Compression Testing...

Is there a thread on this site, or on another website, or a book, that has detailed, step-by-step instructions for testing compression? (On a Ferguson TO-20, in my case.)

I"ve never done it, but I can read and follow instructions, and it doesn"t sound too difficult.

Are gauges usually available for rent/borrowing from auto parts stores? I have a local parts store, and an O"Reilly"s nearby.

Thanks.
 
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.
 
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?
 
Yeah, just go to the O'Rielly's or autozone, where I bought mine, they will tell you how to use it, they have a choice of adaptors to screw into sparkplug or injector holes. Don't bother with the ones that have a rubber grommet and need to be pressed with muscles, they just don't work.
$35 should get you a test kit that will last for ages.
 
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.
 
(quoted from post at 09:21:29 03/05/13) Is there a thread on this site, or on another website, or a book, that has detailed, step-by-step instructions for testing compression? (On a Ferguson TO-20, in my case.)

I"ve never done it, but I can read and follow instructions, and it doesn"t sound too difficult.

Are gauges usually available for rent/borrowing from auto parts stores? I have a local parts store, and an O"Reilly"s nearby.

Thanks.

1. Remove all spark plugs
2. Be sure fuel is shut off
3. Attach battery charger to battery (the test can be skewed by lower battery voltage, esp on the last cylinders you test)
4. Set throttle lever to full open
5. Screw the gauge in and crank the motor over

A compression test is more of a comparison test between cylinders rather than a pass/fail kind of deal. The maximum pressure is a determined by things like compression ratio, speed of the starter motor, ambient temperature, and other factors. You want to compare the readings of the cylinders to each other rather than looking for a specific number, although anything less than 90 psi does indicate a problem. A 10% or less variation between cylinders is ideal, but the motor will still be OK with anything less than 20%.
If you have two cylinders with lower compression that are side-by-side, that usually indicates a blown head gasket between those two cylinders.
If you have a [u:77b11e398f]much [/u:77b11e398f]lower reading with a dry cylinder than a wet one, you probably have a ring seal problem.
You can use a two-gauge cylinder leak down tester for more meaningful results.
Remove the plug for the cylinder you're testing and bring the piston to the top of the compression stroke. Install the tester and shoot air into it. Don't put too much air in or the piston will be pushed down by the pressure, but get at least 50 psi. You may have to hold the crank nut to keep the piston up.
Compare the readings between the two gauges. One gauge will read the air pressure you're applying to the cylinder, the other gauge shows how much the cylinder is holding. I.E., if you're putting 50 psi in and the other gauge reads 40 psi, you have 20% leak down. Another benefit of this test is you can tell where the cylinder is leaking. If you feel air coming from the exhaust pipe, the exhaust valve isn't sealing. Same for an intake valve if you feel air coning out of the carb. Pull the dipstick and, if air's coming out of it, the ring seal is suspect. If the head gasket is bad, you can feel the air coming out of the adjacent spark plug hole if you pull the plug out of that cylinder. Sometimes you can find a cracked head using this tool. If the crack's large enough, you'll see air bubbles coming up in the coolant when you remove the radiator cap and look in the radiator.
Hope this helped,
BillL

EDIT: Jason types faster than me. :D Good info from him, too.
 
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?
 
(quoted from post at 12:04:00 03/05/13)
One (probably dumb) question: why do you open the throttle all the way?

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:
BillL
 
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.
 
I always disconnect the air cleaner at the carb so that a partially clogged air cleaner does not affect the results. I would also conduct the test with the engine at operating temperature and would also conduct the tests both dry nd "wet"(add several ounces of motor oil to each cylinder and repeat the test) recording both numbers.

Very low compression readings are generally indicative of a stuck open or burned valve. Low pressures that don"t improve from dry to wet are generally indicative of leaking valves.
 
Its all relative,is the engine warm, is it cold outside, blaa, blaa, blaa.Mine reads cold 120. At idle it wants to choke down, give it gas it comes to life! My other 35 has 140 cold, at throttle you feel the difference, then the standard 35 at 140 compression is hard to crank but has momumental power cold hot or in between.But I don't wanta know the hot numbers.
 

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