ENGINEERING GUIDES / 25 OCTOBER 2026

Metric vs. Imperial: Selecting ID / OD Clearance for Shafts & Bolts

A practical bore-clearance worksheet for M4–M30 bolts, ball screws and imperial shafts.

Magatom Dynamics force sensing division · Engineering selection note

Nominal thread size is not the required sensor ID.

A smooth thru-hole bore surrounds a bolt or shaft without threaded engagement. For M8 hardware, 8 mm is the nominal thread diameter; it is not an assurance that an 8.0 mm sensor bore will assemble or remain clear. Use the maximum actual diameter, radial misalignment, runout, temperature and a deliberate assembly allowance.

For an imperial shaft, convert the nominal dimension exactly first: 1 in = 25.4 mm. A 1/2 in shaft is nominally 12.7 mm; a 12 mm bore cannot fit it. A 2.75 in bore is 69.85 mm and a 3 in bore is 76.2 mm. Conversion alone does not establish the tolerance stack.

M4–M30 bolt clearance is separate from a precision shaft fit.

These common fine-clearance starting dimensions help define a bolt-passage request. They do not specify the final sensor tolerance, H7/g7 shaft fit or approved loading ring. The complete ISO 273 standard gives general clearance-hole series; special sensor applications need their own drawing. A nominal bolt size is not a minimum guaranteed clearance.

Reference: manufacturer clearance-hole chart. For smooth shafts of 6, 8, 10, 12, 15, 16, 20, 25 or 30 mm, start with actual maximum shaft diameter, runout and thermal growth. Do not substitute a bolt table for a precision rotating-shaft clearance calculation.

For a threaded sensor, specify pitch, tolerance class, engagement and loading direction; a smooth-bore clearance table does not apply.

Build the minimum clearance from the worst case.

Required ID ≥ Dmax + 2 × (radial misalignment allowance + radial motion allowance + assembly allowance), with differential thermal expansion evaluated separately. State whether the supplied runout is a radial amplitude or a total indicated reading before inserting it into the calculation.

Illustration: a 20.00 mm maximum shaft, 0.10 mm radial alignment allowance, 0.05 mm radial motion and 0.10 mm assembly allowance gives 20.50 mm minimum ID before thermal allowance. This is an arithmetic example; the selected bore and tolerance still need engineering approval.

Do not enlarge the bore without rechecking OD and capacity.

The available annular width is (OD − ID) / 2. Increasing ID at fixed OD removes material and changes the stiffness and strain field. Reducing height can also change stress, deflection and moment sensitivity. Custom machining must be reviewed with the sensing design and followed by configuration-specific verification.

For a ball screw bearing support, include the shaft shoulder and bearing-seat drawing. For bolts, include the washer bearing diameter, stack height and installation torque isolation. The sensor must retain the intended loading annulus while the passing hardware stays clear.

Separate the conversion from the sensor drawing.

Geometry and force-path reference: FUTEK LTH300 and its installation manual. The clearance table and formula above are explicitly illustrative selection calculations, not manufacturer dimensional data.

Selection route: custom metric ID / OD review, MT-THL-B bolt force washers, or MT-THL-T threaded interfaces. Send your actual diameter and the available OD and height.