CAMS, LOBES, SHIMS AND ALL THAT
When it comes to valve clearances, there are quite a few components and terms bandied around. Here’s a breakdown of what they all mean…
Cam/Camshaft
A cylindrical shaft that has eccentric section mounted or machined onto it. As the shaft spins, the eccentric lobes push onto cam followers, converting rotational motion into linear movement.
Cam follower/ Bucket follower
A machined and hardened component that the rotating cam lobe pushes on. The cam follower is the first element in the valve train that moves linearly. Modern bike engines often utilise a simple inverted bucketshaped plunger that sits over the top of the valve stem, spring and clearance adjustment shim. Shims are usually circular metal discs that sit under the follower bucket.
Valve clearance/ Cam follower clearance/ Valve-lash
The maximum distance between a cam base circle and the cam follower. The clearance is usually measured with feeler gauges and then altered by replacing the adjustment shims with thicker or thinner ones as required.
Base circle
The round area behind a lobe that maintains a constant specified distance from the cam follower.
Lobe
An eccentric section of a camshaft. The salient lobe properties are lift and duration. Lift is measured as the difference between the diameter of a cam shaft’s base circle and the distance from the tip of the lobe to the rear of the base circle. Lobe duration is the number of degrees of crankshaft rotation from when the lobe lifts the valve off its seat by 0.1mm to when the valve returns to within 0.1mm of its seat.
Valve train
All the engine components responsible for the operation of the valves. For a typical modern bike engine the valve train would comprise: the valves, valve springs, retaining collets and top-hats, clearance adjusting shims, bucket followers and cams.
Valve lift
The amount a valve has lifted from its seat. This value may be the same as cam lobe lift depending on if the cam acts directly on a bucket follower or via a finger or rocker arm with an unequal pivot ratio.