Saturday, February 14, 2009 was the culmination of a series of discussions between Richard Mayor and me. Over lunch one day Richard was surprised to learn that my Healey has nearly 119,000 miles on it and has never had an engine overhaul.
As the discussions progressed we decided we could test the general health of my engine and present the testing protocols as a technical session for the club.
The diagnostics to be performed were a “compression check” and a “leak-down test”. The former analysis consists of removing all six spark plugs and then with the transmission in neutral and the parking brake set, measure the ability of the piston in each cylinder to compress the volume of contained air. Attaching a pressure gauge to each cylinder and then using the bump switch on the starter solenoid to turn the engine over 3-5 times accomplishes this. All six plugs are removed so that the engine can spin freely. At each compression stroke the pressure in the tested cylinder rises until it no longer rises. The peak pressure is then recorded. Each cylinder is similarly tested and recorded (see the data table).
The leak down test is a little more involved. Again, the spark plugs are removed, one at a time, and a special pneumatic system is attached to the test cylinder. The apparatus allows compressed air to be slowly added to the cylinder and the ability of the cylinder to retain 100 psi is then measured and recorded. The testing of each cylinder requires that the cylinder be brought to a top dead center (TDC) of the compression stroke. It is at this point of a four-stroke combustion engine that the intake and exhaust valves are both closed and the cylinder can be pressurized. With the car transmission in 3rd gear, the park brake off, and the strong backs and weak minds of the testers roll the car forward or backward until it is determined that the piston of the subject cylinder is at TDC on the compression stroke. The leak down test requires a bit of precaution. If the cylinder is not exactly at TDC when the compressed air is introduced the car may role as the piston is forced down and away from the TDC position. To avoid surprises and possible injury or damage the park brake is set, the wheels chocked, and five of the spark plugs remain in place.
The table below presents the data recorded. The reference point for the compression numbers is the found in the General Data of the workshop manual for the Healey 100-6 and 3000. A fresh engine should deliver 175 psi in all six cylinders. Therefore, my engine producing a compression ranging from 155 to 162 psi is not too bad. However, the leak down numbers indicates there is a problem in cylinder number 3 with a reading of 26 %. The question is, where is the leak? The answer is usually fairly obvious during the leak down testing. When the cylinder is pressurized and air begins leaking it is audible either at the carburetters (intake valve leakage) or at the tailpipes (exhaust valve leakage). In the case of my car the major leakage was tracked down to piston rings. The sound of leaking air was evident but no sound came from the tailpipe and did not seem to emanate from the intake manifold. But, when the rubber hose from the crankcase breather was removed from the air filter of the back carburetter it was clear that air was passing by the piston rings in all six cylinders, especially cylinder number 3.
Cylinder # Compression % Leak Down
Measured Recorded
1 161 psi 10%
2 156 11
3 155 26
4 160 11
5 155 12
6 162 14
The conclusion of the technical session was that clearly there was measurable wear on my engine due to 119,000 miles of service. However, we concluded there could be many more miles of reliable service yet to be enjoyed before major engine overhaul is necessary. At the time that occurs there are many upgrades that can be done to improve the performance of the Healey six, e.g., lightened fly wheel, improved valve train, race grade exhaust manifold, etc.
