H7/h6 Fit Tolerance: Clearance, Not Transition

H7/h6 fit tolerance is a clearance fit. Not a transition fit, not "basically zero" — a clearance fit, and the deviation table settles the argument in two lines. Half the confusion in RFQ threads about fits comes from people quoting the letter and skipping the number, then blaming the machinist when the shaft won't go in. This is what each character means, what the numbers actually are, and where the code belongs on the document your buyer reads.

What H7/h6 fit tolerance means, in one sentence

H7/h6 is an ISO 286 hole-basis fit in which the hole carries tolerance class H7 and the mating shaft carries h6; both classes start exactly at the nominal size, so the pair can only ever produce clearance, never interference.

That last clause is the whole point. The hole's smallest permitted size and the shaft's largest permitted size are the same number. The tightest possible outcome is zero clearance — metal-to-metal, assembled by hand or with a light tap. It cannot press.

The four characters, decoded

Character	What it controls	H7/h6
Uppercase letter	The feature is a hole (internal) and where its tolerance zone sits relative to nominal	H = zone starts at nominal, runs positive
First number	Tolerance grade (IT grade) of the hole — how wide the zone is	7 = IT7
Lowercase letter	The feature is a shaft (external) and where its zone sits	h = zone starts at nominal, runs negative
Second number	Tolerance grade of the shaft	6 = IT6

Two independent things are encoded: the letter says where the band sits, the number says how wide it is. Change the letter and you change the character of the fit. Change the number and you change the price of the part.

Here is what "change the number" costs, at the same nominal diameter. For a nominal size over 18 up to 30 mm, the ISO 286-2 hole deviations are:

Hole class	Upper deviation	Zone width
H7	+21 µm	21 µm
H8	+33 µm	33 µm
H9	+52 µm	52 µm
H11	+130 µm	130 µm

Same letter, same diameter, same drawing. H11 is six times looser than H7 and a fraction of the cost to hold. If a buyer specifies H7 on a bracket clearance hole because it "looked precise," they are paying for a reamed bore they will never use.

Hole basis vs shaft basis, and why H7 is on nearly every drawing

Hole basis means the hole is fixed at the nominal size and the shaft moves to create the fit. Shaft basis is the mirror: the shaft is fixed (h) and the hole moves. Both are legitimate under ISO 286-1.

Hole basis wins in practice for a boring mechanical reason: a hole is cut by a tool of a fixed size. A reamer, a drill, a broach, a plug gauge — each exists in a discrete size. Shafts are turned or ground to whatever diameter you want with the same tool. So it is cheaper to standardise the hard-to-change side. That is why you will see H7/g6, H7/h6, H7/k6 and H7/p6 across catalogues from four continents, and shaft-basis codes mainly where a piece of drawn bar stock or a commodity shaft is the fixed element.

If you are a supplier and the buyer's drawing uses shaft basis, do not silently convert it. Say so in the RFQ reply.

Clearance, transition, interference: the three outcomes with real numbers

Take a 25 mm nominal diameter (inside the "over 18 up to 30 mm" band). Hole H7 = 25.000 to 25.021 mm. Now vary the shaft:

Fit	Shaft limits (mm)	Result range	Class
H7/f7	24.959 – 24.980	0.020 to 0.062 mm clearance	Clearance — running fit
H7/g6	24.980 – 24.993	0.007 to 0.041 mm clearance	Clearance — sliding fit
H7/h6	24.987 – 25.000	0 to 0.034 mm clearance	Clearance — locational
H7/k6	25.002 – 25.015	0.015 mm interference to 0.019 mm clearance	Transition
H7/n6	25.015 – 25.028	0.028 mm interference to 0.006 mm clearance	Transition, interference-leaning
H7/p6	25.022 – 25.035	0.001 to 0.035 mm interference	Interference — light press

Read the h6 row again. The worst case is 34 µm of clearance — about half the thickness of a human hair. The best case is zero. There is no arrangement of in-tolerance parts that produces an interference. Anyone calling H7/h6 a transition fit is describing k6 or n6.

The transition rows are the ones worth staring at. H7/k6 can come out either way, which is exactly what it is for: a part that must be located precisely but still be removable. If your assembly process assumes it always presses, one batch machined at the low end of k6 will fall out in the buyer's hands.

The four confusions that cost real money

"H7 is a tolerance"

H7 is a tolerance class, not a value. It resolves to a number only once you know the nominal size. H7 is 21 µm over 18–30 mm and a different figure in every other size band. Writing "H7 tolerance ±0.02" on a spec sheet is two contradictory statements in one line. Pick one — the code or the explicit limits — and if you write both, make sure they agree.

"h6 and H6 are the same thing"

Case is load-bearing. Uppercase = hole, lowercase = shaft. H6 on a shaft dimension is not a tight spec, it is a typo, and a machinist reading it will either call you or guess. In a translated Chinese-to-English quotation or a retyped spec table, this is one of the most common corruptions we see — right alongside a lost decimal separator. Check the case of every fit code before the document goes out.

"The fit code covers everything"

It does not. A fit code controls size, and size only. A shaft can be dead-on 24.995 mm at every point you measure and still be bent, tapered, or lobed enough to seize in the bore. Form and orientation come from geometrical tolerancing, which ISO 8015 sets the ground rules for. If roundness and cylindricity matter to your assembly, call them out separately — the fit code will not do it for you.

"General tolerances will cover the rest"

They cover the dimensions you did not tolerance, and only in the way the referenced general-tolerance standard says. That is a separate decision from the fit code, and the general-tolerance landscape shifted when part of the old two-part ISO 2768 arrangement was withdrawn. If your title block still cites a standard that no longer exists in the form you think it does, read tolerance stack-up before your next drawing pack goes out.

What to actually specify: a selection table you can copy

You need	Use	Assembly method	Typical use
Free rotation, room for oil film	H7/f7	Slides freely	Bushings, slow-running journals
Slides and turns, minimal play	H7/g6	Slides by hand	Spigots, plug gauges, sliding pins
Located, no play, removable	H7/h6	Hand or light mallet	Locating pins, spacers, jig bushes
Located tightly, removable with a puller	H7/k6	Light press or tap	Gears on shafts with a key
Located, essentially permanent	H7/p6	Press or thermal	Bearing outer races, bushings

One principle worth memorising: choose the loosest fit that still does the job. Every grade you tighten multiplies the inspection burden and the reject rate, and buyers rarely thank you for precision they did not ask for.

Where the fit code belongs on an export spec sheet

Here is where suppliers lose orders that had nothing to do with machining. The fit is correct, the drawing is correct, and the overseas buyer still asks three rounds of questions — because the fit code lives in a PDF table on page four while the buyer is looking at a photo of the part on WhatsApp.

Put the number where the eye already is. On the product photo, a leader line from the mating diameter reading "Ø25 H7" tells a purchasing engineer more in one second than a full tolerance table does in a minute. The same applies to thread callouts, bore depths, and mating face flatness. If you are exporting fittings or shafts, the identical logic that governs NPT vs BSP thread identification applies here: the spec has to be visible at the moment of decision, not retrievable on request.

The practical constraint is that most people making these images have no CAD seat and no draftsman. Tools that do this well are not photo editors and not AI image generators — an AI generator will happily paint a confident "Ø25" onto a part it never measured. What you want is annotation that snaps to the actual measured edge of the feature in the photo, holds the dimension to the geometry rather than to a text box, and exports at the size your RFQ platform or catalogue template expects. That distinction — a measured number versus a plausible-looking one — is the entire difference between a spec image and a decoration. For a worked example of how that lands on a real part, see the industrial spec diagram format.

And when you are labelling the range rather than a single fit, the general rules in product dimension tolerance cover how to show ± values so buyers trust them.

Pre-send checklist for any drawing carrying fit codes

Every fit code has a nominal size next to it (Ø25 H7, not "H7 bore")
Uppercase/lowercase checked on every code after any retyping or translation
Hole basis or shaft basis stated once, explicitly, in the notes
No dimension carries both a fit code and a contradictory ± value
Grades justified: nothing tighter than IT7 unless a function requires it
Form and orientation called out separately where they matter
General-tolerance standard in the title block is current and correctly cited
The two or three fits the buyer actually cares about are visible on the product image, not only in the PDF

FAQ

Is H7/h6 a clearance fit or a transition fit?

It is a clearance fit. At 25 mm nominal, H7 gives a hole of 25.000–25.021 mm and h6 gives a shaft of 24.987–25.000 mm, so the result ranges from 0 to 0.034 mm of clearance. Interference is arithmetically impossible with two in-tolerance parts. Transition fits start at k6.

What does the number in H7 mean?

It is the ISO tolerance grade, IT7, which sets how wide the tolerance zone is. It is not a fixed value — it resolves to a different number of micrometres in each nominal size band. Over 18 up to 30 mm, IT7 on a hole is 21 µm.

How much clearance does H7/g6 give compared with H7/h6?

At 25 mm nominal, H7/g6 gives 0.007–0.041 mm and H7/h6 gives 0–0.034 mm. The practical difference is the guaranteed minimum: g6 always leaves a gap, h6 may leave none. If the part has to slide reliably or carry a film of lubricant, specify g6.

Why do most drawings use hole basis instead of shaft basis?

Because holes are made by fixed-size tools — reamers, broaches, plug gauges — while shafts are turned or ground to any diameter with the same tool. Standardising the hole and varying the shaft is cheaper to produce and cheaper to inspect.

Does a fit code control roundness and straightness?

No. A fit code controls size only. A shaft can measure within h6 at every diameter and still be too far out of round to assemble. Geometrical characteristics have to be specified separately; ISO 8015 defines the fundamental rules for how those specifications are interpreted.