How to measure enclosure mounting boss spacing

Enclosure mounting boss spacing is specific to the enclosure design; there is no universal pattern you can infer from its outside dimensions. Start with the manufacturer's mechanical drawing for the exact model and revision. If that drawing is unavailable, measure the mounting features, record their uncertainty, and prove the pattern with a rigid template before releasing the PCB. An ingress protection rating does not define mounting dimensions.

What you need

Measure an empty, disconnected enclosure on a stable surface. Remove loose hardware and debris without scraping the bosses or damaging their threads.

Have these items available:

• The exact enclosure identifier and any manufacturer drawing or CAD model.
• Calipers with suitable jaw reach and a checked zero.
• A depth gauge or caliper depth rod for boss height and hole depth.
• Suitable measuring pins for smooth bores, where their fit and alignment can be established.
• A rigid, nonconductive template representing the proposed PCB outline and mounting holes.
• A dimensioned sketch with a defined viewing direction and coordinate origin.

Identify whether each mounting feature is a threaded insert, tapped hole, or unthreaded pilot hole. Do not insert an unknown screw to find out. The article on identifying and sizing PCB mounting boss screws covers that separate task.

Check the caliper specification, not just its display resolution. A display increment of 0.001 in [0.0254 mm] does not establish equivalent measurement accuracy inside a deep enclosure.

Get the correct drawing first

Ask the manufacturer or its distributor for the mechanical drawing covering the exact enclosure variant. Similar-looking boxes can have different inserts, mounting panels, boss heights, or internal ribs.

Look for the mounting-hole detail, not just the overall enclosure dimensions. Confirm whether the view looks into the open base or through its underside. A reversed viewing direction can mirror an otherwise accurate pattern.

Use dimension labels rather than measuring a printed drawing or screen image. Printing, scanning, and display scaling can change apparent distances. A CAD model can help identify geometry, but confirm its revision and whether its dimensions are controlled.

Read these details together:

• Horizontal and vertical hole-center locations.
• Position tolerances or dimensional tolerances.
• Thread or pilot-hole specification.
• Boss-top elevation and usable fastening depth.
• Board outline limits, ribs, lid features, and terminal clearances.

Do not assume that four bosses form a rectangle because the drawing looks rectangular. Use the stated coordinates or dimensions. If tolerances or dimensions are missing, request them before committing a production PCB.

Measure and transfer the mounting pattern

1. Set the viewing direction and datums.

   Sketch the enclosure as viewed into the open base. Label the bosses A, B, C, and D, keeping those labels consistent throughout the measurements. Mark a fixed feature that prevents accidental rotation or mirroring.

   Choose two perpendicular datums for the PCB drawing. One boss center can be the origin, with a second boss defining the horizontal axis. Also record how the pattern sits relative to the enclosure walls. Correct spacing alone does not guarantee that the board outline fits.

2. Choose the feature that actually locates the screw.

   Prefer a documented hole axis or a measurable smooth bore. The outside of a molded boss is not automatically concentric with its pilot hole or insert.

   Avoid measuring across thread crests, countersinks, damaged edges, or a rounded entrance. Those surfaces can give different readings as the jaws move. Do not combine a measurement across boss exteriors with another across hole interiors.

3. Measure the near gap and the far span.

   For two accessible circular features, measure the gap between their nearest edges and the span across their farthest edges. Both measurements must follow the line joining their centers, using the same features at the same height.

   Call the near gap G and the far span O. The center-to-center distance is:

   C = (G + O) / 2

   For example, a near gap of 2.800 in [71.12 mm] and a far span of 3.200 in [81.28 mm] give a center spacing of 3.000 in [76.20 mm]. These example digits illustrate the calculation, not achievable measurement accuracy.

   The method works with unequal feature diameters because their radii cancel. It still requires circular features, correct jaw contact, and measurements along the centerline. Take several readings after removing and repositioning the calipers.

4. Handle deep or threaded features without forcing access.

   Smooth measuring pins can bring accessible measuring surfaces above recessed smooth bores. Confirm that the pins fit closely, stand parallel, and remain seated without damaging the holes. Measure across the pins at a consistent height.

   Long screws are less reliable measuring extensions. Thread clearance, bent screws, and tilted inserts can move their exposed ends away from the actual mounting axes. Do not treat screw-head spacing as a controlled boss dimension.

   If you use threaded extensions for an initial estimate, check readings at more than one height. Changing spacing indicates an alignment problem, although consistent readings alone do not prove accuracy. Use a rigid trial template for fit verification, or obtain a suitable dimensional inspection before releasing a tight-tolerance pattern.

   A paper or card transfer can help establish a rough layout. It is not a substitute for controlled dimensions because marking width, flexing, and distortion introduce uncertainty.

5. Map every boss instead of assuming symmetry.

   Record the horizontal and vertical position of each center relative to your chosen datums. For an apparently rectangular four-boss pattern, measure both horizontal pairs, both vertical pairs, and both diagonals.

   These extra measurements check consistency; equal diagonals alone do not prove that the entire pattern is correct. Investigate disagreement rather than averaging it into a convenient rectangle.

   Record boss-top heights separately. Four correct hole centers will not prevent board stress if the supports are not coplanar. Check blind-hole depth without forcing the depth rod into the bottom or mistaking debris for a solid stop.

6. Set the PCB hole size from a tolerance budget.

   Keep center spacing and clearance-hole diameter as separate decisions. Oversized holes may accommodate small errors, but they cannot correct a misplaced board outline or inadequate screw-head support.

   For a circular clearance hole, the available radial clearance is half the difference between the minimum hole diameter and maximum passing fastener diameter. Include any threaded portion that must pass through the board.

   As an example, a minimum finished hole diameter of 0.160 in [4.064 mm] with a maximum passing fastener diameter of 0.140 in [3.556 mm] leaves 0.010 in [0.254 mm] radial clearance. That allowance must cover the relevant positional errors; it is not an accuracy target for the calipers.

   Include measurement uncertainty, enclosure variation, and PCB fabrication tolerance in the fit assessment. Leave room for washers and screw heads while maintaining copper clearance. One measured enclosure cannot establish production variation across later units.

7. Prove the pattern before releasing the board.

   Make a rigid template using the intended hole pattern and board outline. Seat it on the supports without pulling it into place with screws. Start each verified fastener gently by hand, then check the complete assembly.

   If a hole needs sideways force or the template rocks, stop and investigate. Do not enlarge holes until the original measurement, viewing direction, and support heights have been checked.

Check the board before assembly

Verify more than whether the fasteners enter. The board must remain supported, accessible, and clear of nearby features throughout assembly and service.

Check the following with the template or prototype:

• Every support contacts the board without bending it.
• Screw heads and washers remain clear of components and required copper clearances.
• Fasteners achieve the specified engagement without bottoming in blind holes.
• The board outline clears internal ribs and the door or lid.
• Connectors have room for mating plugs, cable bends, and removal.
• The mounting pattern cannot be installed in the wrong orientation.
• Tall components clear lid ribs, windows, and internal hardware.

Use the specified fastening torque for the actual hardware and boss design. Tightening harder is not a fit correction. The article on setting a safe PCB mounting torque explains how to establish an assembly limit without using the PCB as a spring.

If you generate an enclosure or adapter model, apply the generated enclosure CAD checks before releasing its geometry. A valid solid model still needs verified mounting coordinates and a physical fit check.

Mistakes that spoil the fit or seal

Do not derive internal dimensions by subtracting an assumed wall thickness from the outside dimensions. Molded walls can taper, and corner radii or ribs can restrict the usable board area.

Do not assume that a round boss exterior is centered on its threaded insert. Measure the locating feature, or obtain a drawing that establishes concentricity.

Do not round measurements to a convenient metric or inch pitch without supporting documentation. A pattern that appears close to a familiar increment may still be different enough to bind four screws.

Do not use the screws to flatten a board across uneven bosses. That creates mechanical strain and can damage solder joints, tracks, or mounting holes.

Do not drill through the enclosure wall to rescue an incorrect internal mounting pattern. New penetrations can compromise environmental protection unless the complete mounting and sealing arrangement is evaluated. Keep chips away from the gasket, sealing land, and installed electronics.

Do not cut a boss shorter or deepen a blind hole without an approved design change. Either operation can weaken the support or break through the enclosure wall.

What to put on the drawing or order

Release a dimensioned mounting pattern, not a note saying that the PCB fits a particular box. Include the enclosure identifier and revision, viewing direction, datums, hole coordinates, tolerances, hole sizes, support heights, and fastening requirements.

Distinguish manufacturer-controlled dimensions from dimensions measured on a sample. Record the measurement method and any unresolved uncertainty. Keep the approved fit template or prototype identified with the drawing revision.

If no reliable pattern can be established, use an approved adapter or choose an enclosure with documented mounting geometry. The enclosure selector is a next step when usable dimensions or mounting clearance make the existing enclosure unsuitable; thermal suitability still needs a separate check.

For custom enclosures, we build to customer-defined material, width, height, depth, and cutouts, from one piece. Supply the PCB outline, mounting coordinates, support arrangement, component heights, and cable-entry requirements for drawing review. Do not assume that a custom metal enclosure reproduces molded plastic bosses; have the internal mounting arrangement explicitly defined before approval.

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Catalog sizes that suit the job this article answers, picked from its subject; open one for its full specifications.

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