converting a compression tester to use as a leakdown tester

[rustybronco]

***This idea is not my original work. I got the idea from from this project-> http://www.motorcycleproject.com/mot.../leakdown.html


I started off with the idea I could re-purpose my cylinder compression tester instead of gutting a spark plug, buying a grease hose and, as my air compressor has a means of regulating the air pressure, I wouldn't need the regulator used in the original project as it would have been redundant.

***Note*** you may have to modify what I have shown so that you may use your compression tester, but the basic idea will still be the same.

This is the compression tester I used.



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Remove the gauge and the schrader valve from the other end.

***Note** If you don't remove the valve, air will not be able to flow into the cylinder!



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This is the adaptor I made up to use with the hose from my compression tester.

Starting from the right, the components are, the air line fitting so it can be connected to an air hose. a 1/4" brass nipple filled with epoxy that has a .040 hole drilled through the center of the epoxy. a brass tee with an air pressure gauge (you can use the pressure gauge from the compression tester just as well). a shut off valve (very important) and a fitting to connect this to the compression tester's hose assembly.



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This is what the assembled leak down tester looks like.

***Note*** my compression tester had a quick connect (disconnect) fitting, making
it easier to use by inserting the hose in the spark plug hole, then connecting the 'tester' to the hose.



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How to use the tester.

1) Remove the spark plug

2) position the piston @ top dead center of the compression stroke.

3) thread the compression testers hose into the spark plug hole.

4) close the shut off valve on the 'tester'.

5) connect the tester to an air source.

6) adjust the pressure regulator on the air compressor so the gauge on the tester reads 100 psi.

7) open the valve on the testor and read the pressure indicated.

if the gauge reads 80 psi you have a 20% leakage in the cylinder (100-80=20). if it reads 95, you have 5% leakage in the cylinder and so on...

I hope this gives you some kind of an idea on how you can make use of what you have on hand, instead of going out and buying new.

enjoy!!!

[Steve]

Thanks for the write-up, RB.

I have heard of leakdown testers being used, but never had the occasion to use one myself.
If I understand the principle correctly, you are basically measuring the leakage that is beyond what the .040" hole can supply?

.

[rustybronco]

Steve, I believe how you said it is the correct way to explain the principal.

one might also think of it as a measured pressure drop. (I hope that's correct)

[Sci85]

Another simple yet elegant solution from the GS vets. I was at the auto parts store today getting some tubing and cleaner and saw that shutoff valve and decided on the spot I was going to do this. Thanks RB for passing it along.

Sci85

[koolaid_kid]

Here is a decent write-up on using one: http://www.carcraft.com/techarticles...r/viewall.html
They can be quite handy, but you need to check several times for each cylinder to make sure you have identified the root cause of the suspected leak.
For a bike, you remove the oil fill cap and check there for leakage around the rings. You remove the carb bank to check for leaks around the intake valves. You check the exhaust tip for leaks around the exhaust valve.

[Normk]

If you're looking for the escape route for cylinder pressure rather than wishing to measure leak-down, an open air hose can be more effective than the restriction from a leak down tester. Having been a professional tech, I have compression testers, leak down tester and air hold adapters, all of which are useful for various diagnosis.

Likely everyone already knows this but just in case, a spark plug hole adapter which is connected directly to shop air supply is useful for finding a leaking valve, piston ring sealing issues, and sometimes even to confirm a blown head gasket into the cooling system. We primarily used these adapters as "air holds" when changing valve guide seals during the 1960's, '70's &'80's although the practice seems to have mostly gone away due to better valve guide sealing materials in automotive engines. This was never common practice for motorcycles for a number of reasons but this is becoming too long.

If wishing to simply find where the compression pressure is escaping, a leak down tester can have too much restriction where-as an open adapter works well. Many people seem to misunderstand the purpose of leak down as simply listening for leaks but this is not the purpose. Compression and leak down are separate tests for which each has definite advantages.

[Normk]

Oh, just read the link to leak down test and offer a caution. The article suggests applying pressure to the cylinder and then using a breaker bar and socket on the crankshaft nut (talking automotive but could do the same on a bike) but watch out! If you have 100 PSI on the piston and rock the crankshaft off TDC, the connecting rod will come out of alignment with the cyinder centre line and the air pressure will try to push the piston down.

If your fingers are somewhere they shouldn't be, you can get a very nasty smash on the knuckles! If you're going to play around with that technique, back the pressure way down and get a feel for how much force is involved. You can move the pressure up a bit to as needed but do it in steps so that you don't get hammered!

Try that on a 7 litre V8 with 150 PSI on the piston and it can be live changing!

I think the author expected that we understand basic physics....