Valve Train

The Rover V8 standard valve train consists of Steel Rocker shafts held in place by four pedestals and holding 8 rocker arms per head. The rocker arms are generally of aluminium construction although there are some steel versions on the market that look very similar to the standard items. The rocker arms themselves are fitted with a 'cup' at the pushrod end and a steel shim or pad at the valve end. These pads are slightly curved to allow the valve to 'slide' across the top of them. The standard construction is not the best of rocker assembly designs. The pads tend to wear a groove from the top of the valve and can often fracture or simply break away resulting in very noisy valve gear and subsequent follower and engine damage. There are aftermarket options available including the 'Yella Terra' roller rocker system and so called 'Group A' adjustable rockers but they are expensive and difficult to get hold of.

The standard engine is fitted with hydraulic camshaft followers (lifters or tappets) and these provide a degree of 'pre-load' within the valve train. Basically speaking the follower is oil filled and has a small spring at its centre. The centre of the follower is free to move within the outer shell and this is the part where the pushrod seats. The 'cushioning' effect of the oil filled follower means that the valve, follower , pushrod and rocker remain in contact at all times (effectively).

Pushrods are of solid construction with 'balls' on either end. As the cam rises and pushes the follower up the pushrod then transfers this movement to the rocker arm which in turn rotates and pushes the valve (with its spring) down into the chamber, opening the valve and allowing the air / fuel mixture to be drawn into the cylinder.

The effective 'pre-load' that the follower is subjected to is very important. Too much and the valve will be held open and too little will allow slack in the valve train which will cause gaps at high rpm and subsequent impact damage on all of the valve train components. The procedure for checking and setting the pre-load is as follows:

Valvetrain assembly

Setting The Pre-load

1) The 'pre-load' measurement is taken at the top of the follower and is the distance that the inner plunger is depressed below its retaining circlip. This is really very difficult to measure and we therefore have a slightly re-worked method for making the measurement.

2) With camshaft and followers fitted and the pushrods located on the followers, start to fit the rocker assemblies but do not tighten them down. Check that the camshaft lobe for number 1 inlet is 180 degrees opposite the follower (the follower is at the lowest point of its travel). Slowly tighten the rocker assemblies by turning each of the four bolts in succession (half a turn at a time) until the gaps at the valve side and the pushrod side are closed. At this point you should have no depression on the inner plunger in the follower (it should be at the top against the retaining clip) and the pushrod should be easily spun between your fingers but not able to move vertically up or down easily. Then using either feeler gauges or an accurate vernier caliper, measure the gap between the rocker pedestal and the cylinder head. This measurement will tell you what the pre-load will be without the rocker ratio being taken into account, in order to get the accurate pre-load measurement then use the formula;

Pedestal gap measurement x 1.6 = Pre-load at follower

3) we are looking in most cases for a pre-load of between 0.040" (40 thou) and 0.100" (100 thou) although this is not always the case so please ask if your installation is unusual or non standard.

4) Let us assume that your measured pedestal gap is 0.100" (100 thou). This would mean a pre-load of 160 thou which is too much and therefore the pedestals need 'shimming' to reduce the pre-load. By using the same calculation in reverse it follows that a 0.050" (50 thou) shim will reduce the pedestal gap to 0.050" and give us a pre-load of 0.080" (80 thou) which is acceptable. The shims generally come in thickness's of 0.016", 0.032" and 0.050" so it does not take a genius to adjust the pre-load to within parameters for most instances.

5) What if the pre-load is too low? This is more of a problem! There are a couple of decent solutions; a) have the pedestals machined down by 0.030" to 0.050" and then repeat the pre-load checking procedure or b) use adjustable pushrods to alter the length of the pushrod and therefore the pre-load on the follower. Both of these options have drawbacks! The pedestal machining is expensive, difficult and not always very accurate. The use of adjustable pushrods is a much better solution but most adjustable pushrods are slightly thicker than standard Rover versions and may need the guide holes in the cylinder heads 'opening' slightly to allow for contact free movement (this is a simple drilling exercise!).