How does a Compression "Leak Down" tester work? (2024)

Warm/hot engine set cylinder to be tested at TDC compression. Have someone stand on the rear brake with G/B in 4th, connect leak-down tester, set supply air pressure at 60 to 80 psi (pushes compression ring out) check the other gage to see the difference between supply pressure and what the cylinder will hold, should be 10% of less difference, but becomes moot, to a point, if both cylinders are equal. piston should be at TDC when you test; 5 degrees (+-) is, probably OK, more than that the readings become less meaningful.

WARNING: If you hold the engine with a socket on the drive side and don't hold it as if your life depended on it while SLOWLY increasing supply air pressure serious damage can be done to you and/or the motorcycle.

Best.

I have the same one as in Jimbo's post. It works well.
It's a low pressure type, needing about 20 psi. Hook up to a standard air supply, mine is 110psi, then use the tester's regulator to adjust to the " test" position on the gauge. This is done with the tester disconnected from the machine hose. Have the machine hose threaded into the spark plug hole and ready to go. Have the engine locked as described above. I jam the brake with a 2x2 or similar stick of wood.
Once the regulator has been adjusted to get the tester needle on " test", hook up the tester to the quick coupler on the machine hose, then take note of the reading. It will show a higher % loss than high pressure testers do. As long as the needle is in the green, you are doing well.
High up in the green is better than low down, obviously.
If you have a lot of leakage you will be able to hear where it's coming from.

Glen

A Leak Down Compression Checker is also commonly called a Pressure Differential Compression Tester, which is used in the aircraft industry. If you do a computer check on a pressure differential compression tester you will probably find a number of hits describing how they work. Here is an example link:
Recipricating Engine Differential

I hope this is of some help,
Peter Joe

jimbo said:

but there is no explanation on how it works......

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Post 7

jimbo said:

My question is how it works not how to use it. And I thought the beginning pressure was 100psi.

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jimbo said:

My question is how it works not how to use it. And I thought the beginning pressure was 100psi.

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jimbo said:

this is how I believe it works. There are two gauges with the same range. On the red cased set the one gauge has a normal 100psi scale and the other has a percentage scale that reads 100% to 0% . But they are the same otherwise. When compressor air is introduced, the operator needs to adjust the air pressure by the built in regulator to a exactly 100 psi. reading on the first gauge. There must be a orifice between the two otherwise they would always read the same . So the compressor will maintain the air volume at the necessary 100 psi , because the orifice restriction only allows a portion of it into the cylinder. Different sized orifices would change the overall test outcome wouldn't they?

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I realize you want to know what's happening internally, but , if the unit is the same as mine, you won't get any useful info from it if you regulate to 100 psi.
Use the regulator to bring the second gauge up into " set" range, then hook onto the bike hose.
This requires about 20psi on my tester
This info is in the instructions that came with my unit.
My apologies if you have a high pressure tester. I don't think you have.
The low pressure testers are claimed to be more sensitive to leakage than the high pressure units. I'm not sure why that is so, but having used both types, it seems correct.

Glen

FWIW, we typically used Snap-On leak down testers which specified 90-100 PSI. At the time - this was a few years ago - the low pressure units were not considered suitable for serious engine work because it was felt they they didn't sufficiently pressurize the affected components - particularly the piston rings. Perhaps this has changed with the modern low-pressure testers or maybe it was determined after numerous comparisons, that there wasn't much difference in actual readings. But I have no experience with a low pressure leak down units - still use one of the Snap-Ons for personal engine work.

In any case, a really important part of leak down testing is that the engine be at/close to operating temperature. A leak down test conducted when the engine is cold can be very inaccurate, indicating a problem where none exists or, on the other hand, NOT indicating a problem that does exist.

We never used complicated and unnecessary equipment like that.
We used the bottom of a spark plug and welded an Aro male coupling to it. With a hot engine, we locked the engine at TDC and fitted an airline to it at 120psi. Listen at the carb and the exhaust and you should expect no noises. Listen at the breather and you will here slight hissing noise if all is well. A rush of air indicates a stuffed piston or rings.,
Even a good engine will leak a bit past the rings. With experience, we could tell how worn an engine was, We also used a wee device we fitted over the filler cap which indicated how much pressure was in the crank case. Ideally with a correct breather, a good engine would have a vacuum in the c/case. A worn engine will indicate pressure. A badly worn engine was obvious when you took the filler cap of as your could hear it. A worn engine also exhibited excessive signs of "Salad Dressing" inside the tappet cover and filler cap.

LEAKS AT CARB OR EXHAUST TOLD US WHICH VALVE WAS LEAKING.

Salad Dressing. A combination of water and oil mix. An emulsion.

At 80 psi my BSA clutch slips.
At 120 I would expect quite a few clutches or brakes will slip. I was using a high pressure unit and sometimes struggling to keep things locked.
The low pressure unit doesn't take much to hold, but it gives you the info needed.
It's very simple to use for those of us not born knowing the exact sound of worn rings or valves vs serviceable.
It puts a number on it.

Glen

"Differential testing is far superior to a conventional compression test."

Absolutely!! We always considered a generic compression test to be 1/3 of a "Test." The other 2/3 was a leak down test. OTOH, if a compression test showed a low cylinder(s) then a teardown was required anyway and there was no reason to do a leakdown until after the engine was rebuilt/run-in. After rebuild/run-in, 5% leak-down was the maximum acceptable for an engine to leave the shop.

Here's vid of my Norton going through a leak down test. Regular rings will leak a little more but the process is the same. Note the necessary extension on my breaker bar and the "differential" between the two gauges and what happens when I rotate the crank. The leakdown tester is from Aircraft Spruce.