Standard Dowel Pin Dimensions
Dowel pins locate and align parts in assemblies. Unlike bolts (which have clearance), dowel pins are precision ground to tight tolerances and pressed into reamed holes. They’re the backbone of fixture design, mold tooling, and any assembly that needs to come apart and go back together in exactly the same position.
Standard Dowel Pin Diameters (ASME B18.8.2)
| Nominal Diameter (in) | Diameter Tolerance | Reamed Hole Size | Interference (approx) |
|---|---|---|---|
| 1/16 | +0.0001 / -0.0000 | 0.0625 +0.0003 / -0.0000 | 0.0001 to 0.0004 |
| 3/32 | +0.0001 / -0.0000 | 0.0937 +0.0003 / -0.0000 | 0.0001 to 0.0004 |
| 1/8 | +0.0001 / -0.0000 | 0.1250 +0.0003 / -0.0000 | 0.0001 to 0.0004 |
| 3/16 | +0.0001 / -0.0000 | 0.1875 +0.0004 / -0.0000 | 0.0001 to 0.0005 |
| 1/4 | +0.0001 / -0.0000 | 0.2500 +0.0004 / -0.0000 | 0.0001 to 0.0005 |
| 5/16 | +0.0001 / -0.0000 | 0.3125 +0.0004 / -0.0000 | 0.0001 to 0.0005 |
| 3/8 | +0.0001 / -0.0000 | 0.3750 +0.0005 / -0.0000 | 0.0001 to 0.0006 |
| 1/2 | +0.0001 / -0.0000 | 0.5000 +0.0005 / -0.0000 | 0.0001 to 0.0006 |
| 5/8 | +0.0001 / -0.0000 | 0.6250 +0.0006 / -0.0000 | 0.0001 to 0.0007 |
| 3/4 | +0.0001 / -0.0000 | 0.7500 +0.0006 / -0.0000 | 0.0001 to 0.0007 |
Oversize Dowel Pins
When a reamed hole gets worn or slightly oversized (from repeated assembly), oversize dowel pins save the day. They’re available in standard oversize increments:
| Oversize | Use Case |
|---|---|
| +0.001″ | First repair — slight hole wear |
| +0.002″ | Second repair or moderate wear |
| +0.003″ to +0.005″ | Available for severe cases |
Dowel Pin Design Rules
- Use 2 pins per assembly. One pin locates (tight fit in both parts), the second pin prevents rotation (tight fit in one part, slip fit in the other — the “diamond pin” concept).
- Place pins as far apart as possible. Greater pin spacing = better angular accuracy. Two pins 6″ apart locate better than two pins 1″ apart.
- Pin diameter ≥ 1/3 of the thinner plate. Pins too small for the material will shear under side loads.
- Ream both parts together when possible. This eliminates hole-to-hole tolerance between parts.
- Chamfer the pin holes. 45° × 0.010″–0.020″ chamfer helps the pin start straight.
- Press depth = 1.5–2× diameter. Standard press-fit engagement for adequate holding.
Types of Dowel Pins
- Hardened ground (standard): Alloy steel, Rc 60-62. The default. Use for precision locating.
- Unhardened (commercial): Low-carbon steel, not heat treated. Cheaper, adequate for non-precision applications.
- Spring pins (roll pins): Hollow, spring-loaded. Cheaper, self-retaining, no reaming needed. Not for precision — use for shear pins, hinge pins, retaining pins.
- Taper pins: Self-locking due to taper. Used in handwheels, handles, and cranks. Easy to remove with a drift.
Related: Press Fit Chart | Tolerance Stack-Up Analysis | Drill Size Chart