The best way to insure that threads are in optimum condition is by utilizing thread restoring tools. These tools are designed to clean and restore existing threads, not to cut new ones.
Most typical tap and die sets are designed to cut new threads and their ability to remove material is detrimental to used threads. Male and female threads found throughout the automotive industry have approximately 75% contact between the threads and using a tap and die set will very likely reduce this contact percentage further by removing necessary material from the fastener and/or the threaded receiver. This reduction in mass will affect torque values and ultimately the ability of the fastener to maintain proper holding properties.
Thread restoring sets can be obtained from most, if not all, of the major automotive tool suppliers and are a valuable addition to any tool collection. Finally, all blind holes most be free of residue and all threads and contact surfaces should be lubricated sparingly with clean oil or assembly lube.
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