Press Fit for Machined Parts
The interference fit that locks two parts together without fasteners — how it works, typical interference, press fit vs shrink fit vs slip fit, and how to specify it.
IsoCNC Engineering Team
CNC Machining & DFM Engineers · Updated · 7 min read
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Quick Answer
A press fit is an interference fit — the shaft is machined slightly larger than the hole, and the parts are forced together so friction locks them in place. Typical interference is roughly 0.001–0.002 in per inch of diameter, though the real value comes from a fit designation like H7/p6. It is the standard way to lock bearings, gears, bushings, and dowel pins without fasteners. Shrink fits use heat instead of force; slip fits are the opposite — clearance, not interference.
Friction Does the Fastening
A press fit holds two parts together with nothing but contact pressure. The shaft or pin is machined slightly larger than the hole, and when the two are forced together, elastic deformation creates friction that locks them in place. No fastener, no weld, no glue — just the interference itself. It is the workhorse of permanent assembly: bearings in housings, gears on shafts, dowel pins in plates. But the difference between a fit that holds and one that slips (or one that seizes during assembly) is measured in thousandths, which is why the tolerance matters so much.
How Much Interference?
There is no universal number, because interference depends on size, material, and load — but a widely used rule of thumb is about 0.001–0.002 inches of interference per inch of diameter for many machine fits. Treat that as a starting point, not a standard. The proper way is a fit designation like H7/p6, which bundles the hole and shaft tolerances into one callout, or a calculation from the material and load. Too little interference and the part slips; too much and it seizes or cracks the housing during assembly. Before pushing interference that far, check the material’s stress-strain curve — the contact stress needs to stay well under yield, with margin left for the assembly load.
For real numbers, the ISO 286 hole-basis system answers it directly. The table below gives the two classes shops actually run — H7/p6, the light press an arbor press still handles, and H7/s6, the heavy end that usually wants heat or a hydraulic press. Reading it: interference = shaft size − hole size; the minimum comes from the smallest shaft in the largest hole, the maximum from the largest shaft in the smallest hole.
| Nominal Ø (mm) | H7 hole (µm) | p6 shaft (µm) | H7/p6 interference (µm) | s6 shaft (µm) | H7/s6 interference (µm) |
|---|---|---|---|---|---|
| 6–10 | 0 / +15 | +15 / +24 | 0–24 | +23 / +32 | 8–32 |
| 10–18 | 0 / +18 | +18 / +29 | 0–29 | +28 / +39 | 10–39 |
| 18–30 | 0 / +21 | +22 / +35 | 1–35 | +35 / +48 | 14–48 |
| 30–50 | 0 / +25 | +26 / +42 | 1–42 | +43 / +59 | 18–59 |
| 50–65 | 0 / +30 | +32 / +51 | 2–51 | +53 / +72 | 23–72 |
| 65–80 | 0 / +30 | +32 / +51 | 2–51 | +59 / +78 | 29–78 |
Deviations per ISO 286-2, lower / upper limit. The engineering fits guide explains the letter-and-grade system behind these callouts, and the transition vs interference fit guide shows where the lighter classes (k6, m6, n6) stop guaranteeing interference at all.
Press Fit vs Shrink Fit vs Slip Fit
Three fits that look similar on paper but behave completely differently:
| Fit | Shaft vs hole | Assembly |
|---|---|---|
| Press fit | Shaft larger (interference) | Mechanical force |
| Shrink fit | Shaft larger (interference) | Heating/cooling |
| Slip fit | Shaft smaller (clearance) | Slides freely |
Press and shrink are both interference fits — the difference is assembly method (force vs heat). Slip fit is the opposite: clearance, so the parts move freely. Our engineering fits guide covers the full clearance-transition-interference system. For the two borderline categories compared tolerance band by band — including when a mallet fit stops being enough — see our transition fit vs interference fit guide.
Applications and the Four Factors
Press fits lock bearings into housings, gears onto shafts, bushings and sleeves, and dowel pins — anywhere two parts must stay rigidly together without fasteners. Four factors determine whether the fit works: material properties (softer or more elastic materials need different interference), surface finish (rougher grips more but measures differently), temperature (thermal expansion changes the fit in service), and geometry (thin walls deform more than solid sections). Get any one wrong and the fit fails quietly.
How to Specify a Press Fit
Two ways. The fit designation — H7/p6 in ISO 286, or the ANSI B4.1 equivalent — bundles the hole and shaft tolerances into one callout and is the cleaner choice. Or the explicit limits — nominal diameter plus tolerance limits on both mating parts. Either way, add assembly notes where they matter: press force, heat, lubrication, or orientation. For the tolerance side, see our tolerance guide, and for the holes these fits land in, our reaming guide. And when a drawing leaves non-critical features without their own tolerance callouts, ISO 2768 general tolerances are the fallback that fills the gap.
FAQ
What is a press fit?
A press fit is an interference fit: the shaft or pin is made slightly larger than the hole, and the parts are assembled by force so that elastic deformation creates contact pressure and friction. That friction is what holds the two parts together without fasteners. It is the standard way to permanently lock a bearing into a housing, a gear onto a shaft, or a dowel pin into a hole.
How much interference does a press fit need?
There is no single universal number — it depends on size, material, and application. A commonly cited rule of thumb is about 0.001–0.002 inches of interference per inch of diameter for many machine fits, but that is only a starting point, not a standard. The real value comes from a fit designation like H7/p6 or a calculation based on the material and load.
What is the difference between press fit and shrink fit?
Both are interference fits with the same principle — the shaft is larger than the hole. The difference is assembly: a press fit uses mechanical force, while a shrink fit uses temperature change (heating the outer part or cooling the inner) so the parts slip together and lock when the temperature equalizes. Shrink fits are used where pressing force would damage the parts or when the interference is too large to press.
What is a press fit used for?
The classic applications are bearings pressed into housings, gears pressed onto shafts, bushings and sleeves, and dowel pins locating two parts. Anywhere two parts must stay rigidly locked together without fasteners, a press fit is the standard answer. It is the mechanical alternative to welding, gluing, or bolting.
What factors affect a press fit?
Four things: material properties (softer or more elastic materials need different interference), surface finish (rougher surfaces grip more but measure differently), temperature (thermal expansion changes the fit at operating temperature), and part geometry (thin walls deform more than solid sections). All of these must be accounted for or the fit will be too loose to hold or too tight to assemble.
How is a press fit specified on a drawing?
Specify the nominal diameter plus tolerance limits on both mating parts, or use a standard fit designation like H7/p6 (ISO 286) or the ANSI B4.1 equivalent. The fit designation is cleaner — it bundles the hole and shaft tolerances into one callout. Add assembly notes if needed (press, heat, lubrication, orientation). Our engineering fits guide covers the designation system.
What is the difference between a press fit and an interference fit?
Interference fit is the class name; press fit is the shop name for the same condition — a shaft bigger than the hole across the whole tolerance band, so the parts only separate with force. In everyday use, press fit usually means the lighter interference classes an arbor press can still assemble, while the heavy classes (s6, u6 territory) drift toward being called shrink or force fits. The terminology is one family, and the full system — clearance, transition, interference, with the ISO 286 classes — is worked through in the transition vs interference fit guide.
What is the difference between an interference fit and a transition fit?
Whether overlap is guaranteed. In an interference fit the worst-case shaft is still bigger than the best-case hole — every assembly is tight, so the fit transmits torque or locks location permanently. In a transition fit the tolerance zones overlap: some assemblies land with slight interference, some with slight clearance (H7/k6 is the classic), which is why transition fits locate things — dowel pins, bearing inner races, bushings that still slide in by hand — instead of locking them. The transition vs interference fit comparison runs the numbers at Ø50 mm, including where each class hands off to the next.
What is a typical press fit tolerance for a 25 mm shaft?
A 25 mm shaft falls in the 18–30 mm ISO 286 range, so the table above applies directly. The standard light press fit, H7/p6, puts the hole at 25.000–25.021 mm and the shaft at 25.022–25.035 mm — 1 to 35 µm of guaranteed interference, and an arbor press assembles it. The heavy class, H7/s6, puts the shaft at 25.035–25.048 mm for 14–48 µm of interference; that one usually wants heat or a hydraulic press. Anything past s6 is shrink-fit territory, not a press fit anymore.
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Sources
Standards and references behind the numbers on this page.
- ISO 286-2, ISO code system for tolerances on linear sizes — Part 2: tables of tolerance classes and limit deviations — the standard the H7/p6 and H7/s6 deviation tables on this page are calculated from
- Fractory, Limits and Fits — fit-class definitions and the Ø25 mm worked example that matches the 18–30 mm band in our table
- Interference fit — Wikipedia — press fit vs shrink fit definitions and rule-of-thumb interference ranges
Press Fits Machined to Hold
IsoCNC holds press-fit tolerances to the H7/p6 callout — pins, bores, and housings that lock when assembled. Send the drawing; an engineer reviews every RFQ, typically within 1–2 business days.