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Building a two-stroke leak down tester for under $50, in two stops.

by Ron Smith on 03/11/2017

Because air leaks are particularly risky on a two-stroke it is important to test for them. Unlike a four-stroke engine a two-stroke requires an air tight crank case to operate correctly. It is alternately under compression and vacuum during each cycle.


Tester Assembly

To do that you are going to need a special tool. Motion Pro sells a very nice kit for $289.99 and since its Motion Pro it is undoubtedly a high-quality tool.

But you can build one yourself for under $50. Here is how.

A two-stroke engine has 3 openings. This tool is going be installed into the spark plug hole and seal that off, so you will need to seal off the other two openings, namely the intake and exhaust openings. If your exhaust pipe fits inside the cylinder you will need to purchase an expansion plug to seal it off. If your pipe goes over the exhaust outlet you will need to buy a pipe cap of the appropriate size to seal it.

On the intake side you are going to leave the reed assembly installed so you will need an expansion plug approximately the same size as the spigot on the carburetor.

Before you head out to get the pieces determine which combination of expansion plugs and pipe caps you are going to need and take your measurements. These are going to come in standard sizes so don't worry about getting the exact size to match your measurement, just be sure you are close and get one that is smaller than what you measured. The pipe cap can stretch a fair amount and the expansion plug (as you might guess from the name) expands.

ill. Component  Source Cost
1 Gas Test Guage Hardware Store $14.99
2 Brass Bushing 3/4NPT - 1/4NPT Hardware Store $5.49
3 Pipe Cap* Hardware Store $3.99
4 Expansion Plug Hardware Store $4.99
5 Pipe Tape Hardware Store $0.97
6 Air Valve Holder Auto Parts Store $8.99
7 1/4 NPT Male Air Chuck Auto Parts Store $5.79
8 1/4 NPT Air Nipple (plug) Auto Parts Store $1.85

Tester Parts Illustration

Once you have gathered all the parts you just have to assemble them together into the tester.

Guage Assembly

Using the pipe tape to seal the brass fittings put the busing into the open end of the gas test gauge. Then tape and install the air chuck into the bushing you just installed in the gas test gauge.


Hose Assembly


Install the nipple (plug) into the end of the Air Valve Holder. Pipe tape is not required as there is an O-ring in the hose fitting to create the seal. That’s it, you now have a complete tester.




Up next, how to use your new leak down tester