JIS Thread Callouts on Machined Parts: Pitch, Tolerance Class and Gauging

Drawings / 11 Sept 2026

Threads are where a drawing quietly hands a decision to the shop. M6 looks complete. It is not. It leaves the pitch open on sizes that have more than one, it leaves the tolerance class entirely unspecified, and it says nothing about how the thread will be verified.

On a part that will be gauged at incoming inspection, that is three chances for a rejected lot.

The two standards

JIS B 0205 defines metric screw threads: the basic profile, diameters and pitches. It is where coarse and fine series come from.

JIS B 0209 defines the tolerances: grades, positions and the class combinations that result. It corresponds to ISO 965.

A complete callout uses both. M6 x 1 - 6H names the size, the pitch from B 0205 and the class from B 0209.

Pitch, and why leaving it off is risky

Most common sizes have a coarse pitch and one or more fine pitches:

Size	Coarse	Fine options
M6	1.0	0.75
M8	1.25	1.0, 0.75
M10	1.5	1.25, 1.0, 0.75
M12	1.75	1.5, 1.25, 1.0
M16	2.0	1.5, 1.0
M20	2.5	2.0, 1.5, 1.0

If you write M10 a shop will cut 1.5 pitch, because coarse is the default and that is a reasonable reading. If your mating part is M10 x 1.25, you now have a bin of parts that do not assemble.

Write the pitch. Always. M10 x 1.5 costs nothing extra and removes the assumption.

Fine pitches matter more on stainless than people expect. They give more thread engagement per unit length and better vibration resistance, and they are also more prone to galling, which is covered below.

Tolerance classes

A class is a grade number plus a position letter.

Grade sets how much tolerance there is. Lower is tighter. Grades 4 to 8 are in common use, with 6 as the normal general purpose grade.

Position sets where that tolerance sits relative to the basic profile, and therefore the allowance:

Internal threads: G has a positive allowance, H has none
External threads: e, f, g have progressively smaller allowances, h has none

The common combinations:

Pair	Fit	Use
6H / 6g	General purpose, small clearance	The default. Use this unless you have a reason not to
6H / 6h	Closer, no allowance	Precision assemblies, unplated
6G / 6g	Extra clearance	Plated or coated threads, hot service
5H / 5g or 4H / 4h	Tight	Fine positioning, instrument work
7H / 7g or 8g	Loose	Rough service, easy assembly, hot conditions

If the part will be plated or coated after machining, allow for it. A 5 micrometre plating adds roughly four times that, about 20 micrometres, to the effective pitch diameter, because it deposits on both flanks and the geometry multiplies it. A 6H hole that is plated afterwards may no longer accept a 6g bolt. Specify 6G, or specify that threads are masked.

Depth, and the difference between two numbers

A thread callout needs two depths on a blind hole:

M8 x 1.25 - 6H
Thread depth 16 min
Drill depth 20 min

Give only one and the shop guesses the other. Give neither and you get whatever the standard tool length produced.

A useful default for stainless: thread engagement of about 1.5 to 2 times the diameter. Deeper than 2D adds very little strength and adds real cost, because tapping stainless deep is slow and breaks taps.

Stainless specifics

Galling. Austenitic stainless threads seize against each other. The oxide layer that protects the surface is also what welds under pressure once it is disturbed. It happens during assembly, at moderate torque, and once it happens the fastener is destroyed.

What helps:

A looser class. 6H/6g rather than 6H/6h
Slower assembly speed. Galling is a friction and heat effect
Anti seize or a dry film lubricant, specified on the assembly drawing rather than left to the fitter
Different grades on the two halves. A SUS303 bolt in a SUS304 hole galls less than SUS304 against SUS304
Rolled rather than cut external threads where geometry allows, for a smoother flank

Tapping is where taps break. Stainless work hardens, so a tap that dwells creates a hardened layer ahead of itself. Shops use spiral flute taps for blind holes, correct hole size, thread forming taps on the softer grades, and through coolant where available. From your side, the useful lever is not asking for depth you do not need.

Hole size. A tapped hole is normally cut to about 70 to 75 % thread rather than 100 %. Full depth thread is barely stronger and dramatically harder to tap in stainless. If your drawing specifies a percentage, 65 to 70 % is a reasonable request on SUS316L.

Saying how it will be checked

If the thread matters, say how it will be verified:

M8 x 1.25 - 6H
Gauge with GO / NO-GO per JIS B 0251, record result

That tells the shop to own a gauge for this size and to build the result into the inspection sheet. It also tells them the thread is not a clearance feature, which changes how carefully they set up.

Do not ask for gauge records on every thread on the part. It is real cost per feature. Ask on the ones that carry load or seal.

A complete callout

M10 x 1.25 - 6H
Thread depth 20 min, drill depth 25 min
70% thread
GO / NO-GO gauged, result recorded

Four lines, no assumptions left for anybody.

For the rest of the title block, see JIS B 0405 and B 0419 general tolerances and the JIS drawing checklist.

Have a part with critical threads? Get a quote. The part is placed with a shop that owns the gauges, and the gauge result is recorded at final inspection. Priced quote within 24 business hours.

Questions

Is M6 enough on a drawing?

Only for a non critical thread on a part nobody will gauge. For anything that has to assemble reliably, give the pitch and the tolerance class, for example M6 x 1 - 6H for an internal thread.

What does 6H mean?

It is the tolerance class for an internal thread to JIS B 0209. The number is the tolerance grade and the letter is the position. 6H is the standard general purpose internal class, the equivalent of a medium fit.

What is the external thread equivalent of 6H?

6g. A 6H/6g pair is the normal general purpose combination and gives a small clearance, which is what you want on a plated or coated part. 6H/6h is a closer fit with no allowance.

Do stainless threads need anything special?

They gall. Austenitic stainless threads seize against each other under load, especially with fine pitches and fast assembly. Design with a slightly looser class, and specify a lubricant or an anti seize on the assembly drawing.

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