Can I plate a class 3A thread?

Last updated 10 August 2026

Not without cutting it undersize on purpose. A 3/8-16 class 3A tops out at .3344 on pitch diameter, which is the basic value exactly, so a coating has no allowance to live in the way a 2A thread's does.

Class 2A carries an allowance: its maximum pitch diameter sits a thousandth or so BELOW basic, and that gap is deliberate clearance a thin coat can partly occupy. Class 3A has no allowance at all. Its maximum is basic, so a plated 3A thread that gauged at the top of its band before plating is over the top of it afterwards, by four times the coat.

So the coating has to come out of the .0033 of tolerance, and the plater's range comes out of it too. A .0002 to .0005 window costs .0012 upstream on a thread, which is more than a third of a 3A band spent before the machinist has cut anything.

The practical answers are the ones worth knowing in advance. Specify 2A and let the allowance do its job. Or state the class as applying BEFORE plating, which is what MIL-STD-870 contemplates and what most plated fastener drawings mean. What does not work is calling 3A after plating on a print and expecting the shop to find the room.

Worked

Thread	0.375 inch nominal, 16 threads per inch, class 3A
Pitch	0.0625 in
Basic pitch diameter	0.3344 in
Allowance	none for this class
Tolerance	0.0033 in
Pitch diameter	0.3311 / 0.3344 in

Pitch diameter, not major diameter. A thread is accepted or rejected on pitch diameter, and this is the dimension a gauge checks and the one a walk-back has to move.
Class 3A is a close fit with no allowance, so the maximum pitch diameter is the basic 0.3344 exactly.
Major and minor diameter limits are not returned. They come from a different set of formulas which this engine does not carry, and reporting a partial thread specification as a complete one is worse than reporting less.
3/8-16 UNC: the tolerance is computed at the length of engagement its own published table uses, which for UNC is the major diameter.

Open this case in the calculator, and change the numbers.

Computed by ctrlPlanner. ASME B1.1, Federal Screw Thread Specification H28.

This is one thread. Pair it with walk_back_tolerance to get the size to cut before plating, and note that ctrlPlanner does that pairing for every threaded feature on a print at once, and redoes it when the drawing revises.

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Can I plate a class 3A thread?

Worked

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