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Metric Dowel Pin Size Chart (ISO 8734, Hardened, m6)

Guide · Updated October 28, 2026

A dowel pin is a hardened, ground parallel pin used to locate one part on another so it goes back in exactly the same place every time. ISO 8734 (also EN ISO 8734, formerly DIN 6325) covers hardened steel and martensitic stainless dowels from 1 to 20 mm, ground to an m6 tolerance with Ra 0.8 µm or better. The diameter limits, chamfer c and length tolerances below agree between the ISO 8734:1997 preview, the DIN EN ISO 8734 table from Pin Fastener and the OPAC ISO 8734 catalog sheet. Check the current edition or your supplier for critical work.

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Step by step

Pick the diameter: Two dowels locate a part; size them roughly to the screws that clamp it. A part held with M6 screws usually gets 6 mm dowels.
Pick the length: Engage about 1.5 to 2 times the diameter in each part. Length tolerance is ±0.25 mm up to 10 mm, ±0.5 mm to 50 mm and ±0.75 mm to 100 mm.
Pick the fit: Press the dowel into one part (hole about H7 or slightly under) and slip the mating part over it (hole about F7/G7 or a reamed slip size).
Drill and ream: Drill 0.2 to 0.3 mm under, then ream to size. Ream both parts in one setup, or transfer locations with the parts clamped.
Vent blind holes: Use a pull-out dowel (ISO 8735, internal thread) or grind a flat on the pin so air can escape a blind hole.
Press it in: Press the chamfered end in square with an arbor press. Do not hammer hardened pins; they can chip.

ISO 8734 dowel pin dimensions (mm)

Columns are: nominal diameter d, d max and d min (m6), the approximate end chamfer c, and the shortest catalog length. One end is chamfered about 15°; the other is usually domed or radiused. Some makers supply both ends radiused.

Size d | d max (m6) | d min (m6) | Chamfer c (≈) | Shortest l
1 | 1.008 | 1.002 | 0.2 | 3
1.5 | 1.508 | 1.502 | 0.3 | 4
2 | 2.008 | 2.002 | 0.35 | 5
2.5 | 2.508 | 2.502 | 0.4 | 6
3 | 3.008 | 3.002 | 0.5 | 8
4 | 4.012 | 4.004 | 0.63 | 10
5 | 5.012 | 5.004 | 0.8 | 12
6 | 6.012 | 6.004 | 1.2 | 14
8 | 8.015 | 8.006 | 1.6 | 18
10 | 10.015 | 10.006 | 2 | 22
12 | 12.018 | 12.007 | 2.5 | 26
16 | 16.018 | 16.007 | 3 | 40
20 | 20.021 | 20.008 | 3.5 | 50

Standard lengths l are 3, 4, 5, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95 and 100 mm, then 20 mm steps. Length tolerance is ±0.25 mm up to 10 mm, ±0.5 mm over 10 to 50 mm and ±0.75 mm over 50 to 100 mm. Small pins stop short and large pins start long, so check the supplier range.

Hole sizes for dowel pins (H7 press, slip fit)

The m6 band sits just above nominal, so an H7 reamed hole gives a light press: a 6H7 hole is 6.000 to 6.012 mm against a 6.004 to 6.012 mm pin, interference 0 to 0.012 mm. For a heavier press, go a few microns under nominal. For the slip side, ream about 0.01 to 0.02 mm over the pin max (roughly F7/G7) so the part lifts off cleanly. See ISO 286 hole basis fits for the full tables.

Worked example: doweling a fixture plate

A 20 mm plate is held to a base with four M8 screws. Use two 8 mm dowels at opposite corners, spread as far apart as possible. Engage 16 mm in the base and 14 mm in the plate, so pick l = 30 (the next standard length). Clamp the plate, drill both parts through at 7.8 mm, ream the base 8H7 (8.000 to 8.015 mm) and the plate to a slip size about 8.025 to 8.035 mm. Press the pins into the base chamfer first. The plate now lifts off and drops back within a few microns.

Worked example: identifying a pin from the drawer

A shiny pin measures 5.008 mm and is glass hard to a file. 5.008 falls inside the 5 mm m6 band (5.004 to 5.012), so it is a 5 mm ISO 8734 dowel. If it had measured 4.995 mm it would be an h-tolerance pin, likely ISO 2338 h8, and would slip in an H7 hole instead of pressing.

Dowel pins vs other pins

Use a dowel when you need repeatable location in hardened or ground parts. Use a taper pin when the part must come apart and go back without wearing the hole. Use slotted or coiled spring pins when a drilled hole is good enough.

Common problems

Pin will not start: hole drilled but not reamed, or no lead-in. Ream to size and break the hole edge.
Pin pushes back out of a blind hole: trapped air. Use a pull-out dowel or one with a vent flat.
Plate binds on the second dowel: holes were not reamed together. Re-ream in one setup or open the second hole to a slot.
Pin cracked on install: it was hammered. Press hardened pins with an arbor press.

FAQ

What tolerance is an ISO 8734 dowel pin?

m6 on diameter. A 6 mm pin measures 6.004 to 6.012 mm, so it is always slightly oversize and presses into an H7 hole.

What is the difference between ISO 8734 and ISO 2338?

ISO 8734 pins are through-hardened (about 550 to 650 HV30) or case hardened steel, or martensitic stainless, ground to m6. ISO 2338 pins are unhardened steel or austenitic stainless in m6 or h8 and are not meant for hardened locating work.

What size hole for a 10 mm dowel pin press fit?

A 10H7 hole (10.000 to 10.015 mm) gives a light press with a 10m6 pin (10.006 to 10.015 mm). For a slip fit in the mating part, ream about 10.013 to 10.028 mm (F7) or use a slip-fit reamer.

Do dowel pins carry load?

They locate; screws clamp. Dowels take shear if the part tries to slide, but do not rely on them to hold the joint together.

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