Fits & Clearances
Drill sizes and hole preparation; clearance, transition and interference fits; schedules of fits and clearances; limits of bow, twist and wear; go/no-go gauging.
Fits, clearances & tolerances
Fits & clearances, summary notes
- A fit is the dimensional relationship between a shaft and a hole, and there are three classes: clearance, transition and interference.
- A CLEARANCE fit leaves the shaft smaller than the hole, so the parts assemble freely and can rotate or slide, a plain bearing.
- An INTERFERENCE (press) fit makes the shaft larger than the hole, so the parts are forced together and held by friction, bushes and bearing races.
- A TRANSITION fit falls between the two, giving either slight clearance or slight interference within tolerance, where accurate location matters more than free movement.
- Clearance = hole size − shaft size. A positive result is a clearance fit, a negative result an interference fit.
- Fits are specified on a hole basis (hole size fixed, shaft varied) or a shaft basis. The hole basis is the more common, because holes are harder to vary than shafts.
- Measuring: micrometers and bore gauges for absolute sizes, feeler gauges for running clearances, dial gauges for end-float, backlash and run-out, and go/no-go gauges for a quick pass/fail. Compare the result against the schedule of fits and clearances in the maintenance manual.
- Components must be clean and at a stable temperature when measured, because thermal expansion changes the reading, and the instrument itself must be in calibration.
- ⚠ Exam trap: an interference fit has the SHAFT LARGER than the hole. Reversing that is the standard distractor.
- Clearance
- clearance = hole size − shaft size
- Sign rule
- positive → clearance fit · negative → interference fit
- Interference fitting
- heat the hole or chill the shaft to assemble
A hole measures 25.02 mm and its shaft 25.05 mm. Identify the fit.
Clearance = 25.02 − 25.05 = −0.03 mm. The negative result means the shaft is larger than the hole, so this is an interference (press) fit.
Why is a bearing housing sometimes heated before a race is fitted?
Heating expands the housing bore, temporarily turning an interference fit into a clearance so the race can be positioned without force. On cooling the bore contracts onto the race and the interference is restored, giving the grip the design relies on.
Fits & clearances concept map
Fits & Clearances
Fits & clearances quiz
Fits & Clearances, quiz
1. A hole measures 25.021 mm and its shaft 24.980 mm. The clearance is:
0.041 mm0.021 mm0.980 mm2. A torque wrench of 300 mm is fitted with the 100 mm extension shown, in line with the handle. To deliver 30 N·m at the fastener the wrench must be set to:
22.5 N·m40 N·m30 N·m3. A shaft larger than its hole produces a:
Interference fitClearance fitTransition fit4. Clearance is calculated as:
Hole size − shaft sizeShaft size − hole sizeHole size + shaft size5. A clearance fit is used where:
Parts must rotate or slide freelyA permanent assembly is requiredAccurate location matters most6. A transition fit gives:
Either slight clearance or slight interferenceAlways clearanceAlways interference7. Hole 25.02 mm, shaft 25.05 mm. The fit is:
InterferenceClearanceTransition8. A bearing housing is heated before fitting a race in order to:
Expand the bore so the race can be positionedSoften the housing materialBurn off the old lubricant9. Running clearances are most directly measured with:
Feeler gaugesA micrometerA tensiometer10. End-float and backlash are measured with a:
Dial gaugeVernier caliperSteel rule