COMPRESSION LOSSES

Hawera Star, Volume LI, 11 June 1932, Page 7

COMPRESSION LOSSES

DETECTION AND REMEDY. TESTS FOR EACH. CYLINDEiR

Loss of compression in one or more cylinders of the engine will always have an adverse effect on its performance and petrol mileage, and the car-owner is well advised to stop this trouble before progressive wear and tear may make it necessary to have expensive repairs carried out. Nowadays there is such a big reserve of power in many car engines that minor compression losses may often be overlooked, but it is a good plan to test the compression in each cylinder from time to time. The most satisfactory course is to make proper use of a compression gauge, and all well-equipped service stations will have such an instrument available; but a fairly good idea of the general condition of the cylinders can be gained by using the hand-crank to bring each piston in turn on to its compression stroke, noting if any one in particular seems to offer less resistance than the others. It often happens that loss of compression can be traced to a simple and easily cured valve fault. With a compression gauge one can make accurate tests of each cylinder and should any reading be appreciably below that of the others, full investigation into the cause must be made. To ascertain whether the leakage is at a valve, or past the piston rings, the gauge is unscrewed from the spark plug hole and a little heavy oil is injected on to the top of the piston. This has the effect of sealing the piston temporarily should there be any leakage at the rings, the gauge is next replaced and the engine cranked. If it is then observed that a higher reading is secured, it may naturally be assumed that the piston rings or cylinder walls are at fault, but if there is no change in the reading, then it is most probable that a defective valve is the cause of the trouble. Another easy method of diagnosis is to slip a short length of rubber tubing or hose pipe through the oil filler of the crankcase, taking care that its end does not dip into the lubricant. Then get a helper to bring the piston in the suspected cylinder on to its compression stroke and, applying the other end of pipe to the car, listen for any sound of air leaking past the rings. If this symptom is absent, the natural conclusion is that a valve is the cause of the trouble.

Leakage of compression between the cylinder wall and the piston rings must always be regarded as a serious defect, for, although it may be only slight, it may rapidly get worse and worse until the cylinder has worn to an elliptical shape, and will have to be

rebored. When it has been ascertained that one or more of the cylinders is losing compression past the rings, the most economical course is at once to consult a really skilled garageman. It is a mistake to think that the fitting of new piston rings will always prove to be a remedy, although some motor mechanics seem to regard that treatment as a panacea for the mechanical ills referred to. The truth is that if there is an appreciable elliptical wear of the cylinders, new piston rings will prove worse than useless unless the cylinders are rebored to their true circular form.