Cylinder compression testing is a valuable tool that can reveal
a great deal about the internal condition of the engine. This
is especially true
of older engines, or engines with a questionable maintenance history.
If components such as piston rings and valves are doing their job,
compression readings will be up to specification and consistent
across all cylinders. A cylinder that is more than 25% below
the highest compression reading indicates a problem.
A number of variables affect the results of a compression test,
Cranking Speed Of Motor (typically affected by starting battery
During a compression test all spark plugs are removed to allow the
engine to crank over at a speed well above normal cranking speed.
This increased cranking speed ensures that all cylinders reach their
maximum pressure and also reveals how many compression strokes it
takes to get there. For a cylinder that takes two strokes
to reach maximum pressure, leakage past valves and rings is likely
to be low. However a cylinder that requires five or more compression
strokes is probably leaking.
In the marine environment, cranking speed of the engine is usually
the most problematic in terms of conducting a compression test.
A marginal cranking battery can be quickly run down; this is especially
true if the vessel is designed with a dedicated starting bank and
In addition to a run-down battery, other factors will determine
if a compression test is feasible for your vessel. Inboard
engines are sometimes installed so close to hull structures or to
another engine (in the case of twins) that removal of the spark
plugs is extremely difficult or impossible without some disassembly.
Finally, since the test involves open pistons and the potential
for escaped fuel, proper ventilation is a must to conduct a safe
Because of the variables involved with conducting this test, I will
be unable to advise you if it feasible until I have actually seen
The fee for this test is $150.00 per engine tested.
a complete breakdown of pricing please view the
Pricing Schedule page for more