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On Fiat OHC engines, the adjusting shims—called bis- cuits’—are recessed in the top face of the tappet. If a clear- ance is incorrect, a special tool is used to lever the tappet away from the camshaft, the old bis- cuit is removed and a new one of the correct thickness is inserted in its place.

On Vauxhall OHC engines, no shims or special tools are needed. This means that adjustment is a straightforward DIY job. The gap is altered by a small transverse Allen screw in the tappet, with a ramp on one side. The illustrations show how the gap is adjusted.

Step 1. Identify the inlet and exhaust valves.

2. Check the clearance on each one with the cam lobe pointing away from the tappet.

3. To alter the clearance, rotate the tappet until a niche appears. The access hole for the screw lies below the niche and aligns with a cut-out in the head casting.

4. Use a small Allen key to turn the screw one full turn at a time, anti-clockwise to increase the gap, or clockwise to decrease it. Each complete revolution of the screw alters the gap by 0.003in (0.08mm).

5. After adjustment, re-check the gap with a feeler gauge.