4 THINGS YOU NEED TO KNOW… The cam pro­files af­fects the amount of power and torque made

Motorcycle News (UK) - - Garage -

OHV, SOHC or DOHC?

In re­ally old en­gines the cam would be driven di­rectly from the crank­shaft via a chain, which then used a com­pli­cated sys­tem of pushrods and rock­ers to op­er­ate the valves – these were called over­head valve en­gines (OHV). Then came over­head cam en­gines that had just one in­let and one ex­haust valve per cylin­der, all opened with a sin­gle camshaft – hence Sin­gle Over Head Cam (SOHC). But since de­sign­ers have started us­ing two in­let and two ex­haust valves, you now need two camshafts to op­er­ate them, these are Dou­ble Over Head Cam en­gines (DOHC).

It’s all about tim­ing

Tim­ing is es­sen­tial. The cam has to open and close the valves at ex­actly the right time or as they open they could hit the pis­tons as they ride up to the top of the cylin­der, de­stroy­ing the en­gine. Mod­i­fied en­gines of­ten have the cylin­der head skimmed or thin­ner head gas­kets, which bring the valves closer to the pis­ton, so when build­ing a tuned en­gine, it all has to be looked at as a whole.

Valve train

The cam it­self doesn’t ac­tu­ally push di­rectly onto the valve. Some sys­tems have a rocker arm which is an­chored at one end and pushes down on the valve at the other, while most mod­ern en­gines use shims in a bucket which sit on top of the valve. These sys­tems, called the valve train, help to min­imise the bat­ter­ing the valve would get from the cam at high RPM and also give ad­justa­bil­ity, as there has to be a clear­ance be­tween the valve and camshaft lobe base when the valve is shut.

Ser­vic­ing

Ever won­dered why your bike has a re­ally ex­pen­sive ser­vice ev­ery other time you take it to your dealer? It’s nor­mally be­cause your valve clear­ances need check­ing and ad­just­ing. Tank, body­work, rocker cover and cams all need to be re­moved if any of the shims need chang­ing in or­der to keep the cor­rect clear­ance. Too loose and you’ll lose power, too tight and the valve train will be stressed and the valves could po­ten­tially touch the pis­tons.

Cams push down on buck­ets

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