Ever won­dered why th­ese strange me­tal­lic sticks have such an im­pact on how your clas­sic runs? Fuzz ex­plains how they work

Classic Car Weekly (UK) - - Living With Classics - FUZZ TOWN­SHEND

Es­sen­tial to the op­er­a­tion of the Otto, or four-stroke cy­cle, the camshaft is one of the most in­ter­est­ing and phys­i­cally ma­nip­u­la­ble com­po­nents con­tained within a re­cip­ro­cat­ing pis­ton en­gine.

It is con­nected and syn­chro­nised to the crank­shaft by means of a chain, gears or a belt and this syn­chro­ni­sa­tion is key to the cor­rect op­er­a­tion of the en­gine. In­deed, an in­cor­rectly syn­chro­nised camshaft can cause not only rough run­ning, but col­li­sion be­tween the valves and the pis­tons.

The main func­tion of the camshaft is to control the tim­ing and ex­tent of the open­ing and clos­ing of the in­let and ex­haust valves. In this way, it con­trols the ‘ breath­ing’ of the en­gine and does so by the use of ma­chined el­lip­ti­cal lobes, formed around a cen­tral shaft core.

Th­ese lobes turn the even ro­ta­tion of the core of the shaft into an ec­cen­tric, lift­ing and fall­ing ac­tion which, when trans­ferred to the en­gine’s valves, causes them to open and close. The ex­act shape of the lobes con­trols the rate and amount of lift and fall of the valves, which then de­ter­mines the gas flow through each cylin­der.

Cam shapes for ev­ery­day car en­gines gen­er­ally fea­ture a lower over­all rise, giv­ing lower and smoother valve open­ing, which pro­vides for a more man­age­able en­gine for or­di­nary driv­ing con­di­tions.

High-rise cams of­fer in­creased valve open­ings, both in lift and time which, on in­let valves and when used in con­junc­tion with a forced in­duc­tion method (such as a su­per­charger) gives the op­por­tu­nity to in­duce far greater amounts of air and fuel into the com­bus­tion cham­ber and thus for the en­gine to pro­duce more power.

Con­versely, the shape of the cam op­er­at­ing the ex­haust valve can be formed to al­low large amounts of spent com­bus­tion gases to be rapidly ex­pelled.

Camshafts are gen­er­ally made from chilled cast iron, which gives durable run­ning sur­faces for cam fol­low­ers to op­er­ate upon and su­pe­rior wear char­ac­ter­is­tics.

Bil­leted camshafts are ma­chined from solid, round steel bar and are time-con­sum­ing and there­fore ex­pen­sive to pro­duce. To pro­vide a hard­ened sur­face to each cam, the shaft is ni­trided, which is a heat treat­ment process. Ni­tro­gen is dif­fused into the sur­face of the steel to pro­vide a ‘case-hard­ened’ sur­face, on which the cam-fol­low­ers op­er­ate.

Worn cast camshafts can take a small amount of re­grind­ing, but the ni­trided case-hard­en­ing of bil­leted cams would be ground away if ma­chined and so gen­er­ally this lat­ter type must be re­placed if it shows signs of wear.

It is not only the in­let and ex­haust valves that are op­er­ated by the camshaft, how­ever. On most ve­hi­cles built up un­til the late 20th cen­tury, the dis­trib­u­tor is of­ten driven from a worm gear ma­chined into the camshaft, or its drive is taken di­rectly from the end of the camshaft, the lat­ter espe­cially on over­head cam en­gines.

Also, me­chan­i­cal fuel lift pumps are nor­mally op­er­ated by an­other cam, formed into the main camshaft and so this au­to­mo­tive part is one of the most im­por­tant com­po­nents in a fourstroke en­gine.

‘This is one of the most im­por­tant com­po­nents in a four-stroke en­gine’

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