Crankcase leak test

by Had Robinson
updated March 17, 2023

Leaking main engine seals are rare. However, leaks in the main seals, particularly, will make the engine harder to start and can lean it out at the high end.  Just the same, 2 stroke engines are messy and there will always be some oil and gunk on the engine.  It must not be excessive where it drips while running.

This method uses a soap solution to test for leaks.

Remove the engine from the frame and then remove the carburetor and redrive or whatever covers the seal on the drive side of the engine.  If you have the correct puller, remove the flywheel.

If the flywheel has not been removed, that part of the engine may need to be submerged in water.  The same would apply to the drive side main seal if it has a clutch that has not been removed.  Because the engine will be internally pressurized, we do not need to be concerned about water entering any part of it.  The only part of the engine that does not need to be submerged or tested for leaks is the exhaust port.  If water gets into the reed valve opening, it is of no concern because it will not get any further than the valve.

It is a good idea to have a low pressure gauge to be sure you do NOT over-pressurize the crankcase.

Engines with an internal pulse port  When the carburetor and gasket is removed, the small pulse port going to the crankcase will be exposed.  The proper fitting (a conical rubber tip) on an air hose can be used to pressurize the crankcase.

Engines with an external pulse port    Connect the air hoses as in the photo below.  This is the best way to pressurize the crankcase.

Use leak detector solution (or a liquid soap solution) and apply it all around the main seals (black arrow in photo) and, if desired, around the reed valve body and where the cylinder attaches to the crankcase.  This engine had 100's of hours on it and the main seals were still good.

If you cannot access the flywheel seal, you will have to partially submerge the engine in water (while maintaining crankcase pressure) and look for bubbles.  If you do not or cannot maintain crankcase pressure when partially submerging the engine in water, there is a good chance water will enter the engine.  Water will not harm a coil in good condition.  However, keep as much of the secondary wire as you can and the spark plug cap out of the water.  You do not want any water in the spark plug cap.

Pressurize the crankcase to 3 psi max.  Pressures over 6 psi could damage the reed valve.   Any leak of the seals will be immediately seen.

Replace any seals or gaskets that leak.  Reassemble the engine.

Note:  When replacing the cooling shroud on a Top 80 or a similar engine, put RTV between the large washers on the cylinder head studs and shroud.  This will help cut down the wear of the shroud at these points due to engine vibration.