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Tractor Talk Discussion Forum

Compression test

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02-22-2021 16:13:53

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Can you do a compression test on a motor that has set several years and get an accurate reading?? I have a motor for my Oliver 88 that I bought 4 or 5 years ago and it has set in the shed all of this time. I gave it a few squirts of oil in the plug holes and turned it once in a while so be sure it was free yet. I have it in the tractor and 3 of the old plugs that were in it are oil fouled pretty good. I'd like to do a compression test before I try to start it or would I get a better reading if I start it first, run it a bit and then do the test? I usually take the plugs out and crank it 10 puffs and read it and then give them a squirt of oil and repeat to compare the readings. What say ye??

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02-23-2021 06:27:00

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 Re: Compression test in reply to super99, 02-22-2021 16:13:53  
Not a hard job and it can easily be done before you spend all the time installing everything needed to run the engine.

Could reveal major issues like;

-a hole in a piston

-a valve seized open

-a major head gasket failure

If nothing else it will give you a little piece of mind that it will run.

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02-23-2021 03:42:35

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 Re: Compression test in reply to super99, 02-22-2021 16:13:53  
You seem to indicate that it wouldn't be difficult to get it running. A test on one that has run recently would be much more meaningful.

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02-23-2021 02:03:49

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 Re: Compression test in reply to super99, 02-22-2021 16:13:53  
I did a compression and leak down test on a 2 cylinder engine. Both cylinders were almost identical. Passed both tests. The exhaust temperature on the bad cylinder was 200 degrees less.

So to answer your question can I get a good comparison test, maybe, maybe not.

Go to your Dr., the may do many tests to find your problem.


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02-22-2021 20:24:24

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 Re: Compression test in reply to super99, 02-22-2021 16:13:53  
Id put a pint of atf in each hole let it sit a few days maybe a week or so then roll it over then run the test

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02-22-2021 19:50:08

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 Re: Compression test in reply to super99, 02-22-2021 16:13:53  
I would do an oil change if you haven't already.

Get it running, be sure the thermostat is in and working, replace the plugs and do everything needed to make it run as it should. If the carb needs attention, get it right too.

Put some time on it under load if possible. Listen to the engine, watch the oil pressure, look for under load smoke, look at the crank case blow by. If it will idle smooth is a good sign. These things will really tell as much or more than the compression test.

Then set the valve lash.

Then do the compression test. Do it with a warm engine, dry first, then with a few squirts of oil in each cylinder. Spin the oil out before taking the reading. Be sure the battery is fully charged and capable of running the tests without breaking down, Cranking speed is important.

An engine with bad compression will show as poor idle, puffing out breather tube, and unequal compression readings.

An engine with bad oil rings will show as blue smoke under load, oil consumption, and plug fouling. It can have bad oil rings and still have good compression.

Blue smoke on start up is valve guides.

You can also tell a lot about an engine by the sludge build up. A sludged engine is not healthy. Sludge is a result of leaking compression rings. Severe sludge is a result of bad rings, poor maintenance, possibly running cold and/or under worked.

In rare cases, a sickly engine can be somewhat restored by correcting needed maintenance items, several oil changes using 15w 40 diesel oil, and properly working and loading the engine. If it's borderline, give this a try before jumping into it.

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02-22-2021 19:11:08

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 Re: Compression test in reply to super99, 02-22-2021 16:13:53  
U got it in the tractor so may as well start it now warm it up and check it It only takes 5 puffs per cyl. After that you are only building couple lbs at a time and it dont matter. No sense running the battery down The first two puffs pay attention to. It only takes one compression stroke to start the engine. But u can still check it if u want cold. The valve seats might have junk in them so might not get accurate test.

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02-22-2021 16:33:22

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 Re: Compression test in reply to super99, 02-22-2021 16:13:53  
Take all the plugs out and spin it over for 20 or 30 seconds to blow all the oil out.

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02-22-2021 16:30:43

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 Re: Compression test in reply to super99, 02-22-2021 16:13:53  
I'd spin it over with the plugs out then put maybe a 1/4 cup of ATF in each cylinder and let it sit a few days. Then spin it over with the plugs out again then check compression. I have an Oliver 60 engine that tested very low on compression but after doing the ATF thing compression came up to over 125 per cylinder

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