Guide
BT vs SK vs CAT vs HSK Tool Holders: What Is the Difference?
BT, SK and CAT holders all use the 7/24 steep taper and are pulled into the spindle by a pull stud; HSK-A is a hollow short taper clamped from inside, with no pull stud. They differ in flange, length, thread and units, so order the shank your spindle and tool changer are built for. The figures below are PTP's shank tables.
Last updated 29 October 2026
How do BT, SK, CAT and HSK differ?
| Shank | Standard | Taper | Held in the spindle by | Units | PTP tool holders |
|---|---|---|---|---|---|
| BT | MAS BT · ISO 7388/2-J · JIS B6339 | 7/24 steep taper | MAS-BT or JIS B6339 pull stud and drawbar | mm | BT30, BT40, BT50 |
| SK (ISO) | DIN 69871 · ISO 7388/1-A | 7/24 steep taper | DIN 69872 pull stud and drawbar | mm | SK30, SK40, SK50 |
| CAT | CAT ANSI / ASME B5.50 | 7/24 steep taper, V-flange | ANSI/ASME B5.50 retention knob and drawbar | inch | CAT40, CAT50 (codes on request) |
| HSK-A | ISO 12164-1 · DIN 69893-1 | Hollow short taper | Clamped from inside by the spindle; no pull stud | mm | HSK-A50, A63, A100 |
| DIN 2080 | DIN 2080 · IS 2340 | 7/24 steep taper | Drawbar thread (G) | mm | Shank dimensions only |
What are the dimensions of each shank?
Gauge (or taper) diameter D, flange diameter D1, length L and the thread G, by size. Each shank page has the full table and drawing.
| Shank size | Gauge / taper Ø (D) | Flange Ø (D1) | Length (L) | Thread (G) |
|---|---|---|---|---|
| BT30 | 31.75 mm | 46 mm | 48.4 mm | M12 |
| BT35 | 38.10 mm | 53 mm | 56.4 mm | M12 |
| BT40 | 44.45 mm | 63 mm | 65.4 mm | M16 |
| BT45 | 57.15 mm | 85 mm | 82.8 mm | M20 |
| BT50 | 69.75 mm | 100 mm | 101.8 mm | M24 |
| SK30 | 31.75 mm | 50 mm | 47.8 mm | M12 |
| SK40 | 44.45 mm | 63.55 mm | 68.4 mm | M16 |
| SK45 | 57.15 mm | 82.55 mm | 82.7 mm | M20 |
| SK50 | 69.75 mm | 97.50 mm | 101.75 mm | M24 |
| CAT30 | 1.250 in | 1.812 in | 1.875 in | 1/2"-13 UNC |
| CAT40 | 1.750 in | 2.500 in | 2.687 in | 5/8"-11 UNC |
| CAT45 | 2.250 in | 3.250 in | 3.250 in | 3/4"-10 UNC |
| CAT50 | 2.750 in | 3.875 in | 4.000 in | 1"-8 UNC |
| HSK-A50 | 38 mm | 50 mm | 25 mm | M16×1 |
| HSK-A63 | 48 mm | 63 mm | 32 mm | M18×1 |
| HSK-A80 | 60 mm | 80 mm | 40 mm | M20×1.5 |
| HSK-A100 | 75 mm | 100 mm | 50 mm | M24×1.5 |
| ISO 30 (DIN 2080) | 31.75 mm | 48 mm | 70 mm | M12 |
| ISO 40 (DIN 2080) | 44.45 mm | 60 mm | 95 mm | M16 |
| ISO 45 (DIN 2080) | 57.15 mm | 77 mm | 110 mm | M20 |
| ISO 50 (DIN 2080) | 69.75 mm | 97 mm | 130 mm | M24 |
Are BT40 and SK40 holders interchangeable?
They share the 7/24 taper and the 44.45 mm gauge diameter, but not the rest: BT40 (MAS 403) has a 63 mm flange and a length of 65.4 mm, SK40 (DIN 69871) a 63.55 mm flange and 68.4 mm. They also take different pull studs: MAS-BT studs with a 45°, 60° or 90° head on BT, DIN 69872 studs with a 15° head on SK.
Order the shank your spindle and tool changer are built for.
What does 'ISO 40' mean?
Most often an SK40 holder to DIN 69871 / ISO 7388-1, common on European-built machines; PTP's SK ordering codes start 47ISO… The name is also used for the older DIN 2080 taper, which has a 60 mm flange on size 40 instead of 63.55 mm.
HSK-A, HSK-B or HSK-F?
All three are hollow short tapers to ISO 12164-1 / DIN 69893-1. For the same flange size the forms differ; the PTP shank tables for size 63:
PTP's HSK tool holders and gauges are HSK-A (HSK-A50, A63 and A100).
Which pull stud does each shank take?
The thread G in each shank table is the pull-stud thread of a standard holder. HSK needs none.
| Taper | Pull-stud thread (G) | PTP pull studs | Every stud for this taper |
|---|---|---|---|
| BT30 | M12 | MAS-BT, MG MEGA Strength | BT30 pull studs |
| BT35 | M12 | MAS-BT | BT35 pull studs |
| BT40 | M16 | MAS-BT, MG MEGA Strength, JIS B6339 | BT40 pull studs |
| BT45 | M20 | MAS-BT | BT45 pull studs |
| BT50 | M24 | MAS-BT, JIS B6339 | BT50 pull studs |
| SK30 | M12 | DIN 69872 | SK30 pull studs |
| SK40 | M16 | DIN 69872 | SK40 pull studs |
| SK45 | M20 | DIN 69872 | SK45 pull studs |
| SK50 | M24 | DIN 69872 | SK50 pull studs |
| CAT30 | 1/2"-13 UNC | ANSI/ASME B5.50 | CAT30 pull studs |
| CAT40 | 5/8"-11 UNC | ANSI/ASME B5.50, PSCAT | CAT40 pull studs |
| CAT45 | 3/4"-10 UNC | ANSI/ASME B5.50 | CAT45 pull studs |
| CAT50 | 1"-8 UNC | ANSI/ASME B5.50 | CAT50 pull studs |
| HSK-A50 | No pull stud | Clamped by the spindle | – |
| HSK-A63 | No pull stud | Clamped by the spindle | – |
| HSK-A80 | No pull stud | Clamped by the spindle | – |
| HSK-A100 | No pull stud | Clamped by the spindle | – |
Questions about tool holder shanks
What is the difference between BT40, SK40 and CAT40?
All three are 7/24 tapers with a 44.45 mm (1.750 in) gauge diameter. BT40 (MAS 403) has a 63 mm flange and an M16 pull stud; SK40 (DIN 69871) a 63.55 mm flange and an M16 pull stud; CAT40 (ANSI/ASME B5.50) is dimensioned in inches, with a 2.500 in flange and a 5/8"-11 UNC pull stud.
BT shank dimensions SK shank dimensions CAT shank dimensions
Does an HSK holder need a pull stud?
No. The spindle clamps the HSK hollow taper from inside, so there is no pull stud. For through-spindle coolant an HSK-A holder takes a coolant tube.
Is 'ISO 40' the same as SK40?
Usually: 'ISO 40' most often means an SK40 holder to DIN 69871 / ISO 7388-1. The name is also used for the older DIN 2080 taper, which has a 60 mm flange on size 40 instead of 63.55 mm. Check which one your spindle takes.