Guide
What press fit tolerance do dowel pins and bearing seats need?
A press fit tolerance is the pair of size limits, one on the hole and one on the part pressed into it, chosen so that the pin or bearing is always slightly larger than its bore. The gap between the smallest possible interference and the largest possible interference decides whether the part holds, whether it can be assembled without damage, and whether a bearing still turns freely once it is seated.
By Saola Manufacturing engineering team · Published · Updated
This guide covers the two press fits found on almost every machined assembly, the hardened dowel pin and the rolling bearing: the interference numbers from ISO 286, the force they imply, what changes in a die-cast boss, and what to put on a drawing for an overseas shop.
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How do ISO 286 hole and shaft classes turn into an interference value?
Subtract the largest hole from the smallest shaft to get the minimum interference, and the smallest hole from the largest shaft to get the maximum. ISO 286-2 lists the upper and lower limit deviations for each tolerance class and size range, so both numbers can be read straight from its tables.
In an ISO callout the letter places the band relative to nominal (H starts the hole band at nominal; p, r and s lift the shaft band above it) and the number is the IT grade that sets its width. In the hole-basis system the hole stays H7 and only the shaft letter changes.
What interference do H7/p6, H7/r6 and H7/s6 give at common sizes?
H7/p6 gives between zero and a few tens of micrometers of interference, while H7/s6 guarantees a real minimum interference at every size; per ISO 286-2, at a 10 mm diameter H7/p6 runs from 0 to 24 µm and H7/s6 from 8 to 32 µm.
| Nominal size (mm) | H7 hole band | H7/m6 (transition) | H7/n6 (transition) | H7/p6 (locational interference) | H7/r6 | H7/s6 (medium drive) |
|---|---|---|---|---|---|---|
| over 6 up to 10 | 0 to +15 | −9 to 15 | −5 to 19 | 0 to 24 | 4 to 28 | 8 to 32 |
| over 10 up to 18 | 0 to +18 | −11 to 18 | −6 to 23 | 0 to 29 | 5 to 34 | 10 to 39 |
| over 18 up to 30 | 0 to +21 | −13 to 21 | −6 to 28 | 1 to 35 | 7 to 41 | 14 to 48 |
| over 30 up to 50 | 0 to +25 | −16 to 25 | −8 to 33 | 1 to 42 | 9 to 50 | 18 to 59 |
First, H7/p6 has almost no guaranteed grip: at the loose corner of both bands the pin is the same size as the hole. That is why ASME B4.2 calls it a locational interference fit, meant for accurate location with minimal force, not for carrying torque or axial load. If the joint has to transmit load through friction alone, r6 or s6 is the more honest choice.
Second, the spread between minimum and maximum is the sum of the two tolerance bands. Per ISO 286-2, a 30–50 mm H7/s6 fit swings across 41 µm, which means the press force at the tight corner can be several times the force at the loose corner. Run every force and stress check at both ends.
Inch drawings built on ANSI B4.1 use its FN force-fit classes instead; the logic is the same, but do not mix an inch FN class with a metric H7 hole on one joint.
What hole tolerance should a hardened dowel pin press into?
A hardened dowel pin to ISO 8734 is ground to the m6 shaft class, so the press fit comes from making the hole slightly smaller, not from choosing the pin. Pair it with an M7, N7 or P7 hole in the part that should grip it, and a clearance hole in the part that must come off.
Because the pin diameter is fixed by the pin standard, the designer only controls the hole. Put the standard m6 pin into an H7 hole and the table above shows a transition fit: per ISO 286-2, a 6–10 mm pin can be 9 µm loose or 15 µm tight, which is fine for a slip-fit locating hole but not for a pin that must never walk out. Tightening the hole class moves the whole range toward interference. Per ISO 286-2, a 10 mm M7 hole runs 0 to −15 µm, N7 runs −4 to −19 µm and P7 runs −9 to −24 µm, so an m6 pin gives 6–30, 10–34 and 15–39 µm of interference respectively.
For two plates located by dowels, press the pins into one plate and slip-fit the other. The pinned plate holds the dowels permanently; the mating plate slides on and off during service without dragging the pins out. For blind holes, add a vent or a flat on the pin so trapped air does not push the pin back out or crack a thin wall.
Per ISO 8734, the standard covers nominal diameters from 1 to 20 mm. Outside that range, or for unhardened pins, match the hole to whatever class the pin standard actually uses.
How are shaft and housing fits for rolling bearings different?
The bearing bore tolerance sits entirely below nominal, so a shaft class that would be a transition fit in an H7 hole, such as k5 or m6, becomes a light interference fit under a bearing. The fit is chosen from the ANSI/ABMA-7 shaft and housing fitting tables according to which ring rotates relative to the load.
NASA’s interference-fit study of ball bearings gives a clean worked example (NASA TM-2010-216913): a 50 mm bore bearing in the ABEC-5 tolerance class has a bore deviation from 0 to −8 µm, and an m6 shaft from the ABMA-7 table runs +9 to +25 µm, so the loosest combination is 9 µm tight, the tightest is 33 µm, and the average is 21 µm.
The ring that rotates relative to the load direction is the one that needs the interference; a loose rotating ring creeps around its seat and frets. For the typical case of a shaft turning under a fixed radial load, that means an interference fit on the shaft and a looser fit in the housing, and the reverse for a stationary shaft with a rotating housing.
Tighter is not automatically better. The same NASA study found that tensile hoop stress from the inner-ring fit shortens fatigue life, with life factors from 1.00 where there was no net interface pressure down to 0.38 in the heaviest case (NASA TM-2010-216913). An interference fit also shrinks the bearing’s internal clearance, so a heavy seat fit may call for a bearing with a larger initial clearance. Choose the lightest fit that stops the ring from creeping under the worst load, not the heaviest one the table allows.
How do you estimate the pressure and press-in force for a fit?
Compute the contact pressure from the thick-walled cylinder (Lamé) equations using the maximum and minimum interference, then multiply that pressure by the contact area and a friction coefficient to estimate the force. The result is a planning figure, not a guarantee, because friction during pressing is the least certain input.
For a solid pin of diameter d pressed into a hub of outside diameter D, with diametral interference δ, the interface pressure is p = δ / (d × [ (1/Ehub)((D² + d²) / (D² − d²) + νhub) + (1/Epin)(1 − νpin) ]). When pin and hub are the same material this reduces to p = Eδ(D² − d²) / (2dD²), which is the form NASA uses for a bearing ring on a solid shaft. The press-in force is then roughly F = μ × p × π × d × L, where L is the engaged length and μ the friction coefficient.
The NASA bearing study shows how much geometry matters. Four different 50 mm bearings on the same average m6 fit developed interface pressures from 6.64 to 13.13 MPa, simply because their ring sections differed (NASA TM-2010-216913). The same report deducts a small allowance for surface finish, because asperities flatten during pressing.
Also check the hub. The hoop stress at the bore of the hub is p × (D² + d²) / (D² − d²), and at the maximum-interference corner it must stay below the yield strength of the hub material with margin. Heating the hub or chilling the pin eases assembly but does not change the final stress.
Can you press fit pins and bearings into die-cast aluminum or zinc?
Yes, but the textbook fit tables were written for steel and machined bores, and a die-cast boss behaves differently in three ways: it relaxes, it expands more with heat, and it may have porosity just under a machined bore. Design the fit for the retained load after relaxation, and verify it with a press-off test.
Relaxation is the main difference. NADCA describes relaxation as the form of creep that applies to interference fits, where the stress and the retained load drop while the dimensions barely change, and recommends additional retention such as bonding or staking at press-fit joints where creep can occur (NADCA, thermal creep). NADCA’s temperature guidance notes that creep does not occur below an alloy’s threshold temperature and speeds up progressively above it, and zinc die-casting guidance treats relaxation of press fits as expected rather than exceptional.
For zinc, the International Zinc Association’s design guidance is unusually specific. It recommends a starting interference of 0.0005 to 0.0010 mm/mm of diameter for a rolling bearing pressed into a zinc casting, to be confirmed by test, and warns that press-off load cannot be predicted from textbook cylinder formulas because zinc relaxes, friction data are scarce and a steel part can shave metal from the bore during assembly (IZA, interference fits). For sintered bushings it describes deliberately expanding the boss beyond its elastic limit, by more than 0.25% of diameter, so the bushing ID comes out consistent regardless of the as-cast hole size, and it suggests small crush ribs, typically about 0.4 mm, where full round interference would overload a rolling bearing.
For aluminum, the second issue is thermal. A steel pin or bearing ring expands less than the aluminum around it, so the interference falls as the assembly warms up, which can let a bearing outer ring creep in a housing that was tight at room temperature. Size the fit so there is still interference at the maximum operating temperature.
Third, machining a seat into a casting removes the dense skin and can open subsurface porosity. Keep the boss wall uniform, as NADCA’s boss guidance suggests, and ask for bore sizing on the machined surface rather than relying on the as-cast hole. Cast versus machined tolerances are compared in the ISO 2768 and NADCA guide, and alloys in the ADC12 vs A380 guide.
How should press fits be called out for an overseas supplier?
Give the fit as a tolerance class and as explicit limits, name the mating part, and say how the fit will be verified. A supplier in Vietnam, Mexico or anywhere else then quotes and inspects the same numbers you calculated.
- Write both forms. A callout such as Ø10 N7 followed by the limits in parentheses removes any doubt about which edition or table the shop reads, and it lets an inspector check the part without looking anything up.
- Name the mating part. State the pin standard or the bearing part number next to the hole. A supplier who knows the mate can flag a mismatch early.
- Specify form and finish on the seat. Roundness, cylindricity and a surface finish limit on the bore matter as much as size for a press fit.
- Fix the measurement conditions. Ask for bore sizes measured with a bore gauge or air gauge at the reference temperature defined in ISO 1, and for the measured values on the inspection report, not just a pass mark.
- Say who presses. If the supplier installs the pins or bearings, specify the press method, lead-in chamfer, lubricant if any, and a press-off or push-out check on first articles.
The tighter shaft classes deserve a note on process. Per ISO 286-2, a p6 or s6 shaft at 30–50 mm has a band of only 16 µm, which is narrower than a routine turned band and often points to grinding or careful finish turning with in-process gauging. Saola runs CNC turning and CNC milling at its lead CNC plant and sources die castings from partner foundries for machining there, so shaft seats, bored housings and cast bosses can be reviewed together. The RFQ package guide lists what else to send, and you can send the drawing for a quote with the press-fit features marked.
Frequently asked
Is H7/p6 a press fit?
It is the lightest ISO interference fit, called a locational interference fit in ASME B4.2. At its loosest corner there is essentially no interference, so it locates parts accurately but should not be relied on to carry load by friction alone. Use r6 or s6 when the joint must hold.
Should the dowel pin or the hole carry the tighter tolerance?
The pin is a purchased standard part ground to its own class, so the hole is where the designer sets the fit. Choose the hole class against the pin class, and keep the second hole in a two-plate joint as a slip fit so the plates can be separated.
Will a press-fit pin loosen in a zinc die casting?
The retaining force will drop over time because zinc relaxes under sustained stress, faster at higher temperature. Design for the relaxed load, add staking, adhesive or a crimped lip where retention is critical, and confirm with press-off tests on aged samples.
Can a press fit be specified with ISO 2768 general tolerances?
No. General tolerances are far too wide for a fit and would allow anything from a loose slip to a cracked boss. Every press-fit diameter needs its own tolerance class or explicit limits on the drawing.
Sources
- ISO 286-2:2010, Geometrical product specifications (GPS), ISO code system for tolerances on linear sizes, Part 2: Tables of standard tolerance classes and limit deviations for holes and shafts, ISO, 2010
- ISO 8734:1997, Parallel pins, of hardened steel and martensitic stainless steel (Dowel pins), ISO, 1997
- ASME B4.2, Preferred Metric Limits and Fits, ASME, 1978 (reaffirmed 2020)
- Interference-Fit Life Factors for Ball Bearings, NASA/TM-2010-216913, F. B. Oswald, E. V. Zaretsky and J. V. Poplawski, NASA Glenn Research Center, 2010
- Interference fits, zinc die casting design guidance, International Zinc Association, accessed September 2026
- Thermal creep, NADCA Design, accessed September 2026
- Bosses, NADCA Design, accessed September 2026
- Effects of temperature, NADCA Design, accessed September 2026