Bearing Bore Tolerance: Fit Classes, Roundness and Thermal Growth

A bearing bore that measures 30.000 to 30.021 mm on a bore gauge can still destroy a bearing in a week. The diameter is only one of four things the ring cares about. Roundness, taper, shoulder perpendicularity and the fit class you chose against the ring's own tolerance decide whether the bearing runs at its rated life or at a third of it.

Understanding bearing bore tolerance means understanding that you are not machining a hole. You are machining the seat that sets the ring's internal clearance.

Diameter is the easy part

A 6206 bearing has a 30 mm bore and a 62 mm outside diameter, both toleranced by the bearing maker. A normal-precision ring typically holds bore diameter to 0/−10 µm and OD to 0/−11 µm. Note the direction: the ring is always at or under nominal, never over.

That asymmetry matters. An H7 housing bore at 30.000 to 30.021 mm paired with a shaft at 30.000 gives an apparent clearance of 0 to 31 µm. The bearing's own bore tolerance eats part of it, and the result is a seat that ranges from a light push fit to a rattle. This is why bearing seats are usually specified with a tighter, shifted tolerance band than a general H7.

The rule for the drawing: specify the fit as a class, then state the resulting interference or clearance in microns. A class without a number invites a shop to sit at the easy end of the band forever.

The fits you will actually use

For a 30 mm nominal seat, these are the bands that cover most rotating equipment. Shaft fits are for a rotating inner ring; housing fits are for a stationary outer ring.

Housing fits sit below nominal because the bearing OD never exceeds nominal, so a symmetric band would always be loose. And the interference must stay smaller than the ring's internal clearance, or you preload the bearing before it turns.

See also: Gearbox Housing Machining: Bore Alignment, Seal Bores and Material Choice and Keyway Machining: Broaching, Slotting or Milling?.

Roundness, taper and shoulder perpendicularity

A bore can be on diameter at top, middle and bottom and still be a triangle in section. Three-jaw clamping on a thin housing does exactly that, and the ring conforms: 0.01 mm of out-of-round transfers roughly 0.008 mm of ovality into the outer ring, which shows up as once-per-revolution vibration.

For a bearing seat, hold roundness at 30 percent of the diameter tolerance, and never worse than 0.005 mm on a 30 mm bore. For precision spindles, 0.0025 mm. Check it at three axial stations, not one, because a boring bar will produce a barrel or a taper that a single-section measurement misses.

Shoulder perpendicularity is the callout most often missing. The shoulder locates the ring axially, and any tilt shows up as misalignment. On a 30 mm bore with a 62 mm OD bearing, 0.01 mm of shoulder runout tilts the ring by about 2 arcminutes, which is more than the misalignment allowance of most deep-groove bearings. Hold the shoulder to 0.005 mm for precision work and 0.01 mm for general duty.

Press fit versus slip fit

Press fits are not free. Assembly force for 20 µm of interference on a 30 × 12 mm seat runs roughly 5 to 15 kN, and that load passes through the balls and raceways unless you press on the fitted ring. Never press through the rolling elements.

Three practical consequences:

• Gall time. Pressing a dry steel ring into an aluminum bore scores both surfaces. A light oil film or a 0.01 mm dab of anti-fretting paste prevents it and does not change the fit.
• Heat is better than force. Heating an aluminum housing to 80 to 100 °C opens a 62 mm bore by 0.08 to 0.11 mm, which turns a press fit into a drop fit. Never heat above 120 °C: it anneals 6061-T6 and changes the bore permanently.
• Slip fits need retention. A js6 seat will creep under load unless the ring is clamped axially or held by a shoulder and snap ring.

On housings that get serviced, a slip fit with retaining compound beats a light press, because a light press that has been apart twice is no longer a press fit.

What heat does at operating temperature

Aluminum expands at 23.6 µm/m·K, bearing steel at 11.5. On a 62 mm bore a 40 °C rise opens an aluminum housing by 62 × 23.6e-6 × 40 = 0.059 mm while the ring grows 0.029 mm. The 30 µm difference is larger than most fits in the table above.

So 20 µm of interference in an aluminum housing becomes a clearance fit at temperature. The ring creeps, the bore frets, and the debris reaches the raceway. Fix it with a steel insert, a P7 interference, or an operating temperature low enough that growth stays under the interference.

For a steel housing, the differential is zero at steady state, and the fit stays close to what you machined. That is one reason precision spindle housings are steel or cast iron even when the rest of the machine is aluminum.

Specifying it so it gets made

Put six things on the drawing: nominal diameter with fit class, the interference or clearance range in microns, roundness, taper per 25 mm, shoulder perpendicularity and finish. Ra 0.8 µm suits a pressed seat; finer adds cost and reduces the friction a light press relies on.

State the housing material and the operating temperature. A Ø62 seat in 6061 at 80 °C is a different part from the same seat in 4140 at 80 °C, and the fit that works for one will fail for the other.

Then ask for the measurement method. A bore gauge set with a ring reads diameter but not roundness; a CMM with a 2 mm stylus at 16 points reads both. First articles report diameter at three stations, roundness at each, and shoulder runout, because those numbers predict assembly better than one diameter. If you send the bearing part number with the housing drawing, we can check the fit against the actual ring tolerance before quoting — send the file here or review how we work.

Frequently asked questions

Should a bearing housing be H7 or K7?

H7 for a stationary outer ring in a light-duty housing where the ring is clamped axially, K7 when the ring must not creep. H7 alone gives 0 to 32 µm of clearance against a real bearing OD, so the ring can rotate in the bore under load and fret.

How much interference is too much?

Keep interference under about 60 percent of the bearing's internal clearance, or roughly 15 to 25 µm on a 30 mm bore for normal duty. Beyond that you remove clearance, raise friction and heat, and shorten life before the machine ever runs.

Does the bore need to be round if the diameter is in tolerance?

Yes. A ring conforms to an out-of-round bore, so the outer raceway becomes out-of-round too and the load zone distorts. Hold roundness at 30 percent of the diameter tolerance and check it at three axial stations.

Why did my aluminum housing lose its press fit?

Differential expansion. Aluminum grows about twice as fast as bearing steel, so 20 µm of interference on a 62 mm bore disappears after a 27 °C rise. Use a steel insert, a heavier interference class, or a clamping feature that retains the ring.

Related reading

• Surface Finish Callout: Ra, Rz and How to Specify per Surface2026-10-05
• Thin Wall Machining: Deflection, Support Methods and Realistic Tolerances2026-09-29
• Deep Pocket Machining: Depth-to-Diameter Limits, Corners and Deflection2026-09-30