in that cylinder, and recheck the compression. If 
the compression goes up significantly, you have a ring problem. If it 
doesn't go up significantly, you have a valve problem and get by very 
nicely by working on the head. (For your sake, I hope it is a valve)  
IMHO, significantly as I used it above, is around 20 lbs. or more. Some 
guys use an air check instead of my oil-in-the-cylinder method. 

To do that, they make an adaptor out of an old spark plug that will take 
an air compressor fitting. They bring the piston to TDC on the firing 
cycle as determined by looking at the rotor position and apply air to the 
cylinder. They then listen to where the air is escaping, if it is the 
rings, the air comes out the crankcase openings such as the oil filler 
opeining. If it is an exhaust valve, the air escapes from the exhaust 
pipe, and if it is an intake valve, the air comes back through the 
manifold. I have trouble hearing the air, so I don't use this method, but 
lots of guys swear by it. 

John H.