ScootDawg Forums - Leak Down Tester

Leak Down Testor

You read all the time of folks that can do a compression test and find they have low compression. The next step is to figure out what is the cause without doing an extensive tear down. That is where a Leak-down Tester comes in very handy. Most do it yourselfers don't want to spend a lot of money on a tool that they will not use very often. A good leak-down tester from Snap-On will run a few hundred dollars. For about $7 worth of fittings and a couple cheap gauges ( around $10 a piece or less, I had 2 in my shop) you can build your own to do the same job.

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Parts list.

One 160 lb Gauge with regulator ( a gauge off a spray gun works great)
One 160 lb gauge
One 3 way coupler
One adapter (for gauge too 3 way)
Two fitting adapters
In and Out Quick Disconnect Air Couplers
Teflon Tape
JB Weld or Epoxy
#60 numbered drill bit.

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What you do first is make the orifice out of one of the fitting adapters. Fill the adapter with JB Weld or hard epoxy and let it sit and dry. After it has hardened take you #60 drill bit and drill a hole through the JB Weld. This will be placed between the gauge with the regulator and the 3 way coupler.

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The rest of the assembly is straight forward. Use Teflon tape and assemble tight. Order is (left to right) Input coupler, gauge with regulator, fitting adapter ( the one with the orifice tube you made) , 3 way coupler with 2nd gauge, 2nd fitting adapter and output adapter.

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This is the only other pieces you need. I use the hose and cylinder adapter from my compression gauge, make sure you take the schader valve out of the hose. An alternative to this is to make your own.

You will need:
a male quick disconnect
two 1/4 to 1/8 reducers
a grease gun pump hose ( available at most parts stores for around $5)
an old spark plug.

Yes there is a use for those old worthless Chinese spark plugs and you get to smash it! Put the plug in a vise, make sure you protect the treads with wood or something, then smack the porcelain end with a hammer breaking it off. Then remove it from the vise and grind off the rolled end above the hex ( the part you put a socket on LOL) and grind off the ground electrode. Put it back in the vise and use a drift to drive out the remaining porcelain and electrode. If it is stubborn you may need to grind more of the area off above the hex. Now tap the open hex end with a 1/4 pipe tap. You now have an adapter to screw into your cylinder head.

Now screw one of the 1/4 to 1/8 inch reducers into the adapter, then the grease gun hose, then the other 1/4 to 1/8 inch adapter and finally the male quick disconnect. Make sure use use Teflon tape. You have now just made your own hose and adapter assembly for your leak-down tester for less than $8 and an old useless Chinese plug!

Using a Leak-down Tester.

Now that you have built it, it is time to use it.

The first thing is to make sure you have the motor at TDC by checking the pointer at the crank. Next screw your hose assembly in the spark plug hole and connect to your leak-down tester. Connect your air compressor to the other end of your leak-down tester and adjust the 1st gauge to 100psi. The difference between the two is your leak-down percentage. All motors will have some leakage, 10 percent leakage is acceptable. If the crank shaft turns when you apply air, disconnect the air and reset to TDC and either have a 2nd person hold it there or use ratchet and socket against something. Want you want to listen for is air escaping out of the motor.

Common faults:

Air out the intake - bad intake valve or valve adjustment
Air out the exhaust - bad exhaust valve or valve adjustment
Air into the crankcase - bad rings or scared up wall.
Air out the side of the motor - bad head gasket, warped head, or hole.

Enjoy.