Right on the Rocker

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POSTED ON October 30, 2017

Here is an excellent tool that will help any engine builder set up the perfect rocker geometry in any engine with a far greater level of accuracy than could be achieved in the past.

This small dial gauge makes checking rocker geometry simple and effective. Before the tool can be put to use, some simple calculations must first be made. Number one is to know the total valve lift, which is the lift at the camshaft multiplied by the rocker ratio. This information is normally available on the cam card but make sure the rocker ratio you are running is exactly the same as that on the card. Once you have the total lift figure, divide it by two and write down that figure. On the Windsor engine pictured, the total lift was 0.610-inch so the number we need is 0.305-inch. Next, we must calculate half the valve stem diameter (the radius). In this case the diameter is the very common 11/32 of an inch so the radius is 0.172-inch. The final figure we need is half the diameter of the roller at the end of the rocker arm. In this case it is 0.312-inch. By finding the radius of the roller and the valve stem it is possible to calculate the exact centre of the valve in relation to the centre of the roller rocker.

First reading is 0.088-inch – the pedestals must be lowered for correct geometry. After adjustment – 0.140-inch – spot on!

At half total valve lift the roller should be exactly in the centre of the valve stem. According to our calculations, subtracting the valve stem radius from the roller tip radius gives 0.140-inch, so at half valve lift; this is the figure we are chasing. On our test engine, the special dial indicator is slid under the roller tip and the alloy jig is pressed firmly against the side of the valve stem which protrudes above the retainer. The dial indicator then rests on the roller tip. The resulting measurement is the difference between the valve stem and the roller tip. From our calculations we know that number should be 0.140-inch but in this case it is only 0.088-inch. This means that the roller tip is too close to the inner edge of the valve stem at half valve lift. In an engine equipped with adjustable rockers on studs, it would be a simple matter of fitting a slightly shorter pushrod to correct the geometry but on this engine things are a little more complicated. Because the rocker arms bolt to Jesel pedestals it is necessary to remove shims from under the pedestals; it may even be necessary to machine the base to lower the rocker arms. After removing the required thickness of shims to lower the roller rockers and therefore move the tip of the rocker further out in relation to the valve stem, the rocker geometry is checked for a second time. This time the measurement comes out right on the money at 0.140-inch. Thus, the tip of the rocker is exactly centred on the valve stem half way through its travel. This ingenious, accurate and time-saving tool is available from A1 Hi Performance in Myaree.

Dial Gauge available from A1 Hi Performance At half valve lift the tip of the roller must be exactly in the centre of the valve stem.

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