re: Compression

[Matt Haas <mhaas@xxxxxxx>]

Assuming a stock engine, a jumped or severely stretched timing chain can 
cause low compression readings across all cylinders since it alters when 
the valves open and close in relation to the pistons. A leak down test will 
help pinpoint what's causing the low readings.

A quick way to check for a stretched timing chain is to pull the spark 
plugs and remove the distributor cap. Slowly turn the engine by the 
harmonic balancer bolt until the rotor starts moving. Then, slowly turn the 
engine in the other direction and see how much you have to turn it until 
the rotor starts moving again. On a new timing chain, the rotor will start 
moving almost right away. Any more than about 10 degrees is bad. Note that 
if you jerk the wrench when you turn the engine counter-clockwise, you may 
loosen the balancer bolt. If that happens, make sue you torque the bolt 
back to specifications.

A jumped timing chain is a little more difficult to check and the best way 
is to pull the timing cover and make sure the marks on the gears still line up.

Also, if your engine has a bunch of miles on it, even, low compression is 
normal and is a sign that the engine wasn't abused. You'll get better fuel 
economy and more power with a rebuild but as long as you aren't using an 
excessive amount of oil (more than about a quart very 3,000 miles) and the 
engine isn't smoking, it would be okay to leave as-is.

Matt

At 05:23 PM 2/12/2006 +0000, you wrote:
> "Normal" compression varies with the engine. A 1978 232 should have 140 
> psi, a 258 150 psi... NEW. The 82 TSM states 120-150 for the 258. I 
> believe that's pressure measured with the engine warmed up. Before you get 
> scared, 95-100 psi is still good. Should be no more than 10 psi variance 
> between adjacent cylinders, 30 psi from highest to lowest.
>
> If you read 85 psi across the board, you're probably still good, and 
> likely measured cold. You can also put a squirt of oil in the plug hole 
> and measure compression. That should increase it just about the same as if 
> the engine is warm. As it warms up some oil gets to the rings and helps 
> sealing, dry rings don't seal good.
>
> --
> Frank Swygert
> Publisher, "American Independent
> Magazine" (AIM)
> For all AMC enthusiasts
> http://farna.home.att.net/AIM.html
> (free download available!)
>
> original message----------------------------------------------
>
> Date: Sat, 11 Feb 2006 16:58:34 -0600
> From: Todd Tomason <jayscore@xxxxxxxxxxxxx>
> Subject: Compression
>
> What is normal for compression, anyway?  I did a compression test on my 
> 258 in
> my Spirit a while back, and it was about 95 lb. on each cylinder.  Is that
> normal or low?
>
> Todd

mhaas@xxxxxxx
Cincinnati, OH
http://www.mattsoldcars.com
1967 Rambler American wagon
1968 Rambler American sedan
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