How to Measure Bolt-Hole Spacing on Wear Parts

A part can have the correct overall length and still be unusable because one hole, pitch or countersink is wrong. A reliable match needs a complete dimensional record and photographs that show how each measurement was taken.

Tools

• Clean steel rule or tape for long spans
• Caliper for hole diameter and thickness
• Straightedge and square
• Thread gauge and known fastener, when safe and appropriate
• Chalk or marker for hole numbering
• Camera and a measurement sheet

Remove burrs and packed material without changing the surface. Measure the part, not a distorted photograph.

Figure 1. Record overall length, hole diameter, pitch, edge distance, thickness and hole type. The drawing is schematic and not a production specification.

Six dimensions that define the fit

1. Overall length and width — measure along straight reference edges.
2. Thickness — take several readings away from worn zones and burrs.
3. Hole diameter — measure across two directions to detect elongation.
4. Pitch — center-to-center spacing between adjacent holes.
5. Edge distance — hole center to the nearest reference edge.
6. Hole form — plain, countersunk, counterbored, square-neck seat or another profile.

ISO 273 covers clearance-hole diameter series for general-purpose bolts and screws, but special applications are selected from their design requirements. A machinery wear part should therefore be matched to its drawing and fastener system, not inferred from one generic clearance table. See ISO 273:1979.

Find a hole center reliably

For a clean circular hole:

Center position = measured near edge to hole edge + hole diameter ÷ 2

For two holes of equal diameter, you can also measure outside-to-outside and subtract one diameter:

Center distance = outside span − one hole diameter

Or measure inside-to-inside and add one diameter:

Center distance = inside span + one hole diameter

Measure across several holes when possible. If holes 1 through 5 are nominally equally spaced, measure center 1 to center 5 and divide by four. The longer span reduces the effect of small reading errors.

Example

Outside-to-outside span across five equal holes: 824 mm
Average measured hole diameter: 24 mm
Center 1 to center 5: 824 − 24 = 800 mm
Four equal pitches: 800 ÷ 4 = 200 mm

Verify at least one adjacent pitch; do not assume every pattern is uniform.

Estimate measurement uncertainty

Suppose a long-span measurement can vary by ±1 mm and the hole diameter by ±0.2 mm. The calculated center span should not be reported as more precise than the tools support. Record 800 mm (approximately ±1.2 mm) rather than 800.00 mm.

This matters when comparing two candidates whose catalog dimensions differ by only a few millimetres.

Detect damaged holes

Take diameter readings at 0° and 90°. A meaningful difference suggests elongation or damage. Also look for:

• A bolt head that no longer seats fully
• Bright fretting marks around one side of the hole
• Cracks radiating from the hole
• Different pitch readings along the same row
• A plate that bows away from the straightedge

Do not use a damaged hole as the only matching datum. Use several intact holes and the outside geometry, then flag the damage for repair assessment.

A supplier-ready measurement record

Required item	Example format
Units	mm
Overall size	1,524 × 203 × 16
Hole count	8
Hole diameter	24.0–24.3
Pitch sequence	152 / 152 / 152 / …
End distance	78 left / 78 right
Hole form	Countersunk; include depth and major diameter
Part marking	Exact stamped characters
Photos	Front, back, edge, marking, ruler in frame

Final checks before ordering

• Confirm metric versus inch units.
• Confirm whether dimensions are nominal or measured on a worn part.
• Match the countersink and fastener head, not just the through-hole.
• Verify orientation, curvature and left/right hand.
• Cross-check the machine model, serial range and OEM part number.

Measurement note: all example dimensions are illustrative. Manufacturing acceptance limits must come from the applicable drawing or supplier.