An enquiry for "an M20 thread gauge" cannot be filled. The letter and number identify one of at least four different threads, and the request omits whether the gauge is for a nut or a bolt. Four elements make a complete metric designation, and every one of them changes the tool.
Take M20×1.5-6H and read it left to right.
M means ISO metric, 60 degree flank angle, the system covered by ISO 965 for tolerances and ISO 1502 for gauging. India adopts the same system through the IS 4218 series, and IS 2334 covers the gauges. If you see UNC, UNF, BSW, BSP, NPT or G in a callout, you are in a different system with different flank angles and different gauges, and no amount of matching diameters will make them interchangeable.
This is the size the thread is named after, not any diameter you can conveniently measure on a finished part. The actual major diameter of an external thread sits slightly below 20 mm, and the number that decides fit is the pitch diameter, which no callout states directly.
Pitch is the distance between adjacent crests. M20 has a coarse pitch of 2.5 mm and fine pitches of 2, 1.5 and 1 mm. A callout with no pitch means coarse by convention, so M20 alone is M20×2.5.
This is where most ordering errors happen. An M20×1.5 gauge will not check an M20×2.5 thread at all. The two are unrelated products of the same nominal size, and a shop that orders by diameter alone receives a gauge that screws into nothing.
| Designation | Pitch | Threads in 20 mm of length |
|---|---|---|
| M20 (coarse) | 2.5 mm | 8 |
| M20×2 | 2.0 mm | 10 |
| M20×1.5 | 1.5 mm | 13 |
| M20×1 | 1.0 mm | 20 |
Two characters, and both matter.
The number is the tolerance grade, which sets how much variation is allowed. Grades run from 3 to 9. Grade 6 is the general-purpose middle, roughly equivalent to a medium fit. Lower is tighter, higher is looser.
The letter is the tolerance position, which sets where that band sits relative to basic size. Capital letters describe internal threads, so a nut or tapped hole. Lower case describes external threads, so a bolt or stud. H means no allowance, the band starts at basic size. G is an internal position with a small positive allowance. For external threads, h has no allowance and g has a small negative one, which leaves room for plating or coating.
A metric tapped hole callout uses a capital letter for that reason. M20×1.5-6H on a drawing is the nut or the tapped hole, inspected with a thread plug gauge. The matching bolt is usually 6g and needs a thread ring gauge.
So 6H is an internal thread of medium tolerance with no allowance, checked with a thread plug gauge. 6g is the matching external thread, checked with a thread ring gauge. A single callout of 6H/6g describes the pair.Getting the case wrong is the second common ordering error. Asking for "6H" when you meant the bolt gets you a plug gauge for a hole you do not have.
Length of engagement, which affects which tolerance group applies on long or short engagements. Left-hand threads, marked LH, since nothing in a plain callout implies right-hand. Multi-start threads, which state lead as well as pitch. Coating, which is why an external thread might be specified 6g rather than 6h in the first place.
State those alongside the designation when they apply. Send the drawing if you have one, since a drawing settles the engagement length and the handedness without a further exchange.
Our thread gauges are made to the designation you give us, and we will query a callout that looks incomplete rather than guess at a pitch.
M is the ISO metric system, 20 is the nominal major diameter in millimetres, 1.5 is the pitch in millimetres, and 6H is the tolerance class. The capital H means an internal thread — a nut or a tapped hole — so it is inspected with a thread plug gauge.
M20 alone means the coarse series, which is M20×2.5. M20×1.5 is a fine pitch: a different thread at the same nominal diameter, and a gauge for one will not check the other.
The number is the tolerance grade; grade 6 is the general-purpose middle of the 3 to 9 range. The letter is the tolerance position: a capital letter describes an internal thread with no allowance, so the band starts at basic size. A lower-case letter describes an external thread — 6g is the matching bolt and needs a thread ring gauge.
Length of engagement, left-hand threads (marked LH), multi-start threads, and coating. State those alongside the designation when they apply, or send the drawing, which settles engagement length and handedness without a further exchange.
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