How To Make An Engine Cylinder Leak Down Tester

You can get a small regulator at Sears (part number 282-16025) that is the same regulator they use in my snap-on official leak down tester. It has the small knob you turn to adjust the pressure, and the red outer locking ring that snaps in place to prevent accidentally changing the setting. The regulator comes with the guage too!

I made another engine cylinder leak tester and I use it to test the late model plastic bike oil coolers and radiator cooling systems. (image below with others) Its cheap, all you need with it is a quick coupling for the supply air, the male part, and a female quick coupling that the adaptor hose to fit the spark plug thread you are using (10mm 12mm 14mm etc) connects into. The ready made 14mm X 1.25 thread snap-on cylinder adapter hose I use is Snap-on #MT26J200. Its the same hose as used with a compression tester, except you take the schrader valve (check valve) out of the end that screws in the sparkplug hole. Connect the male quick coupling you put on the air supply side of the regulator to your air supply line and have the hose that goes to the spark plugs with the male connector on the other end out of the female quick coupling.

Now adjust your new leak down testers regulator knob to read 100 psi on the guage that came with it, since the female quick connector that goes to the engine cylinder hose is closed, you call this 100 psi reading "O" percent leakage. With the motor on exactly top dead center (TDC) on the compression stroke (you do each cylinder seperately) you can be sure you are on compression and not on the exhaust stroke by having the tester engine cylinder hose screwed into the spark plug hole and feel the end of the hose with your finger as you turn the engine over by hand coming up on the compression stroke, the air will blow your finger off the hose. (unless that cylinder has no compression at all).

With the piston at TDC and the valves closed when you apply air to the combustion chamber through the spark plug hole, the guage on the tester should read 100 psi or call it 100%. That would be zero percent leakage, because no air is going through the tester, but all motors leak some, even if they are in perfect condition. Any leagage above the vicinity of 8-10% (your guage will now read 90 psi) should be easy to locate by the sound of air whistling out the intake port/carb if the intake valve is not seating (worn out, no lash or bent etc) or by listening for leakage at the end of the exhaust pipe for the exhaust valves, or sometimes you can feel or hear it blowing at the crankase breather vent if the air is going past the piston rings. A faulty head gasket will leak at the cylinder and head junction. The leak down tester is a great tool to tell exactly what is going on in a motors top end, and to pin point problems that may exist. The amount of psi you read on the guage that is less than the 100 psi you originally set it at is called the same number in percent that the motor is leaking on each cylinder.

For example: If your guage reads 80 psi when air is going in to the cylinder at top dead center on the compression stroke (when both valves are closed) it is said to be leaking 20 percent. This is not good, 20% of the available potential energy for power to push the piston down is not being used.

Bob Bertaut Sr.

This Sears regulator comes with the guage, any place that sells painting supplies or compressors should have it, Graingers too. You need a male & female quick coupling (available at any auto parts) and a hose adaptor for the spark plug type your engine uses.

You can adapt this to check other systems, cracked heads, bad gaskets etc.

This version I made is adapted to check high pressure oil coolers for GSXR Suzuki's.


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