Temperature, Coatings and Fit Problems
Thermal expansion, plating and anodize build-up, surface finish, and why parts that are in tolerance still fail to fit.
A fit is set on the drawing at one temperature, with clean, bare surfaces, on parts that are perfectly formed. Real parts run hot and cold, wear coatings, carry roughness and have form error. Each of these shifts the fit, sometimes by more than the whole tolerance. This page covers the effects that most often turn a good design into a bad assembly, with the Machinery’s Handbook data on thermal expansion and surface texture, and worked numbers. It is part of the Fits & Tolerances guide.
Temperature and size
Almost everything gets bigger when it is heated. For a change of temperature ΔT, a length L changes by:
ΔL = α × L × ΔT
α = coefficient of linear thermal expansion (per °C or per °F)
Sizes on drawings are taken to be at the standard reference temperature of 20 °C (68 °F). Away from that, every dimension has changed, and parts of different materials have changed by different amounts. The change matters when it is comparable to the tolerance: a steel part 100 mm long grows 11 µm for every 10 °C, as much as an IT7 tolerance on a 25 mm size.
Expansion coefficients
The Handbook’s Table 8 gives typical coefficients of linear thermal expansion for common materials (in units of 10⁻⁵ per °C); Table 13 gives them for specific metal alloys. They are shown here in micrometers per meter per °C (the same number as 10⁻⁶ per °C).
| Material | α (µm/m/°C) | Source (Handbook) |
|---|---|---|
| Glass | 7 | Table 8 |
| Steel (general) | 11 | Table 8 |
| Gray cast iron | 10.4 | Table 13 |
| Ductile iron | 10.6 to 11.6 | Table 13 |
| Cast steel | 12.6 | Table 13 |
| Titanium (commercially pure; Ti-6Al-4V) | 9.2; 9.5 | Table 13 |
| Stainless steel 304 (S30400) | 17.3 | Table 13 |
| Copper (C11000) | 17.6 | Table 13 |
| Brass (cartridge, C26000) | 20.0 | Table 13 |
| Bronze | 18 | Table 8 |
| Aluminum alloys (2011 to 7075) | 22.7 to 23.6 | Table 13 |
| Magnesium | 25 | Table 8 |
| Zinc | 31 | Table 8 |
| Nylon, glass-fiber reinforced | 23 | Table 8 |
| Polycarbonate (unfilled) | 65 | Table 8 |
| ABS (unfilled) | 72 | Table 8 |
| Nylon (unfilled) | 81 | Table 8 |
| Acetal (unfilled) | 85 | Table 8 |
| Polypropylene (unfilled) | 86 | Table 8 |
| Polyethylene | 130 | Table 8 |
Metals span about 9 to 31; unfilled plastics 65 to 130, roughly six to ten times steel. Glass or carbon fiber reinforcement cuts a plastic’s expansion by two to four times. Volume expansion is three times the linear value.
What temperature does to a fit
The size of a fit changes with temperature as the difference between the hole’s and the shaft’s growth. For a basic diameter D, hole and shaft temperatures Th and Ts above 20 °C:
Change in clearance = D × ( αhole × ΔTh − αshaft × ΔTs )
- Same material, same temperature: nothing changes. The fit is preserved because both parts scale together.
- Different materials (a steel shaft in an aluminum housing, a bronze bush on a steel shaft): the fit changes whenever the temperature moves from 20 °C. A hot aluminum housing loosens its grip on a steel bearing; a cold one clamps it.
- Same material, different temperatures (a shaft warmed by friction inside a cool housing, or a hot bearing journal): the shaft grows more than the hole and the clearance shrinks. This is the usual cause of seizure.
- Interference fits lose interference when the hub gets hotter than the shaft (or the hub material expands more), and gain it when it gets colder.
Measuring at the right temperature
Measurements are meant to be made at 20 °C, with the gauge and the part at the same temperature. A part that is a little warm, straight from the machine or held in a warm hand, reads larger than it is and can be rejected when it is good, or accepted when it is bad if the error is the other way. Let the part soak in the measuring room, and keep it out of sunlight and away from the machine’s coolant.
Coatings and plating
A coating adds its thickness on every surface. On an outside diameter the diameter grows by twice the coating thickness; in a hole it shrinks by twice the thickness. Whether it matters depends on how thick the coating is compared with the fit’s tolerance. The thicknesses below are typical; always check the supplier’s data and the drawing’s statement of whether the dimension applies before or after coating.
| Coating (typical) | Typical thickness | Typical diametral change |
|---|---|---|
| Zinc plating | 5 to 13 µm | 10 to 26 µm |
| Electroless nickel | 5 to 25 µm | 10 to 50 µm |
| Hard chromium | 5 to 50 µm | 10 to 100 µm |
| Anodizing (Type II, sulfuric) | 5 to 25 µm (about half grows outward) | up to about 25 µm, from 50% growth |
| Hard anodizing (Type III) | 25 to 75 µm (about half grows outward) | up to about 75 µm, from 50% growth |
| Powder coat or paint | 25 to 100 µm | 50 to 200 µm |
- Say before or after. The drawing should say whether the size applies before coating (and what the coating thickness is) or after. If not stated, many shops measure the finished part, which can make an uncoated machined size wrong.
- Allow for it in the limits. Machine the shaft smaller (or the hole larger) by the coating allowance so that the finished part lands in tolerance.
- Threads need it most. A thread flank is slanted, so a coating of thickness t on the flanks changes the pitch diameter by about 4t. The Handbook explains that Class 2A threads have an allowance built in for coatings, and that threads should be kept within basic size after plating (see the Fits in Practice page).
- Anodizing grows and eats. Anodic coatings both grow outward and consume the base metal; roughly half of the thickness is growth.
Surface finish and fit
The Handbook makes the link between roughness and tolerance in a single rule. Roughness is the average deviation of the surface from its mean line, and the total peak-to-valley height of the surface is approximately four times the average roughness Ra. If the roughness height eats into the dimensional tolerance, the measurement of the size is uncertain by more than the tolerance. The conclusion:
Ra ≤ tolerance ÷ 8
(the profile height, about 4 × Ra, is then no more than half the tolerance)
If the average roughness exceeds one eighth of the dimensional tolerance, the whole tolerance will be taken up by the roughness height. In most cases a smaller roughness specification is found; but on parts with very small tolerances it is necessary to specify a suitably small roughness so that useful dimensional measurements can be made.
- Press fits flatten the peaks of the roughness when the parts are assembled, so the effective interference is smaller than the measured one. A rough pair loses more.
- Running fits need the film of oil to be thicker than the roughness: a very rough journal wears in, opening the clearance.
- Seals and bearings seats have their own finish requirements, normally given by the manufacturer.
| Size tolerance | Largest Ra by the 1/8 rule | Peak-to-valley height at that Ra (≈ 4 Ra) |
|---|---|---|
| 9 µm | 1.13 µm | 4.5 µm |
| 13 µm | 1.63 µm | 6.5 µm |
| 21 µm | 2.63 µm | 10.5 µm |
| 33 µm | 4.13 µm | 16.5 µm |
| 52 µm | 6.50 µm | 26.0 µm |
The five tolerances are IT5, IT6, IT7, IT8 and IT9 for a 25 mm size (the 18 to 30 mm row of the ISO table). The process page lists which processes reach these finishes.
Why parts in tolerance still do not fit
| Cause | What happens | What to check |
|---|---|---|
| Temperature | The parts were measured warm, or run hot or cold in service | Measure at 20 °C; calculate the fit at the operating temperature |
| Coating | Plating or anodizing added thickness after the size was cut | State before or after coating; allow for it; check on finished parts |
| Form error | Out-of-roundness, taper or bow: a hole is not a perfect cylinder, so a pin only fits where the form allows | Check roundness and straightness; apply Rule #1 or a form tolerance (see the Form Tolerances page) |
| Location error | The holes are the right size but not in the right place | Check position, not just size (see Position and Location) |
| Burrs and edges | A burr or a sharp edge stops a part going in | Deburr; chamfer lead-ins; give edge-break dimensions |
| Surface finish | Roughness is a large share of the tolerance, or flattens in a press fit | Check Ra against the 1/8 rule; specify a finish |
| Stack-up | Each tolerance is fine alone but they add up | Do a stack-up (see Tolerance Stack-up Analysis) |
| Stress relief and springback | Parts change shape after machining or after a clamp is removed | Rough, stress-relieve, then finish; measure free state |
| Moisture and aging (plastics) | Nylon and other plastics absorb water and swell; moldings shrink after molding | Condition before measuring; allow for it in the design |
Worked examples
Example 1: a bearing in an aluminum housing
A steel bearing outer ring, 90 mm diameter, sits in an aluminum housing with an H7 bore (clearance of 0 to 50 µm with a normal-class ring). The assembly warms from 20 °C to 100 °C. Take α = 22.7 µm/m/°C for the aluminum (Handbook Table 13, alloy 2011) and 11 for steel (Table 8).
- Housing bore growth = 90 × 22.7 × 10⁻⁶ × 80 = 163 µm.
- Steel ring growth = 90 × 11 × 10⁻⁶ × 80 = 79 µm.
- Clearance gained = 84 µm, about 2.4 times the IT7 tolerance. A fit that was a snug clearance at 20 °C is now loose enough for the outer ring to turn: this is the Handbook’s “soft and resilient housings” problem. A steel liner, or a tighter fit at 20 °C, are the remedies.
Example 2: a shaft that runs hot
A 50 mm steel shaft in a steel bush runs with the shaft 40 °C above 20 °C and the bush 15 °C above (the shaft is warmed by friction and the bush cooled by the housing). Take α = 11.7 µm/m/°C for both.
- Change in clearance = 50 × 11.7 × 10⁻⁶ × (15 − 40) × 1000 = −14.6 µm.
- An H7/g6 fit at 50 mm has a minimum clearance of 9 µm and a maximum of 50 µm. The 15 µm of lost clearance is more than the minimum (9 µm): this shaft would seize at the tight end of the fit. This is why running fits get looser for higher speeds and larger temperature differences (the Handbook’s free running fits are for large temperature variations).
Example 3: a plated shaft
A 20 mm shaft in a 20H7/g6 fit is zinc plated to 8 µm per side. What must the shaft be before plating?
- 20g6: es = -7 µm, ei = -20 µm, so the finished shaft is 19.980 to 19.993 mm.
- The plating adds 2 × 8 = 16 µm to the diameter, so the shaft before plating is 19.964 to 19.977 mm.
- If the plating thickness varies by ±3 µm, the diameter varies by ±6 µm; that is almost half of the 13 µm shaft tolerance, so the plating thickness is itself a tolerance to be controlled.
Example 4: how warm is too warm to measure?
A steel part is measured 5 °C above 20 °C with a gauge at 20 °C. A 100 mm feature reads larger by 100 × 11 × 10⁻⁶ × 5 × 1000 = 5.5 µm. If the tolerance is ± 0.01 mm (20 µm total), this is over a quarter of the whole band. Let the part soak, or correct for temperature.
Troubleshooting
| Symptom | Likely cause | Try |
|---|---|---|
| A fit is right cold and loose (or tight) when running | Different materials expand differently | Calculate the fit at the operating temperature; choose a fit, or materials, with matching expansion |
| Parts fit after machining but not after plating | The coating’s thickness was not allowed for | Machine to the pre-plating size; control plating thickness |
| A shaft seizes in the bush after a warm-up | Clearance too small for the temperature difference | Use a looser class or add a temperature allowance |
| An outer ring spins in an aluminum housing | The housing expanded more than the steel ring | Steel liner or tighter fit; keep housings cool |
| Plastic parts that fit in the workshop jam in service | High expansion and moisture swell | Allow for α and moisture; use reinforced grades |
Related
- Fits & Tolerances: the overview and quick reference.
- How Fits Work: clearance and interference fits.
- Press, Shrink and Expansion Fits: using temperature to assemble a fit.
- Tolerances by Process and Surface Finish: what roughness each process gives.
- Fits in Practice: bearing seats, keys, threads.
FAQ
Why is 20 °C the reference temperature?
It is the international standard reference temperature for dimensional measurement (ISO 1), so a size on a drawing means the size at 20 °C. At other temperatures a part’s size changes.
Does a steel part in a steel housing change its fit with temperature?
Not if both are at the same temperature: they expand together. If one is hotter than the other, the fit changes by the difference in their growth.
How thick is a typical coating?
It varies from a few micrometers for zinc and bright nickel to tens of micrometers for hard anodizing and chromium, and more for powder coat and paint. The typical values here are a guide; ask the supplier.
What is the 1/8 rule for surface finish?
The Handbook says that if the average roughness Ra exceeds one eighth of the dimensional tolerance, the roughness will use up the whole tolerance, because the peak-to-valley height is about four times Ra.
Should I design the fit at the operating temperature?
Yes, check it there. Calculate the clearance or interference at the temperatures the parts will really reach, with the actual materials, and make sure the fit stays within what the application tolerates.
Do plastics change size with moisture?
Some do. Nylon, for example, absorbs water and grows. Condition plastic parts before measuring, and allow for the change in the design.
References
- Machinery’s Handbook, 31st edition, Industrial Press: “Thermal Properties of Materials” (Table 8, coefficients of linear thermal expansion; Table 13, typical thermal properties of various metals), “Surface Texture” (relation of surface roughness to tolerances) and “Unified Screw Threads” (coated threads); “Mounting Bearings” (soft and resilient housings).
- ISO 1, Geometrical product specifications (GPS): standard reference temperature for the specification of geometrical and dimensional properties.
Last reviewed: October 2026. Coefficients are typical values; use the figures for your actual alloy.