Mercury F8CZ-6505-AA Valve - Exhaust

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A thicker bucket tappet must be fitted.

5. If adjustment is needed, change the shim thickness by depressing the cam follower, removing the old shim, and fitting a new one. 6. Before changing a shim, lower the piston for the relevant cylinder from its TDC position by turning the crankshaft approximately 90° clockwise to avoid the risk of bending the valve. 7. Dealer technicians use service tool 303-563A, which is bolted onto the cylinder head above each camshaft using two of the camshaft bearing cap bolt locations. A spacer may be required to allow the tool to be bolted onto the head. 8. Take care not to scratch or damage the cam follower and its housing. Remove the shim with a small screwdriver or magnetic probe, then slowly release the pressure applied to the tool. Do not rotate the camshaft while a shim is removed. 9. Record the thickness of the shim, which is engraved on the side facing away from the camshaft. If the marking is missing or illegible, a micrometer will be needed to establish shim thickness.

1. Examine the head of each valve for pitting, burning, cracks, and general wear, and check the valve stem for scoring and wear ridges. 2. Rotate the valve and check for any obvious indication that it is bent. Look for pits and excessive wear on the tip of each valve stem. Replace any valve that shows signs of wear or damage. 3. If the valve appears satisfactory, measure the valve stem diameter at several points using a micrometer. Any significant difference in the readings indicates wear of the valve stem, and the valve(s) must be replaced if conditions are apparent. 4. If the valves are in satisfactory condition, they should be ground (lapped) into their respective seats to ensure a smooth gas-tight seal.

The required thickness of the new shim can be calculated based on the desired clearance, measured clearance, and the shim thickness found.