[chris_seven]

When we machine our new rockers we don't allow the arm to pivot and move.

We fix the rocker into a custom made fixture and machine the radius relative to the centre that we determined from the CMM measurement.

By rotating on a shaft the variation in clearance will affect the curvature and the 'squareness' of the heel of the rocker,

We machine the rocker to a precisely controlled surface finish and then isotropically superfinish.

Prior to machining we double temper the rockers at 650 degC.

The surface finish of the rocker pad is relatively important in terms of oil film retention with a cross-hatch pattern being the most effective.

This is difficult to achieve on a small volume production basis so we use a surface treatment process that not only produces a hard surface (1000 HV) but also creates a slightly porous layer that holds an oil film and improves lubricity.

This surface treatment could be applied to a cast steel rocker but is only cost effective on a batch basis. It does produce a limited amount of growth of around 0.003mm per surface. (0.11 thou)

The surface treatment uses a diffusion process carried out in a PVD Chamber at 595 degC which eliminates any thermal distortion.

A typical Chill Cast Iron Camshaft has a surface hardness of 60HRC + 9750 HV) so wear will distribute evenly between the rocker and the cam.

A DLC Treated rocker would have a surface hardness of around 2500HV and would wear the cam approximately 2.5 times faster than the rocker arm.

It the surface of the rocker is significantly lower than the surface of the cam then adhesive wear 'galling' can occur and the rocker would then wear rapidly.

The bush used in the shaft is also quite important and we use the original Glyco Semi-finished bush which is honed to size after being fitted into the rocker arm.

Rocker arm bushes are only partially lubricated and are not pressure fed so their characteristics are quite important.

The Bi-metallic Glyco bush has a PV of about 2.5 times that of a GBB Metal Polymer bush and should provide much better fatigue life at engine temperatures.

The shaft clearance needed for successful operation of a metal polymer bush is significantly greater than those specified when using a Glyco bush and this is a potential banana skin.

It is customary to reduce the diameter of a the shaft to obtain the recommended clearances but as rocker shafts have to expand into the cam housing this isn't feasible.

It is possible to increase the bore diameter of a Metal Polymer bush by burnishing but this does reduce the bearing life.

Increasing diameter by 0.001" reduces life by 30%, by 0.015" life reduces 60% and a 0.002" reduces life by 80%.

An AMPCO 18 material could make a suitable alternative to the Glyco bush and would use standard clearances.