re: Compression
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re: Compression



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.

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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 =============================================================== According to a February survey of Internet holdouts released by UCLA's Center for Communication Policy, people cite not having a computer as the No. 1 reason they won't go online.








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