Pre-plate pitch diameter calculator

Works out the before-plating pitch diameter limits and the thread gage sizes needed to inspect a part before it is coated. Companion to the three-wire pitch diameter calculator.

Reference tool. Not a certified measurement. UN inch class limits are computed from published ASME B1.1 formulas. Pre-plate values are computed from the geometry of the 60° thread form. All values are provided for reference and estimation only. The controlling documents are ASME B1.1, ASME B1.2 and the customer's print or plating specification; where this page and those documents disagree, they govern. The choice of pre-plate convention comes from the print or plating spec, not from this tool. Verify all values before using them for inspection, acceptance, gage procurement, or the disposition of product. This tool does not compute gagemaker's tolerance, does not reproduce coating thickness tables from ASTM or military specifications, and does not compute metric class limits. No warranty of any kind is made as to accuracy, completeness, or fitness for a particular purpose, and no liability is accepted for any loss or damage arising from its use. Version 1.0 · Pre-plate values computed, not transcribed · Last reviewed August 2026

Inputs

Thread

UN inch series, limits computed Enter after-plating limits manually

Thread

Class

Classes 1A and 1B are not offered. Pre-plate work on a class 1 thread leaves almost no band.

Coating

Process, as written on the print

Label only. Does not change the arithmetic.

Thickness units

Thickness is stated as

Radial, per surface Diametral, on diameter

Plating specs almost always state radial thickness per surface. This is the most commonly misread part of a plating callout. A diametral figure is halved before use.

Thickness given as

A single value A minimum and maximum

Thickness

As in "allow for .0002 zinc".

Pre-plate convention

B Full range, narrowed A Band preserved C Single limit shifted

Formulas and conventions

The 4t relationship

Radial thickness from a diametral figure	`t = t_diametral / 2`
Flank displacement along the pitch line	`t / sin 30° = 2t`
Change in pitch diameter, both flanks	`4t`
External thread, pre-plate	`PD_before = PD_after − 4t` smaller
Internal thread, pre-plate	`PD_before = PD_after + 4t` larger

The three conventions

Convention	Limit at risk	Other limit	Band
A band preserved	shifted by 4 t_max	shifted by 4 t_max	unchanged
B full range, narrowed	shifted by 4 t_max	shifted by 4 t_min	narrowed by 4(t_max−t_min)
C single limit shifted	shifted by 4 t_max	unchanged	narrowed by 4 t_max

For an external thread the limit at risk is the maximum, because plating only moves the pitch diameter up. For an internal thread it is the minimum, because plating only moves it down. Convention C leaves the other limit alone on exactly that reasoning.

When a single thickness is entered, conventions A and B are algebraically identical, since t_min and t_max are the same number. Only C differs.

Note on convention C. You asked for A and B. C was added because it follows the directional logic of ASME B1.1 para. 7.5, which handles coating by decreasing the maximum pitch diameter by four times the coating thickness and does not move the minimum. A print that invokes B1.1 for coated threads implies the tightest of the three bands. C is included so that reading is available, not because it is the default. The correct choice comes from your print or plating specification, not from this tool.

Gage pitch diameters

External, thread ring gage	GO = pre-plate maximum · NO-GO = pre-plate minimum
Internal, thread plug gage	GO = pre-plate minimum · NO-GO = pre-plate maximum

GO always sits at maximum material. These are basic gage pitch diameters. Gagemaker's tolerance, X or W per ASME B1.2 or B1.16M, is tabulated rather than derivable from a formula and is deliberately not computed here. Your gage maker applies it.

Where the after-plating limits come from

UN inch series: computed. Same ASME B1.1 engine as the three-wire tool. D₂ = D − 0.6495190528 p, T₂ₑ = 0.0015 ∛D + 0.0015 √Lₑ + 0.015 ∛(p²), allowance 0.300 T₂ₑ for class 2A and zero for 3A, PD multipliers 2A 1.000, 3A 0.750, 2B 1.300, 3B 0.975. Length of engagement is D for UNC and UNF and 9p for UNEF, per B1.1 para. 5.1.

Metric and everything else: entered by you. ISO 965-1's foreword states that its tolerance values were calculated from the formulae and rounded to the nearest R40 preferred number, but that these rounding rules were not always applied in order to reproduce a smooth progression. The tables therefore contain values no rounding rule will reproduce. Rather than publish metric limits that are right most of the time, this tool asks you to supply them from a controlled source. Manual entry is also the path for UNS, modified pitch diameters, and anything outside the standard UN series.

Not covered

Gagemaker's tolerance. Coating thickness tables from ASTM or military specifications, which you enter yourself. Non-uniform or geometry-dependent deposition, including thickness variation between the crest, flank and root, and the heavier build that occurs at thread crests in electroplating. Plating on UNR or UNJ root forms. Taper threads. Lead and flank angle effects of coating. Classes 1A and 1B.