When a stripped M10 thread halts an assembly line, the fix itself is rarely complicated, but thewrong key locking insert callout can stretch a 30-minute repair into a two week parts hunt.After more than a decade supplying Keensert -style key locking inserts to aerospace primesand Tier-1 automotive shops, we've watched the same thread-standard mistakes cost real money.This guide covers the imperial, metric, and ISO conventions every engineer should read fluentlybefore specifying an insert.
| ISO thread Spec. | ▶ | Equivalent ANSI Spec. | ISO thread Spec. | ▶ | Equivalent ANSI Spec. | ISO thread Spec. | ▶ | Equivalent ANSI Spec. | ||
| M1.6 | ▶ | M1.6*0.35 | M14 | ▶ | M14*2.0 | M30 | ▶ | M30*3.5 | ||
| M2 | ▶ | M2*0.4 | M14*1.5 | ▶ | M14*1.5 | M30*2.0 | ▶ | M30*2.0 | ||
| M2.5 | ▶ | M2.5*0.45 | M16 | ▶ | M16*2.0 | M33 | ▶ | M33*3.5 | ||
| M3 | ▶ | M3*0.5 | M16*1.5 | ▶ | M16*1.5 | M33*2.0 | ▶ | M33*2.0 | ||
| M3.5 | ▶ | M3.5*0.6 | M18 | ▶ | M18*2.5 | M36 | ▶ | M36*4.0 | ||
| M4 | ▶ | M4*0.7 | M18*1.5 | ▶ | M18*1.5 | M36*3.0 | ▶ | M36*3.0 | ||
| M5 | ▶ | M5*0.8 | M20 | ▶ | M20*2.5 | M42 | ▶ | M42*4.5 | ||
| M6 | ▶ | M6*1.0 | M20*1.5 | ▶ | M20*1.5 | M48 | ▶ | M48*5.0 | ||
| M8 | ▶ | M8*1.25 | M22 | ▶ | M22*2.5 | M56 | ▶ | M56*5.5 | ||
| M8*1.0 | ▶ | M8*1.0 | M22*1.5 | ▶ | M22*1.5 | M64 | ▶ | M64*6.0 | ||
| M10 | ▶ | M10*1.5 | M24 | ▶ | M24*3.0 | M72 | ▶ | M72*6.0 | ||
| M10*1.25 | ▶ | M10*1.25 | M24*2.0 | ▶ | M24*2.0 | M80 | ▶ | M80*6.0 | ||
| M12 | ▶ | M12*1.75 | M27 | ▶ | M27*3.0 | M90 | ▶ | M90*6.0 | ||
| M12*1.25 | ▶ | M12*1.25 | M27*2.0 | ▶ | M27*2.0 | M100 | ▶ | M100*6.0 |
Reading Imperial Thread Callouts (UNC /UNF/UNJ)
Imperial threads are written as major diameter - threads per inch.
- 3/8-16 =0.375" diameter, 16 threads per inch (UNC, coarse).
- 3/8-24 = same diameter, 24 threads per inch (UNF, fine - preferred where preloadaccuracy matters).
Aerospace key locking inserts often carry a "J" suffix (e.g., 3/8-16 UNJC), indicating a controlledroot radius for fatigue performance under MIL-I-45914 Rev A testing.
Reading Metric Thread Callouts (M-Series)
Metric threads use major diameter x pitch in mm.
- M10 x 1.5 = 10 mm diameter, 1.5 mm pitch (coarse).
- M10 x 1.25 = same diameter, finer pitch - used where vibration resistance is critical.
ISO shorthand: "M10" alone defaults to M10x1.5. Any non-coarse pitch must be written out.Common pitfall: "M16x2" looks like a fine-thread spec, but 2 mm is the coarse pitch for M16 -the callout is redundant.
Imperial vs Metric: What Actually Differs
A Keenserts cut to UTS will not seat in a 55° Whitworth hole - a problem still seen when retrofitting British equipment built before the late 1970s.
Choosing the Right Key Locking Insert
Imperial KeensertsR (#6 through 1-1/2"): Specified on most U.S. defense drawings - MS51830,MS51831, MS51832. Materials include 4140 alloy (160 KSI min), A286 CRES (140 KSI min), and303/304 stainless.
Metric key locking inserts (M4 through M24): Standard for European, Japanese, and most newglobal builds. Aerospace-grade internal threads conform to Fed Std H28/21 (MJ form), tolerance+0.25 mm.
Series at aglance:
- Miniature (KNC):avionics, electronics - M3 to M6 / 2-56 to 1/4-20
- Thinwall (KN/ KNM): limited boss diameter - M5 to M12 / 10-24 to 1/2-20
- Heavy Duty: general thread repair
- Extra Heavy Duty: soft parent materials under high cyclic load
Pre-Order Checklist
- Thread standard verified against the mating bolt drawing.
- Parent material mapped to insert material for galvanic compatibility.
- Locking vs non-locking specified for the vibration environment.
- Drill and tap sizes pulled from the manufacturer datasheet (insert OD runs slightly over size).
- Installation tool matched to series and size.
Specified correctly, a key locking insert delivers roughly 3-5x the pull-out strength of theoriginal tapped thread, and the wedge keys hold against vibration loads that would back astandard joint loose within hours. That margin is why Keenserts remain the default repair onflight-critical assemblies decades after their introduction.
Need to cross-reference a legacy MS or NA part number to a current key locking insert? Send usthe drawing - we'll match the spec.
