Special compression gauges are available at auto parts stores or small
engine parts suppliers. These will catch and hold the highest pressure
reached so you don't need to be in two places at once.
|NotTaR of small Gasoline Engines and Rotary Lawn Mowers : Compression testing
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Note that this procedure may always yield a very low reading if there is
a compression release mechanism on your engine - which is very likely. In
this case, the crankshaft must be spun in the opposite from normal direction
by the flywheel (counterclockwise when viewed from the flywheel end, with the
- If the compression gauge has a screw thread, install it in the spark plug
hole so it snug - it doesn't need to be really tight. If it just has a
rubber boot, have a buddy hold it in place in the spark plug hole as you
perform the following tests.
- Pull the starter cord several times or use the electric starter in the
- New Tecumseh engines should have a compression reading of at least 80 psi.
If your reading is at least 60 psi (or the specification found in your
engine manual), then compression is acceptable.
- If it is less than expected, squirt a small amount of engine oil in through
the spark plug hole so it coats all around the edge of the piston and
- Repeat the compression test.
- If the pressure reading is now acceptable, then the rings need replacement
or the piston/cylinder are excessively worn. If there is little change,
there is a valve problem.
- If the measurement is very low under both conditions, there may be a blown
head gasket or damaged (punctured) cylinder or head. (Or your engine has
a compression release mechanism which is reducing the reading - see the
For Briggs & Stratton engines, the manufacturer simply recommends spinning
the crankshaft by the flywheel in the opposite from the normal direction with
the normal spark plug installed. A sharp rebound (as it compresses the
trapped air since the compression release mechanism is not active in reverse)
indicates good compression. Little or no rebound indicates low compression
and need for service.
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