At The Wheel

NZ Truth, Issue 1199, 22 November 1928, Page 20

At The Wheel

LISTEN FOR THE KNOCK Use Your Ears To Visualise The Engine Working HOW TO READ MOTOk NOISES

JUST as a doctor can determine by the sounds which are producJ/ ed, the functional faults of heart or lungs, so an expert mechanic visualizes the interior condition of an engine by listening- to it running.

THERE is no magic m this, and it 1; would pay the OAvner to learn to distinguish the meaning ol! -the knocks and rattles which develop m time, m the most silent of engines. :Most modern engines are very quiet when new, the most common source of noise being tappets. These, if adjusted to give, too wide a clearance, make a slight clicking noise. They can be . adjusted accurately with a feeler gauge, which can be bought for a couple of shillings. Correct setting of tappets is important m avoiding valve trouble and ensuring full engine efficiency. The period at which definite noises due to wear can be expected naturally varies to a great extent depending ' both upon the car and the way jn which it has been, driven. To 'test. for worn big-end bearings, the .spark plug of the suspected cylinder should be short circuited. If this bearing is at fault, a heavy knock would be heard as the shorting instrument is applied or. removed. The engine must be. kept turning over fairly fast during the test, so as to ensure a good loud thud. . . Small-end or gudgeon pin bearing knocks are quite easily distinguished from big-end noises. They are usually most noticeable with the engine idling. The noise produced is a hollow

sound which cannot he cut out by shorting the plugs, although the sound is sometimes dulled. Escape of gas past the piston rings gives a regular . metallic knock at low speeds, and this is often associated with piston slap, a continuous rattle which is best • heard by suddenly speeding up the engine when running idle. Slap is due to wear of cylinders and pistons, resulting m too great a clearance. The piston is blown violently against the cjiinder walls when the explosion occurs. Since cylinder wear is uneven, the cylinders wearing to an oval shape, more pronounced at the top, re-boring and the fitting of over-size pistons is required. • Main bearing play usually gives a rumbling sound, and is apt on many engines to pa^s unnoticed. Loose bearings 'should. receive prompt attention. .Once audible slackness develops, the hammering action becomes destructive, and a bearing which could originally easily have been adjusted soon reaches a state when it requires renewal. Broken crankshafts and centre main bearing failures are generally the result of whip' developing through loose bearings.