KNOCK DIAGNOSIS.

Timaru Herald, Volume CXXV, Issue 18542, 12 April 1930, Page 11 (Supplement)

KNOCK DIAGNOSIS.

A spark knock is a ringing metallic knock In all cylinders. It is caused by the spark being too far advanced. It is most likely to happen when the engine takes a hard pull and slows down, or upon a quick acceleration from low speeds. To eliminate this knock it is necessary only to retard the spark. A carbon knock sounds much like a spark knock, in fact, they are both caused by preignition. The development of carbon knock is gradual, the engine is sluggish and many tend to overhaul. It may be relieved by retarding the spark, but the only permanent remedy is to remove the carbon. A loose valve lifter adjusted to give too much clearance gives a light metallic tap, varying with the engine speed either idling or under load. It is unpleasant to hear but does no harm unless excessive, when it shortens the time that the valve remains open. It is easily corrected by adjusting the valve lifter. The clearance should never be adjusted to less than .004 inch, as a little tappet noise is much preferable to having a valve “ride” that is never to close tightly. A loose piston gives a light, sharp metallic knock, usually developing on acceleration or under load, and in one cylinder only. Even with perfect alignment of the connecting rod, a piston may slap with very little clearance; no more than is needed for safe operation. Especially with aluminium alloy pistons, a light slap may show when the engine is cold; it does no harm, but disappears as the engine becomes warm. A worn piston pin causes a knock sometimes like piston slap, occurring twice for each revolution of the engine. Play in the lower connecting rod bearing gives a heavy metallic knock, not as heavy but more metallic than a main bearing knock. It usually appears at a speed of about 30 miles per hour under moderate load. A car can be carefully driven for a reasonable distance with a loose connecting rod, but a new bearing should be installed at the first opportunity. A main (crankshaft) bearing causes a dull, heavy bump, showing under load. An engine should not be driven with a main bearing knock. End play in the crankshaft causes a dull, irregular thud. It can be eliminated by pushing backward on the front end of the crankshaft. It is best remedied by a new centre crankshaft bearing. A car can be driven indefinitely with end play in the crankshaft.