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Old 06-27-2006, 07:15 PM
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Compression Testing ....(Hot vs Cold)?

Hello,

My 289 was rebuilt 4 summers ago now and I'm not sure if my rings are still seating.

The reason I say this is there is a difference in the hot vs cold readings.
Basically the "cold" readings are:
151,149,136,148,155,150,140,133

The "hot" readings are:
160,160,160,165,152,155,155,150.

Is there supposed to be that much of adifference between the hot and cold readings?

Also, when i squirt oil in the cylinders some of the jump up to 170 and 175.

Does this tell me anything about my rings and if they are seated properly?

Thanks, Greg.

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Old 06-29-2006, 02:03 AM
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well as your engine warms up the metal of the rings will expand giving it less of a gap between the cylinder wall and the piston groove. So it just might be that less compression is escaping because there is less area for it to move through. Your compression is defently better when warmed up but i think that could be just from normal wear on the rings themselves. anyone else have any ideas?
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Old 06-29-2006, 05:04 AM
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The generally agreed upon standard is 10%. You can run another test cold by squirting a small quantity of oil in the cylinder, turn the engine over a few times and recheck the compression. If there is 10% differance then the rings are worn.

You say the engine was rebuilt. Was it bored and new pistons installed, or just a ring job?

Vince
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Old 06-30-2006, 12:01 PM
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Actually the difference between hot and cold is usually caused by piston expansion. A cold piston is smaller and rocks back and forth in the cylinder more which allows the rings to not mate to the cylinder walls as well. That is why the oil helps. The ring end gap certainly changes but is not the primary ring compression loss. Some piston materials require a larger clearance and a larger clearance will flop more.

A hot test using the differential pressure testing method is preferred and will indicate where the loss is occuring.

Of course you had all the spark plugs removed and the carb venturi full open when you did the test.
Using the hot cranking method and letting the cylinders pump 5-8 times your readings are acceptable. You should also note how many pumps it took to get the highest reading on each cylinder.
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