Transition Fit vs Interference Fit: How to Choose the Right Fit
Choosing between a transition fit and an interference fit comes down to one thing: whether the joint should ever come apart again. A transition fit holds a part in exactly the right place and lets you take it back off. An interference fit makes the shaft permanently bigger than the hole, so the parts go together once, under force or heat, and stay. This page covers what each fit is, the real ISO 286 numbers at Ø50 mm, how shops machine and assemble them, and how to read the callout on your drawing.
IsoCNC Engineering Team
CNC Machining & DFM Engineers · Updated · 16 min read
Quick Answer
The difference comes down to one question: is the shaft always bigger than the hole? In a transition fit the tolerance zones overlap, so it depends on the pair you happen to make — some go together with a hair of clearance, others with a hair of interference. The fit locates a part precisely and still comes apart. In an interference fit the shaft zone sits entirely above the hole zone, so every pair locks, and the friction between the surfaces carries torque without a key or a bolt.
At Ø50 mm the practical line falls between H7/n6 and H7/p6: n6 spans −33 to +8 µm and most pairs want a light press; p6 spans −1 to −42 µm and never reaches clearance at all.
What Is a Transition Fit?
A transition fit is any fit where the shaft tolerance zone and the hole tolerance zone overlap. Machine every part inside its band and you still don’t know what a given pair will do: one shaft in one bore may end up with two microns of clearance, the next with two microns of interference. Across a batch, the play centers near zero.
Newcomers read that as sloppiness. It’s actually the feature. A part seated in a k6 bore sits where you put it — concentric, no rocking — and when maintenance day comes, it comes back off. That combination of precise location and reasonable removability is exactly what gears, pulleys, and hubs on shafts need.
In the hole-basis system at Ø50 mm, two designations cover nearly all real transition fits:
H7/k6 — fit result −18 to +23 µm. Assembles by hand or with a soft mallet. Hubs, gears, pulleys, dowel-located housings: parts that must sit true and must come off.
H7/n6 — fit result −33 to +8 µm. A light press for most pairs, a slide for the lucky ones. Driven bushes, plugs, armatures: parts that live on the shaft but may need to come off someday.
One limit shapes everything else: a transition fit cannot carry torque. Even the tightest pair in the batch makes only a few dozen microns of interference, and friction from that holds nothing under load. If the joint transmits power, it needs a key or a pin — or a different family of fit.
What Is an Interference Fit?
An interference fit is a fit where the shaft is always bigger than the hole. Every pair that comes off the machines is tight by design. You put it together with a press or with heat, the two surfaces squeeze against each other, and the resulting contact pressure — turned into friction — carries torque, thrust, and vibration with no fastener anywhere in the joint.
What you trade for that grip is serviceability. Choosing an interference fit is a decision that this joint is permanent, and the honest designs admit it up front.
At Ø50 mm the standard offers three levels:
H7/p6 (−1 to −42 µm) — goes together cold in a press. Bushings, bearing outer rings in housings, small hubs. This is the range people usually mean by “press fit.”
H7/s6 (−18 to −59 µm) — wants heat or a hot press. Gears and cams mounted for the life of the machine; the tightest fit you should put on a grey cast iron hub.
H7/u6 (−45 to −86 µm) — a true shrink fit, assembled by temperature alone. Permanent couplings; pulling one apart risks scrapping both parts.
The fine print matters as much as the table. The interference only exists if the machining holds the bands: one oversize reamer, or a turning insert worn through a batch, and the interference your torque calculation depended on is quietly gone. And if there’s any chance of service later, design the exit now — a jacking thread in the housing, a hub sized to be heated off. Hoping to press the joint apart dry someday is not an exit. That’s how shops learn what galled steel looks like.
The Two, Side by Side
The two families use the same IT grades and nearly the same machining. What differs is the guarantee: a transition fit guarantees a location, an interference fit guarantees a grip.
| Dimension | Transition Fit | Interference Fit |
|---|---|---|
| Fit result | Zones overlap, so each pair decides | Shaft always bigger, every pair locks |
| Ø50 example | H7/k6 = −18…+23 µm; H7/n6 = −33…+8 µm | H7/p6 = −1…−42 µm; H7/s6 = −18…−59 µm |
| Assembly | Mallet or light press | Hydraulic press, or heat / cold |
| Disassembly | Comes apart for service | Usually ruins something |
| Location accuracy | Very good | Excellent |
| Torque transfer | None — needs a key or pin | By friction alone |
| Typical designations | H7/k6, H7/n6 | H7/p6, H7/s6, H7/u6 |
| Machining demand | IT6–IT7 both features | Same grades, plus the roundness a press needs |
If you remember one row, remember disassembly. A transition fit you can service; an interference fit you sacrifice — usually both surfaces at once.
The ISO 286 Numbers at Ø50 mm
Fit tables are abstract until you pin a diameter, so this page pins Ø50 mm and reads the whole hole-basis ladder against it, from free sliding to the heaviest shrink fit in the standard. Negative numbers are interference, positive numbers are clearance.
| Designation | Type | Fit result (µm) | Assembly method | Typical use |
|---|---|---|---|---|
| H7/g6 | Clearance (sliding) | +9 … +50 | Slides freely | Spigots, guides, sliding bushings |
| H7/h6 | Clearance (location) | 0 … +41 | Hand assembly | Locating rings, gauge rings |
| H7/k6 | Transition | −18 … +23 | Rubber mallet | Hubs, gears, pulleys that must come off |
| H7/n6 | Transition | −33 … +8 | Light press | Driven bushes, plugs, armatures on shafts |
| H7/p6 | Interference (light) | −42 … −1 | Cold press | Bushings, bearings in housings, hubs |
| H7/s6 | Interference (medium) | −59 … −18 | Hot press | Permanent gears and shafts; tightest for cast iron |
| H7/u6 | Interference (heavy) | −86 … −45 | Shrink fit (large temperature differential) | Permanent couplings; removal risks destruction |
The table carries two footnotes it can’t print. The numbers move with size: IT7 is worth 21 µm over the 18–30 mm range and 25 µm over 30–50 mm, so the same designation is effectively tighter on larger parts. And the bands only mean something if the machined parts actually land inside them when they reach your dock — which is a statement about boring, reaming, and turning quality, not about the standard. Our tolerances guide lists what real shops hold, process by process.
If you need the full system — clearance, transition, and interference families, hole-basis and shaft-basis — the engineering fits guide lays it out. And if you actually came here for bolt holes, what you want is the clearance hole chart.
How to Choose
Three questions make most of the decision.
Does the joint carry torque? A transition fit holds position, not load. If the joint transmits power and there is no key or pin, it must be an interference fit. With a key, a transition fit is fine under steady, one-direction loading — a pulley on a conveyor shaft, say. Under rotating or reversing loads, even a keyed transition fit can work loose: the pairs that got clearance develop play, and play under load grows into fretting. Hubs on rotating shafts, crusher rolls, flywheels — all interference territory.
Does it need to come apart, ever? If service means removing the part, use a transition fit and accept the keyway. If removal would mean scrapping the assembly anyway, an interference fit is cheaper than fasteners and never rattles loose.
What temperatures and materials are involved? When two different metals share a joint, heat rewrites the fit. The classic trap is an aluminum hub on a steel shaft: aluminum grows about 23 µm per metre per °C and steel about 12, and the difference comes straight out of the interference. At Ø50 mm, an 80 °C service rise costs about 44 µm — more than the entire p6 band. A joint pressed tight at room temperature can be a slip fit at running temperature, which is why aluminum housings get heavier fits or steel bushings. Material sets a ceiling too: grey cast iron is brittle in tension, so it rarely goes past s6. For the force-and-pressure arithmetic behind a chosen interference, the press fit guide works the numbers end to end.
Machining: Hole Side and Shaft Side
A fit is won or lost on the final pass. Everything before that is roughing.
Hole side. A drilled hole comes out three or four IT grades too coarse for any fit, so the bore gets finished by boring, reaming, or both. Boring corrects position and size in one setup and lands IT7–IT9; reaming on top of that hits the grade dead on, because an H7 reamer makes an H7 hole by definition. When roundness decides whether the press actually seals, honing is the step after reaming. The reaming guide covers how much stock to leave and what a worn reamer does to a batch.
Shaft side. Finish turning holds k6 and n6 without drama on a decent lathe. The p6, s6, and u6 seats go to grinding after turning, and so does anything that has been hardened. The shaft machining guide covers the whole routing.
Checking. Unglamorous and non-negotiable: micrometers on shaft seats, bore gauges in holes, CMM for form and position. We quote precision features to ±0.025 mm (±0.001 in) against the drawing and run first-article inspection before the batch. The right place to catch a drifting reamer is at the machine — not in your assembly jig, with a bushing that slides in by hand.
Assembly: Press, Heat, or Cold
Transition fits go together with whatever is within reach: hand pressure at the loose end of k6, a mallet through the middle of the family, a light press for n6. The lightest interference class, p6, still goes in cold, on an arbor press or a hydraulic press with a slow ram. How much force that takes is its own calculation — the press fit guide walks through it with real numbers.
Past p6 you are working with temperature. Heat the outer part, or chill the inner one, slide the parts together, and let the temperatures equalize. Steel grows about 12 µm per metre of diameter per °C, which keeps the shop arithmetic civilized. At Ø50 mm, the worst-case u6 fit — 86 µm of interference plus a working clearance for the slide-in — needs a temperature difference near 180 °C between the parts, which makes it an oven or induction-heater job. s6 hot pressing typically runs 150–300 °C.
The cold side works too, and it’s cheaper than it sounds. Dry ice at −78 °C shrinks a Ø50 steel shaft about 0.05 mm — enough to help the light fits along. Liquid nitrogen at −196 °C manages about 0.12 mm and drops many medium fits straight into their bores.
Two cautions from people who learned them expensively. Heating a hardened part past its tempering temperature quietly changes what the part is, so know the heat-treat state of every bore before the torch comes out. And thermal assembly does not wait: parts must be clean, aligned, and seated before the temperatures equalize, because a fit that half-seizes on the way in has already damaged both members.
Reading the Callout on the Drawing
Ø50 H7/k6 reads in three pieces. The Ø50 picks the row in the ISO 286 table. The capital-letter pair is the hole: H7 means an H hole — lower deviation at zero — at grade IT7. The lowercase pair is the shaft, k6 in this example.
The hole is almost always the H, for an economic reason rather than a traditional one: one reamer or one boring target per grade covers every fit in that grade, hole after hole, while the turned shaft is cheap to move around. Shaft-basis fits do exist, mostly where the shaft arrives already ground and you don’t get to choose.
Two habits prevent most fit arguments. Put the designation on the feature instead of a ± tolerance: Ø50 H7/k6 means the same thing in every shop on earth, while ±0.01 on both parts is a custom fit nobody can look up — the unilateral vs bilateral comparison explains that trap in detail. And don’t let general notes do a fit’s job: an ISO 2768 invocation covers the non-critical dimensions, but a fitted bore and its shaft need their own callouts. Once coaxial seats meet a datum framework, you have crossed into GD&T, where the location story gets told differently.
FAQ: Transition Fit vs Interference Fit
What is the difference between a transition fit and an interference fit?
Is the shaft always bigger than the hole? With a transition fit, maybe: the tolerance zones overlap, so some pairs go together with a hair of clearance and others with a hair of interference. With an interference fit the answer is yes, always, and that certainty is exactly what you are buying.
At Ø50 mm the difference is easy to see in the numbers. H7/k6 runs −18 to +23 µm, so it lands on either side of zero. H7/p6 runs −1 to −42 µm and never gets near it. In practice, transition fits come apart when you need them to, and interference fits do not.
When should I use a transition fit?
Use one when the part must sit in exactly the right place and still come off for service. Gears and pulleys that come off during maintenance are the textbook case, and that is H7/k6 territory. Dowel-located housings, driven bushes, and armatures running under light loads are H7/n6 work instead.
One warning before you commit: if the joint must carry torque without a key or a pin, a transition fit cannot do the job. Move up to interference.
When should I use an interference fit?
Use one when the joint is permanent and the design honestly accepts that. Bushings and bearing outer rings pressed into housings take p6, gears and cams mounted for the life of the machine take s6, and couplings you genuinely never separate take u6. The joint is the fastener.
Know the price before you choose: assembly takes a press or heat, disassembly usually wrecks the surfaces, and the interference is only as real as the machining. One oversize reamer turns your press fit into a sliding fit.
What is the difference between a press fit and an interference fit?
None in principle, at least at the light end. Press fit is the shop name for an interference fit light enough to push together cold, which at Ø50 mm is roughly the p6 range, 1 to 42 µm.
Once the parts need heat or cold to mate at all, people call it a shrink fit — same family, heavier iron. The force-and-friction math lives in our press fit guide.
How much interference is typical for a press fit?
Whatever the ISO 286 table says for your diameter, because there is no honest rule of thumb like 0.1 percent; real bands jump around with diameter and grade. At Ø50 mm, p6 delivers 1–42 µm of interference to the joint. s6 delivers 18–59 µm and u6 delivers 45–86 µm.
The right number also depends on the material pair in the joint, the torque it carries, and how close both parts sit to their yield strength before the press goes on.
Can CNC machining achieve interference fit tolerances?
Yes, and at this diameter it is routine work, provided the process chosen matches the feature being held. Holes get bored or reamed into the IT6–IT8 range, and an H7 reamer lands the H7 band by design. Honing takes the bore further when roundness decides whether the press seals. Shafts hold k6 and p6 on a finish turn; anything tighter or harder goes to grinding.
At IsoCNC we quote precision features to ±0.025 mm against the drawing, check bores with bore gauges, and run first-article inspection before the batch. That is where a drifting reamer gets caught — at the machine, not at your bench with a bushing that slides in by hand.
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Fitted Bores and Shafts, Held to Band
IsoCNC machines fitted features to the callout: bores bored and reamed to H7, shafts turned to k6/n6, ground seats routed through qualified partner shops. That work runs under our CNC machining services, with CMM and bore-gauge verification and first-article inspection on every order. An engineer reviews each RFQ, typically within 1–2 business days.