Leak Down Test ??

Alpinegold · Feb 20, 2008

Has anyone heard of a leak down test or maybe it's a link down test. It has something to do with the cylinders and it is different from a compression test. It is measured by cylinder in this fashion. Example of 4 cylinder

8% 10% 11% 20%

Where in the 20% reading would represnt a weak cylinder.

HighTrim · Feb 20, 2008

Re: Leak Down Test ??

Main difference from the leak down and the compression test is that the leak down measures pressure lost, instead of measuring pressure, as does the compression test.

Not really used on outboards though, mainly autos.

When you feed in the compressed air, you normally would like to see 5 to 10 percent leakage, anymore than 30 percent and you have some issues that need dealing with, or in your example, the cylinder with 20 percent leakage could have issues.

ezeke · Feb 20, 2008

Re: Leak Down Test ??

I think it would be fairer to say that it is more commonly used on engines with valves.

Some of the most knowledgeable experts on this forum have even gone so far as to say that they have never done a leak down test on a two stroke.

R.Johnson · Feb 20, 2008

Re: Leak Down Test ??

I have had good results with the leak down test. It will show a problem much better than a compression test, as if there is a leak, you can hear it, as well as see'ing the difference on the guage. Anytime I have seen a 10% difference on the guage, there was a definite problem with the cylinder. To do a leak down test, you must have a good stop tool to lock the engine.

Reelized · Feb 20, 2008

Re: Leak Down Test ??

If you going to try with an outboard,you have to lock the flywheel in place.Otherwise when you pressurize your piston is going to move down to the intake/exhaust ports and not pressurize.Also the spinning flywheel can do some damage to your fingers.

flabum · Feb 20, 2008

Re: Leak Down Test ??

Leak down tests are a more accurate way to see the condition of a cylinder.

A compression test will show you a bad cylinder by how much pressure it can build in a certain number of cycles. If a cylinder comes up slow or doesn't come withing 10 - 15% of the highest cylinder, then you know you got a problem.

A leak down test tells you how much compression is getting past the rings (in a 2 stroke) or past the rings or valves (in a 4 stroke). The lower the percentage, the better the seal is in the cylinder. Most racers look for 5% or less.

With two stroke engines, for most mechanics and D.I.Y.'s a compression tester is adequate to do the job. Four stroke mechanics find the leak down tester invaluable in diagnosing an engine with bad valves or rings.

rodbolt · Feb 21, 2008

Re: Leak Down Test ??

a leak down test simply measures the cylinders ability to seal. the lost air is read on the second guage as the amount of air flow it takes to maintain the test pressure in the cylinder. if at TDC there is no need to lock the flywheel as it will stay at TDC, if off the piston will push down.
if you attempt to hold the flywheel with a breaker bar make sure your medical insurance is up to date

R.Johnson · Feb 21, 2008

Re: Leak Down Test ??

As Rodbolt stated, don't try to hold the flywheel with a breaker bar. I put the piston on the cylinder to be tested at slightly over top dead center, then lock the opposing cylinder with a piston stop tool I made. A compression test can give you a false indication. I have found more than once for the cylinder giving the highest reading, being the one that is damaged, A leak down will show this right away.

Alpinegold · Feb 21, 2008

Re: Leak Down Test ??

Well, I just thought I was out of the dog house. I had to put a new power pack on the port side the other day. He did a compression test the following results:

140 140 140 140

The leak down test showed these results :

8% 10% 12% 20%

He told me he did something to improve the 20% cylinder, but it was something to keep an eye on. ??

The dang thing runs like a spotted ringtailed wampus cat but what am I to expect with the % being so high on the 1 cylinder. It is not in my budget to redo a cylinder right now but will running it ruin my motor.

Alpine

iwombat · Feb 21, 2008

Re: Leak Down Test ??

Leak downs are most useful in 4-strokes to determine the wear pattern of the cylinder at various positions. Generally, you do a leak down on bdc, tdc and middle. This doesn't make much sense on a 2-stroke given the ports are cut into the side of the cylinder. On a 4-stroke you can have decent compression, but still get lots of blow-by and oil fouling if the middle of the cylinder is excessively worn but the rings are in good shape.

Compression tests are really all you need to do for a 2-stroke IMHO.

Alpinegold · Feb 21, 2008

Re: Leak Down Test ??

I sure hope your right Iwombat. I could use a break.

iwombat · Feb 21, 2008

Re: Leak Down Test ??

I'm also suspicious of numbers that get progressively worse in any instance. It points to something systematically wrong in the testing more often than not.

HighTrim · Feb 21, 2008

Re: Leak Down Test ??

Im curious what he did that you have to "keep an eye on" ??

R.Johnson · Feb 21, 2008

Re: Leak Down Test ??

On a two stroke you are looking for the sealing qualities above the ports', or to say, between rings', and cylinder wall. This is where a leak down test really works well, it's a good test for any engine. If the engine were mine, I would remove the head, and take a look at that low cylinder. If it has a scuffed piston, or cylinder wall, running will only make it worse, or more exspensive to repair. That cylinder is low for a reason. I have no idea what may have been done to improve that low cylinder, but my best luck has aways been with a wrench.

iwombat · Feb 21, 2008

Re: Leak Down Test ??

Except, you'd see scuffs and wear issues come up in the compression test too. I just look at the numbers creeping upwards and think testing problem. Then I look at the dead-even compression numbers and really think testing problem.

flabum · Feb 21, 2008

Re: Leak Down Test ??

unless it took 20 revolutions to get the same reading as the other cylinder that got there with only 6

Alpinegold · Feb 22, 2008

Re: Leak Down Test ??

Thanks guys so much for your input. Spring time is almost upon us and that means the crappie will be sponding soon in Texas. I'm going to run that old boat with a little caution and I'll let you know if it blows up. Right now it just screams across the water and that just makes fishing even more fun!

Alpine

Happyhunter710 · Mar 29, 2024

HighTrim said:
Re: Leak Down Test ??

Main difference from the leak down and the compression test is that the leak down measures pressure lost, instead of measuring pressure, as does the compression test.

Not really used on outboards though, mainly autos.

When you feed in the compressed air, you normally would like to see 5 to 10 percent leakage, anymore than 30 percent and you have some issues that need dealing with, or in your example, the cylinder with 20 percent leakage could have issues.

View attachment 7141

What options do you have for a two stroke that failed a leak down test? Is the motor complete scrap at that point or can you reboar the cylinders?.

Leak Down Test ??

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