PISTON SLAP

Evening Post, Volume CXV, Issue 11, 14 January 1933, Page 15

PISTON SLAP

Piston slap is a term associated-by most motorists with a noise in. tho engine when it is started up from cold, only-to disappear when the engine;has warmed up, but few owner-drivers have a clear conception of the "whyl.and wherefore" of the temporary but often disconcerting noise. ' .. The slap is caused by the slack fitting of aluminium pistons in the cylinders, and the noise occurs through th& reversal of the side-thrust on the pistons. This reversal of thrust is at its maximum between the compression and firing strokes. When a piston is moving upwards the connecting rod is not vertical, but at an angle to. the cylinder walls, and this forces the piston sideways. As the piston reaches' top-dead-centre the connecting rod becomes vertical, but on the down stroke it again assumes an angle, but this time the angle is on the opposite side of the vertical, with the. result that .the piston is forced against the opposite side of the cylinder. These sideway thrnsts or slaps against cylinder walls are intensified by the compression arid explosiorf pressures. . ' ' ■ ' Piston sla*p is not necessarily dno to wear, but usually is attributable to the fact that a liberal working clearance lias been allowed for the aluminium, piston to expand as it heats up/Nowadays, through the use of aluminium alloys and the combination of soientinc design with the introduction of special invarsteel struts, most modern aluminium alloy pistons are made with an extremely low co-efficient of expansion. This characteristic enables this type of piston to bo fitted with even less clearance than cast iron pistons, arrd "eliminates' the need for the loose .fitting that causes piston slap.

It is advisable to change, the water in the radiator of a new car^ several times during the first 1000 miles of driving. Any foreign substance which may be in the cooling system will be eliminated in this manner.

Regularly clean the exterior of the sparking plugs to remove any oil or dust deposit on the insulation. The dust may act as a conductor and, in all probability, cause short-circuiting from the terminal to tho body of, the Plug- ''.."'