Variations in lash cap design
Lash caps (also widely known as tappet shims) are probably the most innocuous of valvetrain components. They are inherently stiff and, owing to their size, they weigh very little. There is therefore no real incentive to change their design, because any gain in stiffness or low mass will be negligible. It is far better that the design engineer spends his or her time in the pursuit of loftier goals. The humble lash cap is there simply to provide some basic adjustment in the system such that valve clearances can be set to the correct value.
However, not all lash caps are the same, so here we will look at some of the variations in their design.
The most commonly used type of lash cap is the 'inverted cup'. These are relatively simple to make, and can be bought to suit many valve sizes at only modest cost. The 'base' of the cup is the important feature here; its thickness is carefully controlled and both upper and lower surfaces are carefully machined to a fine finish. The wall of the cup is simply there to keep the lash cap centred on the valve stem.
There are several types of lash cap that are even more simple than the inverted cup. Very small plain discs can be used, but these must be prevented from moving. This can be done easily where inverted 'bucket' followers are used in an overhead cam engine; a small recess is provided in the underside of the follower.
However, any marginal saving in complexity and mass with this design when used with the 'bucket follower' is cancelled out by the extra complexity involved in the design and manufacture of the follower. They are also more difficult to fit than an inverted bucket lash cap, and great care has to be taken in their installation to ensure they remain in place. Again, where bucket followers are used, a recess can be machined into the top of the follower and a large disc is used as a lash cap.
These have proved popular in production engines, and a number of manufacturers use aluminium followers with steel shims. The hardened shim provides the contact face between the cam lobe and the follower, so the aluminium follower skirt acts only as a guide for the follower in the bore. A further variation on the flat disc shim is one where a relatively small shim is prevented from moving because it is restrained by a corresponding bore on the spring retainer.
Turbocharged and supercharged engines sometimes require a special type of lash cap. Occasionally a plenum explosion will occur that reaches such a pressure that the valves open when the cams aren't controlling them. As there is nothing accelerating the follower, the lash cap can find itself with sufficient clearance to escape from its intended location. Such lash caps require special design features to ensure they cannot be separated from the valve, except when the engine is rebuilt.
Some lash caps are provided with a small hole in the centre; this is not to save weight but to ensure there is no air or oil trapped beneath the shim on fitting. If there is trapped air or oil 'holding off' the shim from metal-to-metal contact, incorrect valve clearances can be set. When the air or oil is displaced during service, the valve clearance increases, and performance and reliability can be compromised.
Written by Wayne Ward