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How to Shrink Fit a Part (Heating Temperature and Method)

Guide · Updated October 28, 2026

A shrink fit uses thermal expansion instead of force. Heat the outer part until its bore grows past the shaft, slide it on, and let it cool to grip. Because there is no sliding under pressure, shrink fits avoid galling and scored surfaces, and they can carry more interference than you could press. The key number is how hot the hub has to get.

Check the bore before and after

Measure the cold bore and shaft with gages set on blocks before heating. The free Gage Block Calculator builds the gage block stack for any size you need to set.

Step by step

Measure the parts: Measure shaft diameter and bore diameter at room temperature. Interference δ = shaft − bore.
Add assembly clearance: Plan for the hot bore to be larger than the shaft by a small clearance, often about 0.001 mm per mm of diameter, so the part slides on before it cools.
Find the expansion coefficient: Steel is about 11.5 × 10⁻⁶ per °C (6.4 × 10⁻⁶ per °F); aluminum about 23 × 10⁻⁶ per °C; bronze about 18 × 10⁻⁶ per °C.
Calculate the temperature rise: ΔT = (δ + clearance) ÷ (α × d).
Heat evenly: Use an oven, induction heater or hot oil bath, not a torch on one spot. Check temperature with a contact thermometer or temperature crayon.
Assemble quickly: Wear heat-resistant gloves, slide the part on in one motion to its stop, and hold it in place until it grips.
Cool naturally: Let it cool in air; quenching can crack the hub or shift it on the shaft.

Worked example: 25 H7/p6 steel hub

A steel hub with a 25 mm H7 bore goes on a p6 shaft. The worst case is the largest shaft and smallest bore: δ = 0.035 mm. Add 0.025 mm assembly clearance, for a total bore growth of 0.060 mm. ΔT = 0.060 ÷ (11.5 × 10⁻⁶ × 25) = 0.060 ÷ 0.0002875 ≈ 209 °C. From a 20 °C shop, heat the hub to about 230 °C (about 445 °F). Check the drawing first: that temperature is above the tempering temperature of some hardened parts.

Chilling the shaft instead

Cooling the shaft shrinks it by the same formula. Dry ice at about −78 °C gives roughly 100 °C of change from room temperature: 11.5 × 10⁻⁶ × 25 × 100 ≈ 0.029 mm, which is not enough alone for the example but cuts the heating needed by about half when combined. Liquid nitrogen (about −196 °C) gives more, but frost, brittleness and handling hazards need care and proper PPE.

Temperature limits

Bearing makers commonly limit bearing heating to about 120 °C (250 °F) to protect seals, lubricant and dimensional stability; induction heaters with a temperature probe are made for this. Hardened and ground gears and bushings may lose hardness if heated near their tempering temperature, often 150 to 200 °C for case-hardened parts. If the required temperature is too high, reduce interference within the drawing limits, add cooling of the shaft, or use a press instead.

Checking after assembly

Once cool, check that the part seated against its shoulder with a feeler gage (no gap) and check runout with an indicator. A part that stopped short because it gripped early must usually be removed by heating again, so practise the motion before the hot part is in your hands.

Tips

Press alternative: press fit calculation.
Limits: ISO 286 fits.
Safety: shop safety basics.

FAQ

How hot should I heat a hub for a shrink fit?

ΔT = (interference + clearance) ÷ (α × diameter). For 0.060 mm on a 25 mm steel bore, about 209 °C above room temperature.

What is the expansion of steel?

About 11.5 × 10⁻⁶ per °C, or 6.4 × 10⁻⁶ per °F.

Can I use a torch?

Uneven torch heating can warp or soften the part; an oven or induction heater is better.

How hot can a bearing be heated?

Bearing makers commonly say about 120 °C maximum.

Can I freeze the shaft instead?

Yes. Dry ice or liquid nitrogen shrinks it, often combined with heating the hub.

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