How to Check a Metric Dimension Against an Inch Part

Why this matters

Imported equipment, replacement kits and half the fasteners in a parts bin are metric, and the truck stock, the drill index and most of the hardware store are inch. A lot of sizes are close: 10 mm and 3/8 in, 13 mm and 1/2 in, 19 mm and 3/4 in. "Close" is where the trouble lives. A tech decides a 10 mm bolt is "basically 3/8", tries to push it through a 3/8 in hole, and either it will not go or it gets forced, threads galled and the hole wallowed. Or the near size fits, gets installed, and loosens in a month because the fit it needed was tighter than "close".

The check takes a minute: convert with the exact factor, find the difference and which way it goes, and compare that difference against the kind of fit the joint needs. The last part is the one people skip, and it is the only part that answers the question.

The near-miss table

The inch is defined as exactly 25.4 mm, so every conversion below is exact arithmetic, not an approximation. Divide millimeters by 25.4 to get inches; multiply inches by 25.4 to get millimeters.

METRIC SIZE vs NEAREST INCH SIZE   (1 in = 25.4 mm exact)

 metric   = inches | inch    = mm     | metric is
 -------- -------- | ------- -------- | -------------------
  6 mm    0.236 in | 1/4 in  6.350 mm | smaller by 0.014 in
  8 mm    0.315 in | 5/16    7.938 mm | larger  by 0.002 in
 10 mm    0.394 in | 3/8 in  9.525 mm | larger  by 0.019 in
 13 mm    0.512 in | 1/2 in 12.700 mm | larger  by 0.012 in
 16 mm    0.630 in | 5/8 in 15.875 mm | larger  by 0.005 in
 19 mm    0.748 in | 3/4 in 19.050 mm | smaller by 0.002 in
 25 mm    0.984 in | 1 in   25.400 mm | smaller by 0.016 in

Read the last column before anything else. The direction decides more than the size of the gap. A part that is larger than the hole it has to pass through does not fit at all, however small the difference. A part that is smaller than its hole always passes; the question is whether the extra looseness matters.

The differences in that column are also small in a way that misleads. 0.019 in is about the thickness of five sheets of copier paper. It sounds like nothing. It is the difference between a bolt that slides through a hole and one that will not start.

Step 1: Measure the real part, in its own unit

Measure the bolt, shaft, bore or hole with calipers. Do not go by the size stamped on the box, the label on the bin, or the size the hole "should be".

Why: a hole drilled 3/8 in might be 0.375, or a bit oversize from a worn bit, or undersize where paint has built up. A bolt sold as 10 mm has a thread major diameter a little under 10 mm by design. The fit is between the two real parts, not between two names.

Read it in whichever unit the caliper shows, then convert. A digital caliper that switches units saves the arithmetic, but write down which unit each reading was taken in. A reading of "9.52" with no unit is either a 3/8 in part in millimeters or a nonsense number in inches.

Step 2: Convert with 25.4, and keep enough digits

Use 25.4 exactly. Keep three decimal places in inches and two in millimeters for fit work. 0.01 mm is about 0.0004 in, and 0.001 in is 0.0254 mm, so those two precisions are about the same resolution in each unit.

The slip to avoid: rounding the factor to 25 makes a 10 mm part 0.400 in instead of 0.394 in, a 0.006 in error, which is a third of the very 0.019 in difference you are trying to judge. A shortcut factor is fine for estimating a length of conduit. It is not fine for deciding whether two parts fit.

Step 3: Write the difference with its sign

Subtract in one unit, and write whether the metric part is bigger or smaller than the inch part it is replacing or mating with. A 10 mm bolt is 0.394 in; a 3/8 in hole is 0.375 in; the bolt is 0.019 in larger than the hole.

Why the sign gets its own step: "they are 0.019 apart" is true of a bolt too big for a hole and of a bolt with 0.019 in of clearance, and those two outcomes are opposite.

Step 4: Compare the difference to the fit the joint needs

The same 0.005 in can be irrelevant on one joint and disqualifying on another. Name which kind of fit you are judging:

• Clearance fit (a bolt through a hole that a nut or a tapped part clamps on the other side). Any bolt smaller than the hole passes. The slop matters only if the hole also locates the part, holding it in position, rather than just letting the bolt through.
• Locating fit (a dowel, a hinge pin, a hole that sets where a bracket sits). Extra clearance shows up as play, misalignment or a part that shifts under load.
• Slip or set-screw fit (a pulley, coupling, gear or bearing on a shaft). These are made to a close tolerance on purpose. A hub that is a few thousandths loose on its shaft rocks, frets and loosens no matter how hard the set screw is tightened. Check the part maker's bore tolerance; it is far tighter than any difference in the table above.
• Thread engagement. Metric and inch threads do not interchange, ever, even where the diameters nearly match, because the pitch is different. An M10 coarse thread advances 1.5 mm per turn; a 3/8-16 thread advances 1/16 in, which is 1.5875 mm. The two will start, engage a turn or two, then bind, and forcing them strips one or both.
• Wrench and socket fit. A near-size tool that is loose on the fastener puts the load on the corners and rounds them. A near-size tool that is snug may be acceptable on a fastener that is not torqued hard. Judge it by feel and by the stakes: on a seized or high-torque fastener, use the correct size.

Step 5: Decide

There are three answers, and the fit from Step 4 picks between them:

1. The near size will do. The part passes, the joint is a clearance fit, the hole does not locate anything, the strength of the substitute is adequate, and the equipment's instructions do not specify the hardware.
2. The near size will do with a change. A washer to cover an oversize hole, a sleeve or bushing made for the pairing, a nut on the back of a through-hole instead of a tapped thread.
3. The right part is needed. The substitute does not pass, the fit is a locating, slip or press fit, the joint is threaded, or the hardware is specified.

Do not drill out a hole or substitute hardware on anything where the hardware is part of a listing, a rating or a load path: a manufacturer's mounting kit on listed equipment, a structural connector, an anchor that holds a person (a fall-protection anchorage), a pressure-retaining flange or a lifting point. On those, the specified part is the only part; get it, or stop and call the maker.

Two cases, one check

Case one: a 10 mm bolt and a 3/8 in hole. A tech replacing a fan guard on an imported unit finds the original bolts missing. The bracket holes measure 0.375 in on the calipers (drilled 3/8 in), and the bin of metric hardware on the truck has 10 mm bolts.

• Convert: 10 mm / 25.4 = 0.394 in.
• Difference: the bolt is 0.394 - 0.375 = 0.019 in larger than the hole.
• Fit: clearance fit, but the sign is wrong. The bolt cannot pass.
• Decision: the right part is needed. Either 3/8 in hardware for a 3/8 in hole (which will itself be a very tight clearance, since a hole the same size as its bolt leaves no room) or whatever the unit's parts list specifies. Opening the hole up to take the 10 mm bolt is a modification to the maker's bracket and stays off the table unless the maker says otherwise.

Case two: a 5/8 in shaft and a 16 mm pulley bore. A tech changing a belt-drive pulley finds the replacement in stock has a 16 mm bore, and the motor shaft measures 0.625 in.

• Convert: 0.625 in x 25.4 = 15.875 mm.
• Difference: the bore is 16.000 - 15.875 = 0.125 mm larger than the shaft. In inches, 0.125 / 25.4 = 0.005 in.
• Fit: set-screw fit on a shaft. 0.005 in of looseness is a large gap for this kind of fit.
• Decision: the right part is needed, a pulley with a 5/8 in bore, or a bushed pulley with a bushing made for a 5/8 in shaft. Run with the 16 mm bore and the set screw pushes the shaft to one side of the bore, the pulley runs out of true, the belt wears, and the hub frets until the key or the set screw lets go.

Note what those two cases show. The first difference (0.019 in) is almost four times the second (0.005 in), and the first one is the obvious failure while the second one looks like it would be fine. The size of the gap never answered the question. The sign and the fit did.

Case three, the one that passes. Same as case one, reversed: a bracket drilled for metric hardware, with holes that measure 11.00 mm, and a 3/8 in bolt on the truck. 3/8 in is 9.525 mm, so the bolt has 11.00 - 9.525 = 1.475 mm of clearance against the 1.00 mm a 10 mm bolt would have had. It passes. If it is a plain clamping joint with a washer under the head and a nut behind, and the hole does not locate the part, the near size will do as a temporary repair. Two things still get checked: the 3/8 in bolt is slightly smaller in section (a 3/8-16 bolt's tensile stress area is about 0.0775 sq in, roughly 50 sq mm, against about 58 sq mm for an M10 coarse), so match or exceed the grade of what came out; and the correct hardware goes on the parts list for the next visit.

When the answer flips

• Corrosion, coatings and wear change both parts. A painted hole may be undersize; a worn bolt or an elongated hole may make a near size fit that would not have fit new. Measure what is there.
• Temperature moves the fit on hot equipment, because metal grows when heated. A fit measured cold on a part that runs hot is not the fit it runs at.
• Thread inserts and adapters exist for converting a tapped hole from one system to the other. They are a legitimate fix where the maker allows them, and a hidden failure point where it does not.

How to know you got it right

• Each reading is written with its unit, and the conversion used 25.4.
• The difference is written with its sign, bigger or smaller.
• The fit type is named, and the decision follows from it rather than from how small the number looks.
• A threaded joint was never solved with a near size.

References

• See related: Common Unit Conversions + Math Formulas
• See related: Trade Math and Imperial Fractions Reference
• See related: The Tolerance Stack-Up That Causes the Fault
• See related: Measurement Resolution and Honest Rounding
• Equipment manufacturer's parts list and installation instructions for specified hardware