ENGINEERING REFERENCE CHART
Dowel Pin Size Chart
A dowel pin size chart pairs every common pin diameter with the two numbers that matter when you machine the hole: the actual pin diameter, and the reamer size for the fit you need — press fit or slip fit, in inch and metric. Get pin size, m6 tolerance, reamer size, and standard lengths, all in one lookup.
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Pin Diameter & m6 Tolerance
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Reamer Size for Press / Slip Fit
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Inch & Metric Standard Lengths
Dowel Pin Reference
Pin size, hole, and fit in one place
Pin Ø
Actual diameter
Reamer
Hole size for fit
Press Fit
m6 / oversize
Slip Fit
Clearance reamer
Quick reference
Pin size → reamer size → fit
REFERENCE GUIDE
What Is a Dowel Pin Size Chart?
A dowel pin size chart lists pin diameters alongside their actual sizes, the hole or reamer sizes for press and slip fits, and available lengths — so you can select and machine the correct pin-and-hole pair without calculation.
Dowel pins are the cylindrical alignment pins that hold two parts in exact position. They do not carry load; they guarantee position. The size chart turns a drawing note into a machined hole.
Nominal & Actual Size
Nominal size is the label (e.g., 1/4″ or 8 mm), not the actual diameter. Inch pins are ground +0.0002″ over nominal; metric m6 pins sit above nominal by a few micrometers.
Reamer Size & Fit Type
The hole diameter you ream determines the fit: press, slip, or loose slip. This is the value a machinist actually programs. Choose based on whether the pin must stay fixed or slide freely.
Standards & Lengths
Inch pins follow ASME B18.8.2; metric hardened-ground pins follow ISO 8734. Available lengths vary by diameter — choose the length that engages your stack as required.
DOWEL PIN REFERENCE DATA
Inch Dowel Pin Sizes (ASME B18.8.2)
Inch dowel pins are governed by ASME B18.8.2 — made oversize. The standard series is ground to nominal + 0.0002″. Ream the hole to nominal for a firm press fit. The oversize series (nominal + 0.001″) is for repairing worn holes.
Actual pin Ø shown is the standard-series maximum (nominal + 0.0002″). The oversize column shows the oversize-series maximum, ground to nominal + 0.001″ with standard tolerance included. ASME B18.8.2 publishes full max/min pin tolerances and the exact preferred length list.
| Nominal pin size | Actual pin Ø (max, in) | Ream hole Ø (in) | Hole tolerance (in) | Oversize pin Ø (max, in) | Standard lengths (in) |
|---|---|---|---|---|---|
| 1/16″ | 0.0627 | 0.0625 | +0.0000 / +0.0002 | 0.0635 | 3/16–1/2 |
| 3/32″ | 0.0940 | 0.0938 | +0.0000 / +0.0002 | 0.0948 | 1/4–3/4 |
| 1/8″ | 0.1252 | 0.1250 | +0.0000 / +0.0002 | 0.1260 | 1/4–1-1/2 |
| 5/32″ | 0.1564 | 0.1563 | +0.0000 / +0.0002 | 0.1574 | 3/8–1-1/2 |
| 3/16″ | 0.1877 | 0.1875 | +0.0000 / +0.0002 | 0.1887 | 3/8–2 |
| 1/4″ | 0.2502 | 0.2500 | +0.0000 / +0.0002 | 0.2510 | 1/2–3 |
| 5/16″ | 0.3127 | 0.3125 | +0.0000 / +0.0002 | 0.3137 | 1/2–3 |
| 3/8″ | 0.3752 | 0.3750 | +0.0000 / +0.0003 | 0.3762 | 3/4–3 |
| 7/16″ | 0.4377 | 0.4375 | +0.0000 / +0.0003 | 0.4387 | 3/4–3 |
| 1/2″ | 0.5002 | 0.5000 | +0.0000 / +0.0003 | 0.5012 | 3/4–4 |
| 5/8″ | 0.6252 | 0.6250 | +0.0000 / +0.0004 | 0.6262 | 1–4 |
| 3/4″ | 0.7502 | 0.7500 | +0.0000 / +0.0004 | 0.7512 | 1–4 |
ASME B18.8.2 Standard Series
The pin is always a hair over the nominal size, and the hole is the nominal size. A 1/4″ dowel pin is 0.2502″ at its largest; ream the hole to 0.2500″. If you drill instead of ream, the hole won’t be round or sized to the ten-thousandths that make a press fit work.
End Styles & Hardness
Type A pins have a chamfer at each end — the everyday choice. Type B pins have a radiused end for very small sizes. Hardened versions are ground from bearing steel and heat-treated to 60–62 HRC, so they won’t wallow the hole.
READING GUIDE
How to Read a Dowel Pin Size Chart
Reading this chart is a three-step job — and the third step is the one that separates a good dowel pin installation from a reworked part.
01
Pick the Pin Diameter & System
Match the pin to the load and plate. A good starting rule: pin diameter roughly equals the size of the fastener screws holding the assembly together — so a joint with M6 screws gets an 8 mm or 6 mm pin. Then read the inch table or the metric table, whichever matches your drawing.
02
Read the Pin’s Actual Size
Look up the row. On the inch side, a 1/4″ pin shows an actual maximum of 0.2502″ and a ream hole of 0.2500″. On the metric side, a 6 mm pin shows an m6 actual of 6.004–6.012 mm and an H7 hole of 6.000–6.012 mm.
03
Read the Hole for Your Fit
Decide press or slip, then read the reamer column. For the 1/4″ pin in loose-press, that’s a 0.2498″ reamer; in tight slip, 0.2505″. For a 6 mm pin, press is H7 at 6.000–6.012 mm; a firmer press is P7 at 5.980–5.992 mm. Then machine undersized and ream to size.
Worked Example: Two-Plate Fixture
Your fixture has two steel plates held by four M6 screws and needs to locate repeatably. Select two 6 mm dowel pins. Base plate gets the press: drill undersized, ream H7 to 6.000–6.012 mm, and press the m6 pin in until it engages about 9–12 mm (1.5–2× pin diameter). Top plate gets the slip: ream 6.030–6.080 mm so it drops over both pins with hand force.
6 mm m6 Pin
Base plate: H7 press
6 mm Slip Hole
Top plate: 6.030–6.080 mm
Engagement
1.5–2× diameter (9–12 mm)
LOCATING RULE
One Press-Fit Pin, One Slip-Fit Pin
Use two dowel pins per joint — one press fit, one slip fit. One pin leaves rotation free; two pressed pins over-constrain and bind. The standard answer: one pin pressed into both plates, the second pin pressed into the base but sliding in a slip-fit hole in the top plate.
Inch Dowel Pin Sizes — ASME B18.8.2
| Nominal size | Actual pin Ø (max, in) | Ream hole Ø (in) | Hole tolerance (in) | Standard lengths (in) |
|---|---|---|---|---|
| 1/16″ | 0.0627 | 0.0625 | +0.0000 / +0.0002 | 3/16–1/2 |
| 3/32″ | 0.0940 | 0.0938 | +0.0000 / +0.0002 | 1/4–3/4 |
| 1/8″ | 0.1252 | 0.1250 | +0.0000 / +0.0002 | 1/4–1-1/2 |
| 5/32″ | 0.1564 | 0.1563 | +0.0000 / +0.0002 | 3/8–1-1/2 |
| 3/16″ | 0.1877 | 0.1875 | +0.0000 / +0.0002 | 3/8–2 |
| 1/4″ | 0.2502 | 0.2500 | +0.0000 / +0.0002 | 1/2–3 |
| 5/16″ | 0.3127 | 0.3125 | +0.0000 / +0.0002 | 1/2–3 |
| 3/8″ | 0.3752 | 0.3750 | +0.0000 / +0.0003 | 3/4–3 |
| 7/16″ | 0.4377 | 0.4375 | +0.0000 / +0.0003 | 3/4–3 |
| 1/2″ | 0.5002 | 0.5000 | +0.0000 / +0.0003 | 3/4–4 |
| 5/8″ | 0.6252 | 0.6250 | +0.0000 / +0.0004 | 1–4 |
| 3/4″ | 0.7502 | 0.7500 | +0.0000 / +0.0004 | 1–4 |
Metric Dowel Pin Sizes — ISO 8734 (m6) / DIN 6325
| Nominal Ø (mm) | m6 actual pin Ø (mm) | H7 reamer hole Ø (mm) | P7 press hole Ø (mm) | Standard lengths (mm) |
|---|---|---|---|---|
| 4 | 4.004–4.012 | 4.000–4.012 | 3.980–3.992 | 5–30 |
| 5 | 5.004–5.012 | 5.000–5.012 | 4.980–4.992 | 6–40 |
| 6 | 6.004–6.012 | 6.000–6.012 | 5.980–5.992 | 6–60 |
| 8 | 8.006–8.015 | 8.000–8.015 | 7.976–7.990 | 8–80 |
| 10 | 10.006–10.015 | 10.000–10.015 | 9.976–9.990 | 10–100 |
| 12 | 12.007–12.018 | 12.000–12.018 | 11.971–11.989 | 12–120 |
| 16 | 16.007–16.018 | 16.000–16.018 | 15.971–15.989 | 16–160 |
| 20 | 20.008–20.021 | 20.000–20.021 | 19.965–19.986 | 20–200 |
Inch Rules — ASME B18.8.2
The pin is always a hair over nominal, and the hole is the nominal size. Standard series runs +0.0002″; oversize +0.001″ for repair holes. Type A (chamfered) is the everyday choice; Type B (radiused) for very small sizes. Hardened to 60–62 HRC.
Metric Rules — ISO 8734 / DIN 6325
m6 is a tolerance zone, not a diameter — an 8 mm m6 pin measures 8.006–8.015 mm. Ream H7 for standard press (8.000–8.015 mm), or P7 for a firmer press (7.976–7.990 mm). Ream to nominal, not to the pin you measured.
Dowel Pin Types & Materials
Not every pin is a hardened-ground dowel pin — a dowel pin size chart only makes sense when you know which family you’re reading. The families, in order of how often fixtures are built with them:
| Type | Standard | Tolerance / Hardness | Best for |
|---|---|---|---|
| Hardened ground dowel pins | ISO 8734 / ASME B18.8.2 | m6 / +0.0002″ oversize, 58–62 HRC | Standard fixture pins — press in/out without wallowing |
| Case-hardened dowel pins | ISO 8735 | m6, hard case over tough core | Impact or bending loads |
| Plain parallel pins | ISO 2338 (h8) | h8 (slightly under nominal), unhardened | Light locating pins, part shafts (not pressed dowels) |
| Taper pins | ASME B18.8.2 | 1-in-48 taper, self-locking | Permanent locating, adjustable assemblies |
| Extraction pins | DIN 7979 | Internal thread for slide hammer removal | Tooling where pins are removed often |
| Grooved & spring pins | ASME B18.8.1 / B18.8.2 | Rolled/slotted, flex on installation | Positioning hardware — no reamed hole required |
Materials follow the standards: bearing steel (52100 or 100Cr6) for hardened-ground pins, 303/304/316 stainless for corrosion resistance, and alloy steel for case-hardened pins. Hardness sits at 58–62 HRC, with ground surface finish below 0.4 µm Ra for smooth pressing. The chamfer on each end lets the pin start straight into the reamed hole — when you machine the receiving hole, add a small lead-in chamfer on the hole mouth to keep the pin from scoring the bore.
CNC MACHINING APPLICATION
Dowel Pin Holes in CNC Machined Parts
Dowel pin holes are drilled and reamed — the tolerance that makes the fit work is a reaming tolerance, not a drilling one. On the shop floor, a dowel pin hole is a three-operation job: center drill, drill undersize, then ream to the exact diameter the fit class needs.
The rules that keep dowel pin holes right: drill undersize then ream, ream to the fit class (not to the pin you measured), chamfer the hole mouth, watch edge distance, and drive to the right engagement depth.
Drill & Ream
Process & Tooling
A drill leaves a hole that is oversized, bell-mouthed, and not round. The reamer produces the size and finish the fit depends on. Drill 0.010–0.020″ (or 0.3–0.5 mm) under finish size, then ream. Center drill first to locate.
Ream to Fit Class
Fit Selection
Pick the reamer from the fit class on this dowel pin size chart — H7 for standard metric press, loose-press reamer for inch work. Trust the pin’s tolerance to supply the interference. Measuring one pin and matching the hole to it produces a hole that only fits that one pin.
Edge & Engagement
Design Rules
Chamfer the hole mouth on both plates to prevent galling. Keep at least one pin diameter of material around the hole. Press the pin in until it engages about 1.5–2× its diameter in the base plate — that engagement carries the load, not the full length.
Our CNC Machining Capabilities
Our CNC drilling and machining teams cut dowel pin holes every day, from single fixtures to production runs. Every hole is drilled undersize, reamed to the fit class, chamfered, and verified on CMM before it ships. Where the ream tolerance matters most, our precision CNC machining takes it to the practical limit — and if the hole needs a ground finish, our CNC grinding department closes that loop.
±0.001″
Reamed hole tolerance
CMM
Inspection on every hole
ISO 9001
AS9100 certified facility
DOWEL PIN QUESTIONS
Dowel Pin Size Chart FAQ
Ream the hole to the pin’s nominal size with an H7 or the standard +0.0002″ tolerance: a 6 mm pin gets a 6.000–6.012 mm H7 hole, a 1/4″ pin gets a 0.2500″ hole. Drill undersize first, about 0.3–0.5 mm or 0.010–0.020″ below finish, then ream. Do not drill to finish size; the reamer produces the round, sized bore a press fit needs.
A press-fit pin is installed with force into a hole slightly smaller than the pin and stays put; a slip-fit pin drops into a slightly larger hole and is meant to come out. For locating, press the first pin into both plates, and let the second pin slide in a slip-fit hole so the assembly is not over-constrained. On the inch side, press reams about 0.0002–0.002″ under nominal and slip about 0.0005–0.001″ over.
For a 1/4″ pin, the loose-press reamer is 0.2498″ and the tight-slip reamer is 0.2505″. For an 8 mm pin, ream H7 at 8.000–8.015 mm for the standard press, or P7 at 7.976–7.990 mm for a firmer press. Confirm the exact reamer against your supplier’s catalog.
The m6 tolerance zone sits a few micrometers above the nominal size. An 8 mm m6 pin measures between 8.006 and 8.015 mm, never exactly 8.000. The m6 zone is what makes an m6 pin pressed into an H7 hole a light press fit. ISO 8734 specifies the m6 zone for hardened-ground metric dowel pins.
ISO 8734 is the current metric standard for hardened-ground parallel pins at an m6 tolerance, and it replaced the German DIN 6325 designation, so the two names describe the same pin. ASME B18.8.2 is the inch standard, where hardened pins run +0.0002″ over nominal instead of an m6 zone. ISO 2338, by contrast, is the unhardened parallel pin at h8, a different family that runs slightly under nominal.
Engage the pin about 1.5–2 times its diameter in the base plate, so a 6 mm pin engages roughly 9–12 mm. For the overall pin length, add the top plate’s thickness and the base plate’s engagement, then round up to a standard length from the chart. A pin long enough to engage both plates properly locates better than a long pin that only reaches halfway through a thick stack.
Get an Instant Quote
Need a part that matches a pin on this dowel pin size chart? Upload your CAD file and get an instant quote in 60 seconds, with live pricing, lead time, and free DFM feedback. Our engineers will confirm the pin size and fit class, check the hole for edge distance and press-force limits, and machine the drilled-and-reamed holes to ±0.001″ before CMM inspection at our ISO 9001 & AS9100 certified facility.