Threading · Drawings · Fasteners
How to Read a Thread Callout: M, UNC/UNF and Tolerance Classes
One assembly, two callouts: a tapped hole marked M10×1.25-6H on your own part, and a bought cylinder whose rod end says 7/16-20 UNF-2A. Both are complete instructions. Each character tells the machinist, the inspector and the buyer something specific, and leaving one out changes the part.
This is how to read them, metric and inch, and what the tolerance classes at the end actually control.
> Key takeaways
- An ISO metric callout reads diameter × pitch, then tolerance class, then length of engagement and hand: M10×1.25-6H-LH. No pitch means coarse pitch.
- A unified inch callout reads size, threads per inch, series, class: 1/4-20 UNC-2A. A is external, B is internal, and 2A/2B is the standard commercial fit.
- In an ISO class like 6g, the number is the tolerance grade (smaller is tighter) and the letter is the position: H and h have no allowance, g, f and e leave room, for example for a coating.
Anatomy of a metric thread callout
ISO 965-1 sets the designation of ISO metric threads. Take a full example, M10×1.25-6H-LH, and read it left to right:
> M10×1.25-6H-LH, piece by piece
A few variants you will meet:
- M8: coarse pitch (1.25), and since no class is written, the default medium class applies.
- M10×1-5g6g: two classes for the same external thread. The first applies to the pitch diameter, the second to the major (crest) diameter. When they are the same, write it once: 6g.
- M20×2-6H/6g: a fit. Internal class first, external class after the slash.
- M16×Ph3P1.5: a multi-start thread. Ph is the lead (3 mm of travel per turn), P the pitch (1.5 mm), so two starts.
Unified inch callouts: UNC, UNF, UNEF
Inch threads follow ASME B1.1 and are written in a different order. Take 1/4-20 UNC-2A:
- 1/4: nominal size in inches. Below 1/4", number sizes are used: #10 is 0.190", #4 is 0.112".
- 20: threads per inch (TPI), always written. Pitch = 25.4 / TPI, so 20 TPI is 1.27 mm.
- UNC: the thread series. UNC is coarse, UNF fine, UNEF extra fine. Constant-pitch series (8-UN, 12-UN, 16-UN) are used on large diameters.
- 2A: the class. The number is the fit (1 loose, 2 standard, 3 tight), the letter is A for external and B for internal.
Left hand works the same way as metric: 1/4-20 UNC-2A-LH. You may also see UNR, which applies to external threads only and requires a rounded root, and UNJ (ASME B1.15), with a controlled root radius for fatigue-critical parts, typically in aerospace.
Tolerance classes: what 6H, 6g and 2A actually control
An ISO tolerance class has two parts. The number is the tolerance grade: it sets the width of the zone. The higher the number, the wider the zone, the easier the thread is to make. The letter is the position of that zone relative to the basic profile:
- H (internal) and h (external): the zone starts right on the basic profile. No allowance.
- G (internal) and g, f, e (external): the zone is pushed away from the basic profile, leaving a gap even when both parts are at their limits. From g to e, the gap grows.
Inch classes work on the same idea with less granularity: 1A and 2A include an allowance, 3A does not. Internal classes 1B, 2B and 3B have no allowance.
| Quality | ISO internal | ISO external | Unified (rough equivalent) | Typical use |
|---|---|---|---|---|
| Fine | 4H, 5H | 4h | 3B / 3A | Precise positioning, short engagement, high-precision assemblies |
| Medium | 6H | 6g | 2B / 2A | General engineering. The default, and what catalog parts are made to |
| Coarse | 7H | 8g | 1B / 1A | Quick assembly, dirty or damaged threads, long engagement |
The unified column is an approximation for orientation only. The two systems don't map one to one, so never convert a callout by this table: draw the thread in the system the part is made in.
Why the "g" in 6g matters for plated parts
On a 60° thread, a coating of thickness t increases the pitch diameter by about 4 × t, because it builds up on both flanks. For an M10 bolt in class 6g, the allowance on the pitch diameter is 32 µm. That leaves room for a coating of about 8 µm before the bolt grows past the basic size. This is the logic used for electroplated fasteners in ISO 4042: the thread is made with the g allowance, and after coating it must still accept a gauge at the basic profile (h position). Specify h on a part that will be plated, and the allowance is gone before the first part is coated. Fastener suppliers such as Bossard publish the same pairing: 6g bare or with standard electroplating, 6h after coating, 6e or 6f for thicker coatings.
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ISO 6410-1 covers how threads are drawn: thin line for the root, thick line for the crest, and the callout on a leader. What the callout must also say depends on the feature:
- Blind tapped hole: give the full-thread depth and the drill depth separately, for example M8 ↧16 and Ø6.8 ↧20. The extra drill depth leaves room for the tap's lead and for chips. The tap drill is close to diameter minus pitch: 8 − 1.25 = 6.75, so 6.8.
- Inch equivalent: 1/4-20 UNC-2B ↧.50, with a #7 drill (0.201") for the tap drill.
- External thread: give the threaded length, and say whether a thread runout or undercut is acceptable at the shoulder.
- Left-hand thread: write LH in the callout, not only in a note. See left hand vs right hand thread for where LH threads are actually used.
Common thread callout mistakes
Missing pitch
M10 on a fine thread
Without a pitch, M10 means M10×1.5. If the mating part is M10×1, write it. It happened to me: see the shop-floor story below.
Mixed systems
M6 vs 1/4-20
6.00 mm with 1.0 mm pitch against 6.35 mm with 1.27 mm pitch. Close enough to start by hand, different enough to bind and damage both threads. Same trap with M8 and 5/16-18.
Over-tolerancing
4h "to be safe"
A fine class needs special taps or dies, special gauges and more inspection time. If the function doesn't need it, 6H/6g is the right answer.
Coatings ignored
Bare tolerance on a plated part
Thick coatings eat the allowance. Say whether the class applies before or after coating, and use the allowance positions (g, f, e) for the external thread.
From the shop floor: the M10 that should have been M10×1
A special machine, a pneumatic cylinder ordered with a fine-pitch rod: M10×1. On the rod, a rod end with the same M10×1 thread. Below the rod end, the tool housing it screws into, which I had drawn as M10.
M10 alone means M10×1.5. The housing was made to the drawing, and the rod end couldn't go in.
The fix was to re-tap the housing with an M10×1 tap. Luckily, we had one in stock. The result was passable: cutting a 1 mm pitch through an existing 1.5 mm thread leaves incomplete flanks, so the thread engagement was shorter than designed. With the loads on that machine, it worked. On a more heavily loaded joint, it would have meant a new part.
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The takeaway for the shop floor
Read a thread callout as a complete spec: size, pitch, class, length of engagement, hand. What isn't written takes the default (coarse pitch, 6H/6g, normal engagement, right hand). When the default is what you want, leave it out. When it isn't, write it, because nobody downstream will guess. More technical designs for people who read drawings for a living in the mechanical engineer collection.
FAQ
What does M8×1.25 mean on a drawing?
An ISO metric thread with an 8 mm nominal diameter and a 1.25 mm pitch. Since 1.25 is the coarse pitch for M8, the drawing can simply say M8.
What does 6H or 6g mean on a thread?
It's the tolerance class. 6 is the tolerance grade (medium), the letter is the position of the zone: capital H for an internal thread with no allowance, lowercase g for an external thread with a small allowance. 6H/6g is the default general-purpose fit.
What does 2A and 2B mean on an inch thread?
It's the unified thread class from ASME B1.1. A is external, B is internal, and 2 is the standard commercial fit. Class 3 is tighter, class 1 looser.
What is the difference between UNC and UNF?
Both are unified inch threads. UNC is the coarse series (1/4-20), UNF the fine series with more threads per inch (1/4-28). UNEF is extra fine.
Is M6 the same as 1/4-20?
No. M6 is 6.00 mm with a 1.0 mm pitch, 1/4-20 is 6.35 mm with a 1.27 mm pitch. They can start by hand, then bind and damage both threads.
Sources and standards
- ISO 965-1 — ISO general purpose metric screw threads, tolerances, principles and basic data (designation, tolerance classes)
- ISO 261 — ISO general purpose metric screw threads, general plan
- ISO 68-1 — Basic profile for metric screw threads
- ISO 6410-1 — Technical drawings, screw threads and threaded parts
- ISO 4042 — Fasteners, electroplated coating systems
- ASME B1.1 — Unified inch screw threads (UN and UNR thread form)
- ASME B1.15 — Unified inch screw threads (UNJ thread form)
- Bossard — ISO metric threads, technical information (tolerance classes for commercial screws and nuts, coatings)
About the author
Gabriel Weider is a mechanical engineer with 15 years in special machines, CAD, medtech and regulated environments. He still works full-time as an engineer and designs every grabNade shirt. Read his story.