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Moses Ludel

How-to: Make an Inexpensive Engine Cylinder Leak Tester!

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I talk a lot about using a cylinder leakdown tester for pinpoint engine diagnosis.  When you want to understand an engine's internal condition, the degree of wear or actual cylinder seal, this is the tool.  You can narrow your findings to a bad intake or exhaust valve, worn piston rings, excess cylinder taper, a blown head gasket or an engine casting crack—even more findings if you're creative!


For some, the cost of a leakdown tester is not justified.  Maybe the tool will not be used beyond a one-time test of your vehicle's current engine.  Maybe you're strapped for cash and simply cannot afford the least expensive tester.  Regardless, I will share a very inexpensive alternative if you have access to an oxy-acetylene brazing torch, some air line fittings and an air compressor.


The homemade tool consists of an old spark plug and an air hose fitting.  Here are the steps:


1) Remove the ground strap from the spark plug end;  grind away and wire brush any remaining, rough material.


2) Knock the insulator/electrode out of the plug, wearing eye protection—porcelain is like glass!


3) Use an air coupler fitting with male NPT threads that will roughly screw into the empty steel body of the old spark plug. 


4) Carefully and thoroughly braze the fitting to the spark plug shell;  make this a strong, air-tight seal.  Remove any flux or rough surfaces to prevent blowing debris into the engine's cylinders.


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The homemade tool shown here is a special extension/adapter for leak testing at the hard-to-reach spark plug threads of the magazine's Honda XR350R, XR500R and other four-valve motorcycle engines. The principle is the same for making an inexpensive, homemade cylinder leak tester.


The tool I'm describing works with your air compressor.  Set the line pressure first to 60 PSI, which simulates low compression seal, then boost line pressure to 100-120 PSI.  This higher PSI will create enough pressure to expand the piston rings and force the compression ring(s) outward against the cylinder wall.  This is more like actual engine operation pressures.


Here's how to do the check:


1) Before inserting the spark plug thread air adapter, disable the ignition system and remove the spark plugs for #1 cylinder and its opposite firing cylinder. 


2) Make sure the #1 piston is at TDC on its firing stroke (verify with the distributor rotor position if necessary).  Note that for #1 and its opposite firing cylinder, the crankshaft timing mark for TDC will enable quickly bringing the piston to top-dead-center for the test.


3) With the piston at TDC on its firing stroke, install the homemade spark plug thread adapter.  Seat the plug adapter carefully.  (Use the plug's original gasket to protect the head, especially if aluminum.)


4) Apply compressed air at the lower pressure first.  You will follow up with the higher pressure.


5) Listen for leaks at the crankcase (oil filler cap removed); the tailpipe; and the engine's air intake.


Caution: If you suspect a blown head gasket, remove the radiator cap before applying compressed air to cylinders! Otherwise, you can blow off a radiator hose or cause severe damage to the radiator at these test pressures...This also applies when testing with a ready-made cylinder leakdown tool.


When through with #1 cylinder, you can rotate the crankshaft carefully (ignition disabled!), one rotation of the crank pulley.  This will bring the cylinder opposite #1 to the TDC position for testing.  The reason for using these two cylinders for a quick, general engine condition test is that each of these two cylinders can use the crankshaft pulley's TDC mark for locating exact TDC for that piston.  Repeat the air pressure tests at the second cylinder.


Finding the opposite cylinder to #1 is simple.  Note the engine's spark firing order.  On a Jeep inline six, this would be 1-5-3-6-2-4.  The cylinder opposite #1 is #6, both pistons rise and set in sync.  When #1 is at TDC, #6 is also at TDC, one is at its firing position, the other at the top of the exhaust stroke...For a popular GM or Chrysler V-8 with a firing order of 1-8-4-3-6-5-7-2, the two paired cylinders are #1 and #6.  A Jeep four is 1-3-4-2.  #1 and #4 pistons rise and set simultaneously.  This follows suit for other engines, note your engine's firing order.


This test is for rough results only, as you are not actually measuring the percentage of leak, rather you are trying to find a substantial leak.  In cases where the rings are shot, a valve or valve seat has a burned notch on its edge, or a head gasket is severely blown between cylinders, or into the cooling port(s), this test can be a quick, accurate likeness to what you get from a leakdown test.


The homemade tool can also be used as an air hold for changing valve springs with the engine's cylinder head in place.  With the rocker arms loose and valves closed, regardless of piston position in the cylinder, you can keep the valves in position while you carefully remove the valve springs with an overhead spring compressor.  Do not allow the valve to unseat, and keep air pressure applied.  As a precaution, raise the piston to TDC for this procedure;  that way, the valve cannot drop far into the cylinder. 


If you do any volume of engine work, an inexpensive cylinder leakdown tester like the OTC 5609 tool is cost effective.  I just pulled up this "best buy" on line at $58 plus shipping:




For that price, you may prefer buying the leakdown tool.  A few engine tests, and you will more than pay for the tool in pinpoint diagnostic value.



This is my Snap-On leakdown tester, which has paid for itself many, many times over since 1981.  I have quickly diagnosed major engine problems and internal engine issues without engine or cylinder head removal.  See my additional discussion about this tool in the engine diagnostics tool forum.



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