This blog, in addition to the usual maintenance details for classic mini cars, an attempt is made to explain the "whys" and "wherefores" of the various jobs.

Saturday, 17 October 2009

Valve Clearance Adjustment

If a small, highly-efficient overhead-valve engine is to be kept in good tune it is essential to check the valve clear¬ances at 6,000-mile intervals. The clearance between the tips of the inlet and exhaust valves and the rockers, when the valve is closed and the tappet is on the base of the cam, should be 0-012 in. (12 thousandths of an inch), measured when the engine is cold. It will be necessary, of course, to remove the valve cover.

To make sure that each tappet is on the base of the cam, adjust No. 1 rocker with No. 8 valve (counting from the fan end of the engine) fully open—that is, with the spring fully compressed—No. 3 with No. 6 fully open and continue in this manner by selecting pairs of valves that add up to 9. To take one further example, No. 7 valve would be adjusted with No. 2 fully open.

As a starting handle is not provided, the most convenient method of rotating the engine is to remove the sparking plugs and pull on the fan belt. This enables the correct position of the camshaft to be obtained more easily than the alternative method of engaging top gear and pushing the car backwards and forwards to rotate the engine.

To adjust the clearance, slacken off the adjusting screw lock-nut at the opposite end of the rocker to the valve and insert a feeler blade between the toe of the rocker and the valve tip. The adjusting screw should now be turned with a screwdriver until the correct clearance is obtained and held in this position while the lock-nut is tightened. A firm pressure should be applied to the screwdriver in order to displace all but a thin film of oil from the cupped ends of the tappet and push-rod. A false

Adjusting the valve clearance: 1, slacken the lock-nut and rotate the adjusting screw; 2, check the gap between the rocker and the tip of the valve with a feeler gauge
clearance may otherwise be obtained. Tighten the lock-nut and re-check the clearance.

The action of tightening the lock-nut will usually be found to change the adjustment slightly, so that one or two attempts may be necessary before an accurate clearance is obtained. It is advisable to use two feeler gauges to check the adjustment—one0001 in. too large, which should not enter, and one 0-001 in. too small, which should enter easily.

Remember that if the cylinder-head nuts are tightened down, the clearances will be reduced.

Before replacing the valve rocker cover inspect the cover gasket. If there is any doubt regarding its condition it should be renewed. Make sure that the cover is seating evenly on the cylinder head and is located correctly before replacing the sealing washers and tightening the securing nuts evenly.