Valve spring tension is all that keeps the valves from hitting the pistons. A “swallowed” valve is the mechanic’s equivalent of the great Lisbon earthquake or the gas blowout at King Christian Island, which illuminated the Arctic night for eight months and could be seen from the moon. Thus, I suggest that valve springs be replaced (regardless of apparent condition) during upper engine overhauls.
If springs are to be used, inspect as follows:
• Carefully examine the springs for pitting, flaking, and flattened ends.
• Measure spring freestanding height and compare with the factory wear limit.
• Stand the springs on their ends and, using a feeler gauge and machinist’s
square, determine the offset of the uppermost coil (Fig. 7-35). Compare with the factory specification (in angular terms, maximum allowable tilt rarely exceeds 20). Most keeper failures arise from unequal loading.
• Verify that spring tension falls within factory-recommended norms. Figure 7-36 illustrates the tool generally used to make this determination.
Valve spring shims have appropriate uses, chiefly to restore the spring preload lost when heads and valve seats are refurbished. But shims should not be used as a tonic for tired springs, because the fix is temporary and can result in coil bind.