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Tuesday, August 21, 2018

Differences In Thread Fit - 1A/2A/3A & 1B/2B/3B

Differences In Thread Fit - 1A/2A/3A & 1B/2B/3B

Thread fit measures the looseness of fit between male and female threads.  For external threads the three fit classes are 1A/2A/3A and for internal thread (for example in hex nuts) 1B/2B/3B.  The higher the class number the tighter the fit - 3A/3B fit therefore is the tightest and presents the greatest risk when the parts require any type of plating or additive finish.  In our experience parts that require 3A/3B thread fit are at greater risk of not passing the Go/No-Go gauge test after electroplating, especially at smaller diameters.  Typically this happens because more of the plating material is added to the part at it's extremities, making a consistent plating thickness difficult to achieve and thereby compromising the tight fit tolerance at the end of the screw where the gauges engage.

1A and 1B thread fits are not used very often and are typically only found for thread diameters 1/4" or greater in both coarse and fine thread pitch.  The vast majority of the standards and custom made fasteners are manufactured to a 2A and 2B thread pitch, which allows for the best balance in cost and performance.  Tolerances vary for each thread diameter and we are happy to assist you with greater detail for your situation anytime.  The diagram shown here is an example for 1/2-13 UNC thread pitch.

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