compression test

[John *.?-!.* cub owner]

Scrivet's cub is in my shed waiting on a new clutch hanger and rod, and since it is a smoker I decided to do some testing on it.

First off I did a leak down test, which showed cylinders 2 & 4 to have small leaks at the exhaust valves, and 1 & 3 were leaking past the rings. 2 & 4 were probably also leaking past the rings, but the valve leaks combined with a faulty pressure regulator kept me from telling for sure.

After that I decided to do a little experimenting with a compression test using different cranking speeds.



Tractor had not be run in 3 or 4 days. Testing was done with all plugs removed and throttle wide open. the tractor was split, so I used double jumper cables to make sure I had enough current capacity.

The hand cranking and 6 volt cranking were closer together than I expected, but the difference between cranking with a 6 volt battery and a 12 volt battery was wider than I expected.

[DieselDennis]

Please allow my tangent here John,

But are you saying that even though all the cylinders tested 115-120 on 12V wet, they all had leaks?

My tractor doesn't smoke, but only had compression tests a little above 100, and weren't near that close together as a group.

[Scrivet]

......... I decided to do some testing on it............

Some of the most frightening words I ever heard John utter. This is going to end up with some "might as wells" and a trip to Belleview isn't it?

I think I just need to convert it to twelve volts, it's got a lot more compression there

I can't make heads or tails of the numbers because they all have dollar signs in front of them on my screen. I am surprised at a roughly 25% "increase" in compression between using 6V and 12V. Would not have expected that.

[John *.?-!.* cub owner]

Please allow my tangent here John,

But are you saying that even though all the cylinders tested 115-120 on 12V wet, they all had leaks?

My tractor doesn't smoke, but only had compression tests a little above 100, and weren't near that close together as a group.

The big jump in compression between the dry hand crank and 6 volt and the "wet" (couple squirts of oil in cylinder) are what shows the leaks around rings either from worn rings or worn cylinders or both. It is a 6 volt system, but I had a 12 volt battery handy, so I did my first compression test with it. the smoking and compression readings were at odds, so I redid it with a 6 volt battery and hand cranking, then redid them all with wet cylinders. The test using 6 volt battery and hand crank both confirmed my suspicions, but it cranks over around twice as fast (less time for air to leak out on each stroke) with the 12 volt battery which makes the compression show higher than normal so far as I can tell.

A leak down tester allows you to put compressed air into the cylinder at a controlled rate and shows the quality of the compression by the air pressure on the cylinder side of an orifice, and also allows you to listen for leaks at carburetor, exhaust pipe, and crankcase to tell where air is leaking to diagnose the cause of low compression.

[DickB]

Scrivet's cub is in my shed waiting on a new clutch hanger and rod, and since it is a smoker I decided to do some testing on it.

First off I did a leak down test, which showed cylinders 2 & 4 to have small leaks at the exhaust valves, and 1 & 3 were leaking past the rings. 2 & 4 were probably also leaking past the rings, but the valve leaks combined with a faulty pressure regulator kept me from telling for sure.

After that I decided to do a little experimenting with a compression test using different cranking speeds.



Tractor had not be run in 3 or 4 days. Testing was done with all plugs removed and throttle wide open. the tractor was split, so I used double jumper cables to make sure I had enough current capacity.

The hand cranking and 6 volt cranking were closer together than I expected, but the difference between cranking with a 6 volt battery and a 12 volt battery was wider than I expected.

John. What a nice contribution to the forum. Now, for dummies like me, please explain. Like, I thought to test for compression one bought something from a shop, took out a plug, hand turned the engine, and read on a dial what the psi was. Do that for each cylinder and then what? Well, you're not only checking out compression but able to understand if it is the rings or the exhaust valves. How did you do that?

There's a bit of smoking coming from my tractor. In time I'll have to man-up and figure out what's going on, and how to fix it. So, please educate, John.

Best, DickB

[Eugene]

Wet test seals the rings. A significant increase in psi from the dry test to wet test would indicate the major part of the problem is with the rings. A minor increase in psi from the dry test to wet test indicates valve problem.

One of my Cub's has some blowby and uses a bit of oil. The dry compression test is 105 psi, dead even on all cylinders. An engine rebuild would reduce or eliminate the blowby - - but not significantly increase the power. Not an immediate candidate for an engine overhaul.

Significant blowby is an indication of worn or stuck piston rings. You need to go through the entire engine when conducting an overhaul, not just replacing piston rings.

[Matt Kirsch]

John also did a "leak down test" which is what indicated the loss of compression through the valves. It does not yield discrete numbers, just symptoms. You'd use a tool like this for leak down testing:

http://www.harborfreight.com/cylinder-l ... 94190.html

You use air from your compressor to pump up each cylinder to a fixed pressure, then remove the air and the meter tells you how bad things are. Intake or exhaust depends on which hole you hear the air hissing from (i.e. the carburetor or the muffler).

[John *.?-!.* cub owner]

Eugene and Matt gave you good answers, with the exception that the gauge Matt linked to which is the one I have, you do not remove the air supply, you adjust it to a set point on the right dial with the regulator, then connect the second hose with the adapter. there is a small orifice inside the body between the two gauges that limits the amount of air that can pass through. With each cylinder at TDC (top dead center) read the left gauge. It will give you comparative readings on each cylinder, plus you also listen for air leaking in the intake, exhaust, or crankcase to help determine why the compression is low.

[mchattod]

Funny this subject came up. I just did a compression check on my Cub because, I'm way down on power. My results are now suspect because, this post made me research cranking speed as a factor. My battery was pretty tired when I ran my test. I'm going to re-do the test with a fully charged battery.

I found this great link on compression testing http://www.gregsengine.com/cylinder-compression-testing.html

From John's test, the wet and dry numbers suggest the rings may be worn but, I wouldn't take them too serious since, the test was conducted on a cold engine.

Leakdown testers are great for not only diagnosing the source of problems (valves, rings, etc.). They're also a good health check. One gauge is reading what you put in, the other gauge measures what leaks out. Leak of 20% to 30% bad. Under 20% good. I always input 100psi. Makes the math easier. If your output gauge reads 80psi, you're leaking 20%.

Here's a link on leak down testers: http://www.carcraft.com/techarticles/116_0406_cylinder_leakdown_tester/

[Scrivet]

Anybody wanna take a guess where my engine is residing at the moment?????????

........This is going to end up with some "might as wells" and a trip to Belleview isn't it?...........

By the way Belleview is a town in MO with a great machine shop not the Bellvue Mental Hospital from the Barney Miller TV show.

[John *.?-!.* cub owner]

According to machinist, some of the cylinders were close enough to just re ring, but some were not. I told him to go ahead, it ain't my money.

[BrianRoberts]

I want to add a bit of info when using a leakdown tester always start with using 15 - 20 psi applied to the cylinder and increase as needed to find your leak and determine how bad the leak is. Using 100psi or more to start with can cause valves to seal off in the case of worn valve guides or carbon buildup on the valves and/or seats. I use a dual gauge Snap-On leakdown tester and have found many leaks that would not show using higher pressure. By the way I am a General Motors Technician so I get plenty of time using this stuff. Good post John.

[mchattod]

Thanks Brian!! This post is teaching more and more every day. Your tip on starting at low pressure and John's post on cranking speed have been a real eye-opener. I plan to run my own tests this weekend and start my own post. Until then...