Does anodizing alter the bolt/nut thread pattern and diameter?
Yes. Anodizing is a process that converts aluminum to its oxide. The oxide is thicker than the aluminum that is consumed, which means the dimension of the anodized part changes. The amount of change will depend on the anodizing process conditions (temperature, current density, etc.) and alloy. Under nominal Type II anodizing conditions, the rule of thumb is 2/3 in 1/3 out; for example, a coating that is 0.6 mil thick will have consumed 0.4 mil of aluminum. Under hard coat (Type III) anodizing conditions the ratio changes to ½ in ½ out. Keep in mind that when calculating the shrinkage of a hole, you must double the amounts given because a hole has two sides, for example, the hole diameter reduction for a 0.6 mil Type II coating would be (1/3 of 0.6 mil) x 2 = 0.4 mil. Another example to consider is the hole reduction of a 1.5 mil Type III coating (1/2 of 1.5mil) x 2 = 1.5 mil. Other processes in an anodizing line (including, for example, chemical etching and brightening) will affect dim
