Metric vs Imperial Thread Chart — Will a 1/4-20 Fit an M6? 44 Lookalike Pairs That Do Not Mate

The Chart — 44 Lookalike Metric and Inch Threads, M1.6 to M30

Every pair on the chart is within 1.750 mm in major diameter and looks like the same thread to the eye, and every pair fails in one of two ways: the pitches differ and the flanks bind after a few turns, or the pitches agree and the diameter is wrong. Metric sizes are ISO 261 / ISO 262 coarse pitch unless the label carries a fine pitch (M8×1, M10×1.25, M12×1.5); inch sizes are Unified coarse (UNC) and fine (UNF) with the major diameter converted at 25.4 mm per inch and the pitch as 25.4 / TPI, both rounded to three decimals. Diameter and pitch differences are taken between the nominal majors and the nominal pitches; drift over 5 turns is five times the pitch difference.

EKINSUN LTD is a custom parts manufacturer that machines the thread adapters, standoffs and double-end studs that bridge these pairs — a metric thread one end, a Unified inch thread the other — from one piece, to a drawing, a sketch or the worn part.

Reading the columns: a smaller major always starts in the larger nut, and a larger one never does, so the direction that “works” is fixed by the diameter alone. The pitch difference is what the joint does after that — 0.161 mm per turn on M8 / 5/16-18 is 0.805 mm after five turns, more than half a thread, and the flanks lock. The bridge column names the part that is machined for the pair: a female-to-male thread adapter from M6 to M16, a standoff below that, a double-end stud from M20 up.

The Closest Calls — 22 Pairs Within 0.10 mm in Pitch or Diameter

M2 and #2-64 are the closest pair on the chart in pitch, 0.400 against 0.397 mm; M5 and #10-32 are the close call met most often: 0.800 mm against 0.794 mm, a difference of 0.006 mm, so a #10-32 screw runs several turns into an M5 nut and only its 0.174 mm smaller diameter gives it away, as looseness rather than resistance. The 22 pairs below are within 0.10 mm in pitch or within 0.10 mm in diameter, which is the band where a part feels right by hand and a caliper alone cannot settle it; each row gives the full account of what happens in both directions.

Three of these deserve a second look because they are met in mixed hardware every week. M3.5 and #6-40 are 0.005 mm apart in diameter and 0.035 mm in pitch, close enough that an M3.5 screw feels correct for the two or three turns a thin #6-40 tapped plate holds, and binds in a deeper hole. M8×1 and 5/16-24 sit 0.062 mm apart in diameter and 0.058 mm in pitch, the fine-thread trap on instruments and sensors. M12×1.25 and 1/2-20 share a pitch to within 0.020 mm but are 0.700 mm apart in diameter, so an M12×1.25 stud runs into a 1/2-20 port and carries nothing.

How to Tell Them Apart With a Caliper and a Rule

Counting threads over a 10 mm length separates M8 from 5/16-18 at 8.0 threads against 7.1, and a caliper on the major diameter separates M5 from #10-32 at 5.000 against 4.826 mm; between those two measurements every pair on the chart is identified. Lay a steel rule along the thread, count the crests that fall inside 10 mm, and read the count against the two columns below; when the counts are within 0.5 of each other, measure the diameter instead. The table gives the eight pairs that are most often confused, the count each side gives, and which of the two measurements decides.

A thread pitch gauge does the same job faster: the metric leaves are marked in millimetres, the inch leaves in TPI, and the leaf that sits on the flanks without rocking is the pitch. On a worn or painted thread the caliper reading drops below nominal by the wear, so read the diameter as a band, not a point — an M8 worn to 7.9 mm is still 8.0 threads in 10 mm. For Whitworth sizes, which share the 55° flank with BSP pipe threads and add a third set of lookalikes (1/4 BSW at 20 TPI against 1/4-20 UNC), use the Whitworth thread chart. Where a nut has to be identified by its hex rather than its thread, the wrench size chart lists the across-flats of both systems.

What the Results Mean — Four Outcomes, None of Them a Joint

Every one of the 44 pairs fails in one of the two ways above, pitch fight or wrong diameter, and none of them reaches the flank contact a matched thread gives, so none may carry load; seen from both directions and with the close calls split out, the chart uses four labels:

Does not start — on nominal diameters the bolt’s major is larger than the nut’s, by 0.005 mm (#6-32 in M3.5) to 1.750 mm (1-1/4-7 in M30), and the first crest will not enter the first groove without cutting it; below about 0.1 mm the 6g / 6H tolerance bands let the bolt enter a turn before the pitch difference locks it. Starts, binds within a few turns — the bolt is smaller than the nut and enters, but a pitch difference above 0.020 mm, on a pair more than 0.100 mm apart in pitch or in diameter, accumulates each turn until the crests ride onto the wrong flanks and lock, at 0.805 mm drift after 5 turns for 5/16-18 in M8. Runs in, wrong diameter, no load — the pitches agree to within 0.020 mm, so the bolt screws home freely, but the diameter is short by 0.166 to 0.700 mm and only the crest tips touch the flanks, as with #10-32 in M5 or M4 in #8-36. Engages, feels right, then binds — both differences are within 0.100 mm, the part feels right by hand, but the pitch difference of 0.032 to 0.079 mm still accumulates, 0.160 to 0.395 mm over 5 turns, and the flanks lock, as with #0-80 in M1.6 or 5/16-24 in M8×1.

A screw thread carries load on the flanks of the engaged turns, spread over the length of engagement. A mismatched pair loads a fraction of that: where the pitch differs, the first turn takes the load and the rest are out of step; where the diameter differs, the crests bear on the flanks instead of flank on flank. Under a tightening torque the crests deform, the thread strips from the softer part, and under vibration the joint that felt tight comes loose. Forcing the pair also destroys the nut or the tapped hole, which is the expensive half: an M8 tapped hole in a casting cannot be bought again. The right answer to a lookalike pair is a part with the correct thread on each end, machined for the two systems, not a forced joint.

Pipe Threads Are a Different Trap

G (BSPP, ISO 228) and NPT (ASME B1.20.1) share 14 TPI at 1/2 inch and 3/4 inch, a pitch of 1.814 mm, and still do not seal: G has a 55° flank angle, NPT a 60° flank angle plus a 1-in-16 taper (62.5 mm per metre), so where the counts match the flanks do not seat. The majors sit close enough to start — 20.955 mm for G1/2 against 21.336 mm for 1/2 NPT, 26.441 against 26.670 mm at 3/4 — and the joint screws together a few turns and then leaks or jams. At 1/4 and 3/8 inch the counts differ too, G at 19 TPI (1.337 mm) against NPT at 18 TPI (1.411 mm), and at 1/8 inch G is 28 TPI (0.907 mm) against NPT 27 TPI (0.941 mm). The G / NPT lookalike rows for 1/8, 1/4 and 1/2 inch, with flank angle and taper side by side, are on the thread adapters page; the bridge is a machined G-to-NPT adapter, never a forced fitting.

Where the Chart Stops — the Part With a Different Thread Each End

For all 44 pairs on the chart the catalogue stocks the bolt and the nut in each system and almost never the part that joins the two. A metric board in an inch enclosure, an imported press with a 1/2-13 frame and metric fittings, a probe with a 3/8-16 stem for an M10 boss: the fix is a standoff, a stud or an adapter with the correct thread on each end, machined to the two nominals in the chart. Two jobs bring buyers to this part: the replacement of an adapter that came with a discontinued machine and cannot be bought again, and a new design by an engineer who has to marry a metric assembly to inch-standard hardware and would rather specify one turned part than a bag of washers.

Requests read like this: “I need a standoff 1/2 in OD, 2-1/2 in long, 1/4-20 both ends, 1 in deep.” Or: “We need studs M12 one end, 1/2-13 the other, for an imported press.” Both are turned in one setting: bar to diameter, the metric thread and the inch thread single-pointed to their own pitch, a shoulder or a thread undercut between them so each thread runs out cleanly. Materials by duty: brass CW614N for electronics standoffs and instrument adapters, 303 stainless (1.4305) for free-machining adapters, 304 / 316 (1.4301 / 1.4404) for wet and marine service, 6061 aluminium for weight, 42CrMo4 for studs that carry preload, Delrin (POM) where the adapter must insulate. Small runs of 1 to a few dozen leave in 5 working days; shipping to Europe or the US takes 5–7 working days. A hand sketch with the two threads, the lengths and the hex is enough to quote; for the family of shapes see thread adapter types, hex adapters and threaded rods in the size the catalogue skips. The strength class of the finished stud is a separate question, answered on the bolt grade chart.

Quick Answers

Is 3/8-16 the same as M10?

No: 3/8-16 has a major of 9.525 mm against 10.000 mm for M10, 0.475 mm smaller, and a pitch of 1.588 mm against 1.500 mm, 0.088 mm coarser. A 3/8-16 bolt starts in an M10 nut and binds 0.440 mm out of step after 5 turns; an M10 bolt does not start in a 3/8-16 nut. The diameter, 0.475 mm apart, is the measurement that separates them at a glance.

Is 1/2-13 the same as M12?

No: 1/2-13 has a major of 12.700 mm against 12.000 mm for M12, 0.700 mm larger, and a pitch of 1.954 mm against 1.750 mm, 0.204 mm coarser. An M12 bolt starts in a 1/2-13 nut and binds 1.020 mm out of step after 5 turns; a 1/2-13 bolt does not start in an M12 nut. M12 is also confused with 7/16-14 (11.113 mm, 0.887 mm smaller), which starts in an M12 nut and binds 0.320 mm out of step after 5 turns.

Is 3/4-10 the same as M20?

No: 3/4-10 has a major of 19.050 mm against 20.000 mm for M20, 0.950 mm smaller, and a pitch of 2.540 mm against 2.500 mm, only 0.040 mm apart. A 3/4-10 bolt starts in an M20 nut and binds 0.200 mm out of step after 5 turns; an M20 bolt does not start in a 3/4-10 nut. At this size the bridge is a double-end stud with M20 one end and 3/4-10 the other.

Metric vs Imperial Thread FAQ