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Re: Compression Testing...

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Jason S.

03-05-2013 08:29:21

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Most auto parts stores have one for rent, but even if they don't a decent one doesn't cost that much. Just remove all your spark plugs, open the throttle all the way,screw the gauge in,and spin the motor over until the gauge quits going any higher.Then release the compression on the gauge,unscrew the gauge and repeat on the next cylinder. Then when your done compare the readings from all cylinders. Ideally they should be within 10% of each other.

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Tom in MO

03-05-2013 08:40:11

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 Re: Compression Testing... in reply to Jason S., 03-05-2013 08:29:21  
Thanks, Jason.

The procedure you describe would be for the "dry" test, correct? As I understand, for "wet", you squirt a teaspoon of oil in each cylinder?

What do I learn from the difference between wet and dry? What should each set of numbers be?

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Jason S.

03-05-2013 10:45:19

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 Re: Compression Testing... in reply to Tom in MO, 03-05-2013 08:40:11  
If the compression comes up with oil squirted into the cylinder,that means the rings are worn. If it doesn't come up then that means you are loosing compression due to a valve not seating or a blown head gasket. There are way to many variables to tell you what your compression readings should be. If the engine has been rebuilt, a different thickness head gasket will change it, if was rebuilt with dished instead of flat top pistons it will change it, when they recut the valve seats in the head which sinks the valve lower in the combustions chamber will change it. Also the camshaft is different between a TO20 and a TO30 which will change cylinder pressure. Btw....the TO30 is the better camshaft to have and I think most of the replacements are ground to the TO30 specs. That's why I said as long as the cylinders are within 10% of each other.

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Tom in MO

03-05-2013 11:04:00

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 Re: Compression Testing... in reply to Jason S., 03-05-2013 10:45:19  
Thanks, all - that"s exactly what I was looking for. (Especially the numbered steps, ARK, since that"s how my brain works best.)

The tractor runs fine, and I don"t have any reason to suspect that I have a major problem. (Tiny coolant leak at the head gasket, and a few bubbles in the radiator, as discussed in another thread.) I guess I"m just curious.

One (probably dumb) question: why do you open the throttle all the way?

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03-05-2013 11:22:28

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 Re: Compression Testing... in reply to Tom in MO, 03-05-2013 11:04:00  
Quoting Removed, click Modern View to see

So that the air going into the cylinder is unrestricted. It's another means of getting consistent results from cylinder-to-cylinder.

BTW, there aren't any dumb questions, but I sometimes give dumb answers. :oops:

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Tom in MO

03-05-2013 11:39:19

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 Re: Compression Testing... in reply to Ark68SS, 03-05-2013 11:22:28  
Thanks, Bill - that makes sense.

Regarding unrestricted airflow: it seems like I read somewhere (probably the Internet!) that you unhooked the carb or the air intake to get full airflow?

That sounded a little extreme, and that was one of the reasons I was looking for a clear, concise description of the procedure.

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