ER collet nut thread chart
The single most common reason a replacement nut doesn't fit: the same ER number exists with two different threads. Match the thread to your holder, not to the collet.
A-type — standard hex pattern
The pattern most people mean when they just say "ER nut". Used on the majority of hobby spindles, bench mills and tool holders.
| Nut | Thread | Nut Ø (mm) | Nut length (mm) | Fits collet series |
|---|---|---|---|---|
| ER8-A | M10 × 0.75 | 13.5 – 14 | 11 | ER8 |
| ER11-A | M14 × 0.75 | 19 | 11.3 | ER11 |
| ER16-A | M22 × 1.5 | 28 | 17.5 | ER16 |
| ER20-A | M25 × 1.5 | 34 – 35 | 19 | ER20 |
| ER25-A | M32 × 1.5 | 42 | 20 | ER25 |
| ER32-A | M40 × 1.5 | 50 | 22.5 | ER32 |
| ER40-A | M50 × 1.5 | 63 | 25.5 | ER40 |
| ER50-A | M64 × 2.0 | 78 | 35.5 | ER50 |
Dimensions cross-checked across several nut manufacturers. Two sizes show a small spread between sources (ER8-A and ER20-A outside diameter); either value is within manufacturing tolerance for wrench clearance, but measure yours if you're machining a holder adapter.
A-type nuts
M-type — mini / compact pattern
A slim nut for tight nose geometries. Note the pitch differs from the A-type on almost every size — this is the trap. An ER16-A is M22 × 1.5; an ER16-M is M19 × 1.0. Similar enough to look right in a photo, completely different on the thread.
| Nut | Thread | Nut Ø (mm) | Nut length (mm) | Fits collet series |
|---|---|---|---|---|
| ER8-M | M10 × 0.75 | 12 | 10 – 11 | ER8 |
| ER11-M | M13 × 0.75 | 16 | 12 | ER11 |
| ER16-M | M19 × 1.0 | 22 | 18 | ER16 |
| ER20-M | M24 × 1.0 | 28 | 19 | ER20 |
| ER25-M | M30 × 1.0 | 35 | 20 | ER25 |
UM-type — reduced-diameter / bearing-sealed
A slimmer profile used where an oversized nut would foul the workpiece or the spindle nose. Some UM nuts carry a built-in bearing so tightening doesn't twist the collet — genuinely useful when you care about runout.
| Nut | Thread | Nut Ø (mm) | Nut length (mm) |
|---|---|---|---|
| ER16-UM | M20 × 1.5 (some makers list M22 × 1.5) | 32 | 17 – 20 |
| ER20-UM | M24 × 1.5 (some makers list M25 × 1.5) | 35 | 19 – 20 |
| ER25-UM | M32 × 1.5 | 42 | 20 |
| ER32-UM | M40 × 1.5 | 50 | 22.5 |
| ER40-UM | M50 × 1.5 | 63 | 25.5 |
| ER50-UM | M64 × 2.0 | 70 – 78 | 35.5 – 36 |
How to identify what you already have
- Read the nut. Most are stamped with their type alongside the size — look for a trailing A, M or UM after the ER number.
- Measure the thread's outside diameter. This alone narrows it to one candidate in almost every case: a 22 mm external thread is ER16-A, a 19 mm one is ER16-M.
- Check the pitch with a gauge. ×1.5 coarse and ×0.75 / ×1.0 fine are visually obvious once you've compared them side by side.
- Look at the spanner. If you have the original wrench and it fits, note which pattern it matches — the spanner families are not interchangeable either.
Spanners and hooks
Spanners follow the nut family, not the ER size. This catches people out constantly: an ER25 spanner will not fit an ER25-M nut, because the M-type nut is a different diameter. Check which family your nut belongs to before ordering the wrench.
| Spanner type | Covers | Notes |
|---|---|---|
| A-type | ER8-A through ER40-A | The standard hex-pattern nut spanner |
| M-type | ER8-M to ER20-M | Smaller hook for compact mini nuts |
| UM-type | ER16-UM to ER50-UM | For reduced-diameter and sealed nuts |
| C-type | Castle nuts | Different engaging face again |