Re: engine compression

Posted by Bill Smith on February 28, 2003 at 08:43:08 from (128.242.4.59):

In Reply to: engine compression posted by Chad on February 28, 2003 at 06:30:46:

First off, I might say that if the engine is sitting right (TDC) it is possible to turn the engine slightly with ease by the front pully. It takes a half of a turn in the clockwise rotation for a piston to complete a compression stroke. If turning slow enough, the compression build up will be lost, especially if rings are dry. The best way to do a compression test is to first start the tractor and let it run for a bit first to allow the internal engine parts(rings and so on) be oiled naturally by engine. This will give you a more accurate first test. Secondly, you will probably want to have manifold and carb installed with throttle open. Not haveing them on there will affect the intake. I would guess that not having them would give you a higher reading instead of a lower reading because it would lesson the restriction of air flow to the valve and allow the piston to draw more air during intake. After installing the guage tool to the spark plug hole, ussually turning the engine over with starter so that that cylinder completes 2 or 3 compression strokes will give you a fairly accurate reading. Then you can add additional oil to the cylinder to check compression loss through the rings. Not allowing the engine to be naturally oiled before hand would make this testing inaccurate. Engines that have been sitting for a spell, allows some of the oil from rings and what not to drain back to the pan and become somewhat dryer. Also a cold engine may give you somewhat different readings than a warmed up engine. Compression test would also be more accurate on a warmed up engine. Another reason to pre-run the engine before testing. Bottom line is that compression test is more accurate when cylinder is at normal run conditions. Being dry or cold from engine sitting for who knows how long, is not a normal running condition for the cylinder. Well, only upon start up of tractor. Just my 2 cents worth.

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