VALVE CLEARANCE

Hawera Star, Volume LII, 28 January 1933, Page 12

VALVE CLEARANCE

PERIODIC ADJUSTMENT NEEDED

CONSEQUENCES OF NEGLECT

Every motorist who knows anything about the mechanism of his car is aware that from time to time the valve clearances require chcokinc and adjusting, but there is, perhaps, too common a belief that the benefits derived from that attention are merely more quiet tunning and a slight betterment in the pulling power of'the engine. Actually, neglect of this matter may have serious consequences, apart from the creation of noise and ' lack of liveliness of the

engine. . _ , , \ n undulv close setting of the clearance is far worse than an excessive one, for should it happen that the valve does not return completely to its seating when the tappet has passed off the cam, there will be not only a- very appreciable loss of efficiency, but the face of the valve will soon suffer, because of its exposure to the high temperatures prevailing in the cylinder during the combustion of tiro charge, and will become pitted and eroded. Conversely, itthe clearance is too great, the valve concerned will not be lifted fully, and so in the case‘of an inlet valve, the amount of mixture drawn into the cylinder will be affected, and with an exhaust valve tho essential quick evacuation of the spent gases will be impaired. After an engine has had a top overhaul, and the valves have been ground in. it often happens that they settle slightly, and for that reason it rs customary in such circumstances to err on the side of liberality when first readjusting the clearances, and to enee.c them after the car has run a few hundred miles, to guard against the danger of any of them riding high because of this bedding down- process. The adjusting of the tappet clearances is an easy job, even for the amateur, but unless it is done in' the correct ■way harm, rather than good, will be the result First of all, it is necessary to consult the instruction book of tho car to ascertain w'hat arc the gaps advised by the .manufacturers, taking note whether or not different distances are recommended for inlet and exhaust' valves respectively, and if the adjustment should bo made when the engine is hot or cold. It is most unsatisfactory to' dependon makeshift gauges such as safety razor blades, and tho motorist should ■borrow' or buy a feeler gauge of the right thickness. Before the gap is checked the engine must be turned wi'th the hand crank until the valve to, be tested is completely closed; that is to say after the tappet has passed over •the cam and is at rest. The -easiest method is to get a helper to- crank the engine while one watches the valve stems and tappets of the cylinder concerned. It will be noticed that the exhaust valve will rise and- fa-ll and. immediately afterwards the inlet valve will open and then drop again, so that during a short portion of the cycle of operations both valves in each cylinder arc shut. That period corresponds to the compression and- firing strokes of the piston. The right setting of the engine for tappet adjustment is at abou-t the half-way poi-nt of that -petiod and with a little practice the amateur will be able ,to gauge the position to- a nicety. The tappet settings of the valves of that cylinder should then be adjusted -so that the appropriate feeler gauge can be inserted between the tappet and the stem without much resistance, but care must be -taken- that the set-tin" is not altered. w T hcn the lock-nut. is tightened, and a final check should nlwavs be made.