Dowel Pin Hole Sizes: Press Fit and Slip Fit
Starting-point hole sizes for hardened dowel pins, inch 1/16 to 1 in and metric ISO 8734 2 to 20 mm, with the pin's own tolerance and the fit you end up with. Jump to inch or metric.
These are common shop practice, not a standard. The right hole depends on the material (aluminum and plastics grip differently than steel), the pin's tolerance series, the engagement length and whether the part will be taken apart. Prove the size with a test pin in the real material, and work out the exact limits for your fit with the fit tolerance calculator.
Need fixtures, tooling plates or precision assemblies made? .
Inch hardened dowel pins
| Pin | Standard pin dia | Press-fit hole | Press fit (in) | Slip-fit hole | Slip fit (in) |
|---|---|---|---|---|---|
| 1/16" | 0.0626 to 0.06281.590 to 1.595 mm | 0.0620 to 0.06251.575 to 1.588 mm | -0.0008 to -0.0001 | 0.0630 to 0.06351.600 to 1.613 mm | 0.0002 to 0.0009 |
| 3/32" | 0.09385 to 0.094052.384 to 2.389 mm | 0.09325 to 0.093752.369 to 2.381 mm | -0.0008 to -0.0001 | 0.09425 to 0.094752.394 to 2.407 mm | 0.0002 to 0.0009 |
| 1/8" | 0.1251 to 0.12533.178 to 3.183 mm | 0.1245 to 0.12503.162 to 3.175 mm | -0.0008 to -0.0001 | 0.1255 to 0.12603.188 to 3.200 mm | 0.0002 to 0.0009 |
| 5/32" | 0.15635 to 0.156553.971 to 3.976 mm | 0.15575 to 0.156253.956 to 3.969 mm | -0.0008 to -0.0001 | 0.15675 to 0.157253.981 to 3.994 mm | 0.0002 to 0.0009 |
| 3/16" | 0.1876 to 0.18784.765 to 4.770 mm | 0.1870 to 0.18754.750 to 4.763 mm | -0.0008 to -0.0001 | 0.1880 to 0.18854.775 to 4.788 mm | 0.0002 to 0.0009 |
| 1/4" | 0.2501 to 0.25036.353 to 6.358 mm | 0.2495 to 0.25006.337 to 6.350 mm | -0.0008 to -0.0001 | 0.2505 to 0.25106.363 to 6.375 mm | 0.0002 to 0.0009 |
| 5/16" | 0.3126 to 0.31287.940 to 7.945 mm | 0.3120 to 0.31257.925 to 7.938 mm | -0.0008 to -0.0001 | 0.3130 to 0.31357.950 to 7.963 mm | 0.0002 to 0.0009 |
| 3/8" | 0.3751 to 0.37539.528 to 9.533 mm | 0.3745 to 0.37509.512 to 9.525 mm | -0.0008 to -0.0001 | 0.3755 to 0.37609.538 to 9.550 mm | 0.0002 to 0.0009 |
| 7/16" | 0.4376 to 0.437811.115 to 11.120 mm | 0.4370 to 0.437511.100 to 11.113 mm | -0.0008 to -0.0001 | 0.4380 to 0.438511.125 to 11.138 mm | 0.0002 to 0.0009 |
| 1/2" | 0.5001 to 0.500312.703 to 12.708 mm | 0.4995 to 0.500012.687 to 12.700 mm | -0.0008 to -0.0001 | 0.5005 to 0.501012.713 to 12.725 mm | 0.0002 to 0.0009 |
| 5/8" | 0.6251 to 0.625315.878 to 15.883 mm | 0.6245 to 0.625015.862 to 15.875 mm | -0.0008 to -0.0001 | 0.6255 to 0.626015.888 to 15.900 mm | 0.0002 to 0.0009 |
| 3/4" | 0.7501 to 0.750319.053 to 19.058 mm | 0.7495 to 0.750019.037 to 19.050 mm | -0.0008 to -0.0001 | 0.7505 to 0.751019.063 to 19.075 mm | 0.0002 to 0.0009 |
| 7/8" | 0.8751 to 0.875322.228 to 22.233 mm | 0.8745 to 0.875022.212 to 22.225 mm | -0.0008 to -0.0001 | 0.8755 to 0.876022.238 to 22.250 mm | 0.0002 to 0.0009 |
| 1" | 1.0001 to 1.000325.403 to 25.408 mm | 0.9995 to 1.000025.387 to 25.400 mm | -0.0008 to -0.0001 | 1.0005 to 1.001025.413 to 25.425 mm | 0.0002 to 0.0009 |
Standard-series hardened ground pins (ASME B18.8.2) are nominal + 0.0002 in, plus or minus 0.0001. Holes use the half-thou rule: press fit nominal - 0.0005 to nominal, slip fit nominal + 0.0005 to + 0.0010. Fit columns are hole minus pin; negative is interference. On pins 3/32 in and smaller, hold the press-fit hole near the top of its band.
Metric dowel pins (ISO 8734, m6)
| Pin | Pin dia (m6) | Press-fit hole (H7) | Press fit (mm) | Slip-fit hole (F7) | Slip fit (mm) |
|---|---|---|---|---|---|
| 2 mm | 2.002 to 2.0080.0788 to 0.0791 in | 2.000 to 2.0100.0787 to 0.0791 in | -0.008 to 0.008 | 2.006 to 2.0160.0790 to 0.0794 in | -0.002 to 0.014 |
| 2.5 mm | 2.502 to 2.5080.0985 to 0.0987 in | 2.500 to 2.5100.0984 to 0.0988 in | -0.008 to 0.008 | 2.506 to 2.5160.0987 to 0.0991 in | -0.002 to 0.014 |
| 3 mm | 3.002 to 3.0080.1182 to 0.1184 in | 3.000 to 3.0100.1181 to 0.1185 in | -0.008 to 0.008 | 3.006 to 3.0160.1183 to 0.1187 in | -0.002 to 0.014 |
| 4 mm | 4.004 to 4.0120.1576 to 0.1580 in | 4.000 to 4.0120.1575 to 0.1580 in | -0.012 to 0.008 | 4.010 to 4.0220.1579 to 0.1583 in | -0.002 to 0.018 |
| 5 mm | 5.004 to 5.0120.1970 to 0.1973 in | 5.000 to 5.0120.1969 to 0.1973 in | -0.012 to 0.008 | 5.010 to 5.0220.1972 to 0.1977 in | -0.002 to 0.018 |
| 6 mm | 6.004 to 6.0120.2364 to 0.2367 in | 6.000 to 6.0120.2362 to 0.2367 in | -0.012 to 0.008 | 6.010 to 6.0220.2366 to 0.2371 in | -0.002 to 0.018 |
| 8 mm | 8.006 to 8.0150.3152 to 0.3156 in | 8.000 to 8.0150.3150 to 0.3156 in | -0.015 to 0.009 | 8.013 to 8.0280.3155 to 0.3161 in | -0.002 to 0.022 |
| 10 mm | 10.006 to 10.0150.3939 to 0.3943 in | 10.000 to 10.0150.3937 to 0.3943 in | -0.015 to 0.009 | 10.013 to 10.0280.3942 to 0.3948 in | -0.002 to 0.022 |
| 12 mm | 12.007 to 12.0180.4727 to 0.4731 in | 12.000 to 12.0180.4724 to 0.4731 in | -0.018 to 0.011 | 12.016 to 12.0340.4731 to 0.4738 in | -0.002 to 0.027 |
| 16 mm | 16.007 to 16.0180.6302 to 0.6306 in | 16.000 to 16.0180.6299 to 0.6306 in | -0.018 to 0.011 | 16.016 to 16.0340.6306 to 0.6313 in | -0.002 to 0.027 |
| 20 mm | 20.008 to 20.0210.7877 to 0.7882 in | 20.000 to 20.0210.7874 to 0.7882 in | -0.021 to 0.013 | 20.020 to 20.0410.7882 to 0.7890 in | -0.001 to 0.033 |
Limits computed from ISO 286 deviations for m6, H7 and F7. H7 with an m6 pin is a transition fit: usually a light press, but the tolerance extremes allow up to about 0.013 mm of clearance, so hold the hole in the lower half of H7 when the pin must not move. F7 clears the pin except at the extremes, where the two can touch by 0.002 mm or less.
Using dowels well
The usual pattern is two dowels per joint: each pin pressed into one part and a slip fit in the other, so the parts come apart without driving the pins out. Space the pins as far apart as the part allows; for parts that grow with heat or have a long span, make the second hole a slot or use a diamond pin so the joint locates without binding.
Give each pin enough engagement, roughly one and a half to two diameters in each part, and do not let the screws do the locating: dowels locate, screws clamp. Blind press-fit holes trap air, so vent them or use pull-out dowels with a tapped hole in the end. Aluminum grips less than steel and grows more with temperature, so a press that is snug at room temperature can loosen in a hot machine; test it or step up the interference.
Frequently asked questions
- What size hole for a 1/4 in dowel pin press fit?
- A common starting point is a reamed hole of 0.2495 to 0.2500 in. A standard hardened dowel is 0.2501 to 0.2503 in, so that gives 0.0001 to 0.0008 in of interference. Try a pin in a test hole in the actual material before committing a batch.
- What size hole for a 1/4 in dowel pin slip fit?
- About 0.2505 to 0.2510 in, which clears a standard pin by 0.0002 to 0.0009 in. That is loose enough to assemble by hand and tight enough to locate within a few ten-thousandths.
- What hole for a 6 mm dowel pin?
- ISO 8734 pins are ground to m6, 6.004 to 6.012 mm. The usual press-fit hole is H7, 6.000 to 6.012 mm, which is interference most of the time. For a slip fit, an F7 hole (6.010 to 6.022 mm) clears the pin in almost every case, or keep the H7 hole and use a pin ground to h6 or g6.
- Should dowel holes be drilled or reamed?
- Reamed, or bored or interpolated on larger sizes. A twist drill cannot hold a few ten-thousandths on size or roundness. Drill undersize by the amount the reamer maker recommends, then ream, and ream both parts in one setup when you can so the holes line up.
- What are oversize dowel pins for?
- Oversize pins are 0.001 in over nominal instead of 0.0002 in. They are for repairs, where a worn or reworked hole no longer grips a standard pin.
