For a press fit, ream the hole for a hardened inch dowel about .0001–.0005 in under nominal (a metric ISO 8734 m6 dowel goes in an H7 hole); for a slip fit, ream about .0004–.0007 in over nominal (metric: F7). Hardened, ground dowels are made slightly over their nominal size, so the hole size, not the pin, sets the fit.
The inch table lists ASME B18.8.2 hardened ground machine dowel pins in the standard series (nominal +.0002 in, ±.0001) and the oversize series (nominal +.001 in) used for repair, with typical press-fit and slip-fit reamed hole sizes and the interference or clearance they give. The metric table uses the ISO 286 limits for m6 pins (ISO 8734 / DIN 6325) and works out the real fit in H7, F7 and G7 holes. Both tables give a drill to use before reaming. Use the inch/mm switch to see the inch table in millimetres, and the search box to jump to a size.
Inch dowel pins: hole sizes for press and slip fits
| Nominal | Decimal | Standard pin (B18.8.2) | Oversize pin | Press-fit hole | Interference | Slip-fit hole | Clearance | Drill before reaming |
|---|---|---|---|---|---|---|---|---|
| 1/16" | .06251.587 | .0626 – .06281.590 – 1.595 | .0634 – .06361.610 – 1.615 | .0622 – .06251.580 – 1.587 | .00013 µm – .000615 µm | .0629 – .06321.598 – 1.605 | .00013 µm – .000615 µm | #53 (.05951.511) |
| 3/32" | .093752.381 | .09385 – .094052.384 – 2.389 | .09465 – .094852.404 – 2.409 | .09345 – .093752.374 – 2.381 | .00013 µm – .000615 µm | .09415 – .094452.391 – 2.399 | .00013 µm – .000615 µm | #43 (.08902.261) |
| 1/8" | .12503.175 | .1251 – .12533.178 – 3.183 | .1259 – .12613.198 – 3.203 | .1247 – .12503.167 – 3.175 | .00013 µm – .000615 µm | .1254 – .12573.185 – 3.193 | .00013 µm – .000615 µm | #31 (.12003.048) |
| 5/32" | .156253.969 | .15635 – .156553.971 – 3.976 | .15715 – .157353.992 – 3.997 | .15595 – .156253.961 – 3.969 | .00013 µm – .000615 µm | .15665 – .156953.979 – 3.987 | .00013 µm – .000615 µm | #25 (.14953.797) |
| 3/16" | .18754.762 | .1876 – .18784.765 – 4.770 | .1884 – .18864.785 – 4.790 | .1872 – .18754.755 – 4.762 | .00013 µm – .000615 µm | .1879 – .18824.773 – 4.780 | .00013 µm – .000615 µm | #14 (.18204.623) |
| 1/4" | .25006.350 | .2501 – .25036.353 – 6.358 | .2509 – .25116.373 – 6.378 | .2495 – .24996.337 – 6.347 | .00025 µm – .000820 µm | .2504 – .25076.360 – 6.368 | .00013 µm – .000615 µm | C (.24206.147) |
| 5/16" | .31257.938 | .3126 – .31287.940 – 7.945 | .3134 – .31367.960 – 7.965 | .3120 – .31247.925 – 7.935 | .00025 µm – .000820 µm | .3129 – .31327.948 – 7.955 | .00013 µm – .000615 µm | N (.30207.671) |
| 3/8" | .37509.525 | .3751 – .37539.528 – 9.533 | .3759 – .37619.548 – 9.553 | .3745 – .37499.512 – 9.522 | .00025 µm – .000820 µm | .3754 – .37579.535 – 9.543 | .00013 µm – .000615 µm | U (.36809.347) * |
| 7/16" | .437511.112 | .4376 – .437811.115 – 11.120 | .4384 – .438611.135 – 11.140 | .4370 – .437411.100 – 11.110 | .00025 µm – .000820 µm | .4379 – .438211.123 – 11.130 | .00013 µm – .000615 µm | 27/64" (.421910.716) * |
| 1/2" | .500012.700 | .5001 – .500312.703 – 12.708 | .5009 – .501112.723 – 12.728 | .4995 – .499912.687 – 12.697 | .00025 µm – .000820 µm | .5004 – .500712.710 – 12.718 | .00013 µm – .000615 µm | 31/64" (.484412.303) * |
| 5/8" | .625015.875 | .6251 – .625315.878 – 15.883 | .6259 – .626115.898 – 15.903 | .6245 – .624915.862 – 15.872 | .00025 µm – .000820 µm | .6254 – .625715.885 – 15.893 | .00013 µm – .000615 µm | 39/64" (.609415.478) * |
| 3/4" | .750019.050 | .7501 – .750319.053 – 19.058 | .7509 – .751119.073 – 19.078 | .7495 – .749919.037 – 19.047 | .00025 µm – .000820 µm | .7504 – .750719.060 – 19.068 | .00013 µm – .000615 µm | 47/64" (.734418.653) * |
| 7/8" | .875022.225 | .8751 – .875322.228 – 22.233 | .8759 – .876122.248 – 22.253 | .8745 – .874922.212 – 22.222 | .00025 µm – .000820 µm | .8754 – .875722.235 – 22.243 | .00013 µm – .000615 µm | 55/64" (.859421.828) |
| 1" | 1.000025.400 | 1.0001 – 1.000325.403 – 25.408 | 1.0009 – 1.001125.423 – 25.428 | .9995 – .999925.387 – 25.397 | .00025 µm – .000820 µm | 1.0004 – 1.000725.410 – 25.418 | .00013 µm – .000615 µm | 63/64" (.984425.003) |
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Pin diameters are the ASME B18.8.2 limits: standard series nominal +.0002 in ±.0001, oversize series nominal +.001 in ±.0001. Hole sizes are widely published shop practice, not part of the standard: press fit nominal −.0003 to nominal for pins up to 3/16 in, nominal −.0005 to −.0001 for 1/4 in and up; slip fit nominal +.0004 to +.0007 in. Interference and clearance are worked out against the standard-series pin. * No inch drill leaves stock inside the usual band for this size; the drill shown is the nearest (U leaves a little less, 1/64 under leaves a little more). A metric drill can sit closer; see the reamer pre-drill chart.
Metric dowel pins (m6): H7, F7 and G7 holes
| Dowel | m6 pin limits (mm) | H7 hole (mm) | H7 / m6 fit | F7 hole (mm) | F7 / m6 fit | G7 hole (mm) | Drill before reaming |
|---|---|---|---|---|---|---|---|
| Ø1 mm | 1.002 – 1.008 (+2/+8 µm) | 1.000 – 1.010 (0/+10) | 8 µm int. to 8 µm cl. | 1.006 – 1.016 (+6/+16) | 2 µm int. to 14 µm cl. | 1.002 – 1.012 (+2/+12) | 0.9 mm (0.10 mm stock) |
| Ø1.5 mm | 1.502 – 1.508 (+2/+8 µm) | 1.500 – 1.510 (0/+10) | 8 µm int. to 8 µm cl. | 1.506 – 1.516 (+6/+16) | 2 µm int. to 14 µm cl. | 1.502 – 1.512 (+2/+12) | 1.4 mm (0.10 mm stock) |
| Ø2 mm | 2.002 – 2.008 (+2/+8 µm) | 2.000 – 2.010 (0/+10) | 8 µm int. to 8 µm cl. | 2.006 – 2.016 (+6/+16) | 2 µm int. to 14 µm cl. | 2.002 – 2.012 (+2/+12) | 1.9 mm (0.10 mm stock) |
| Ø2.5 mm | 2.502 – 2.508 (+2/+8 µm) | 2.500 – 2.510 (0/+10) | 8 µm int. to 8 µm cl. | 2.506 – 2.516 (+6/+16) | 2 µm int. to 14 µm cl. | 2.502 – 2.512 (+2/+12) | 2.4 mm (0.10 mm stock) |
| Ø3 mm | 3.002 – 3.008 (+2/+8 µm) | 3.000 – 3.010 (0/+10) | 8 µm int. to 8 µm cl. | 3.006 – 3.016 (+6/+16) | 2 µm int. to 14 µm cl. | 3.002 – 3.012 (+2/+12) | 2.9 mm (0.10 mm stock) |
| Ø4 mm | 4.004 – 4.012 (+4/+12 µm) | 4.000 – 4.012 (0/+12) | 12 µm int. to 8 µm cl. | 4.010 – 4.022 (+10/+22) | 2 µm int. to 18 µm cl. | 4.004 – 4.016 (+4/+16) | 3.8 mm (0.20 mm stock) |
| Ø5 mm | 5.004 – 5.012 (+4/+12 µm) | 5.000 – 5.012 (0/+12) | 12 µm int. to 8 µm cl. | 5.010 – 5.022 (+10/+22) | 2 µm int. to 18 µm cl. | 5.004 – 5.016 (+4/+16) | 4.8 mm (0.20 mm stock) |
| Ø6 mm | 6.004 – 6.012 (+4/+12 µm) | 6.000 – 6.012 (0/+12) | 12 µm int. to 8 µm cl. | 6.010 – 6.022 (+10/+22) | 2 µm int. to 18 µm cl. | 6.004 – 6.016 (+4/+16) | 5.8 mm (0.20 mm stock) |
| Ø8 mm | 8.006 – 8.015 (+6/+15 µm) | 8.000 – 8.015 (0/+15) | 15 µm int. to 9 µm cl. | 8.013 – 8.028 (+13/+28) | 2 µm int. to 22 µm cl. | 8.005 – 8.020 (+5/+20) | 7.8 mm (0.20 mm stock) |
| Ø10 mm | 10.006 – 10.015 (+6/+15 µm) | 10.000 – 10.015 (0/+15) | 15 µm int. to 9 µm cl. | 10.013 – 10.028 (+13/+28) | 2 µm int. to 22 µm cl. | 10.005 – 10.020 (+5/+20) | 9.8 mm (0.20 mm stock) |
| Ø12 mm | 12.007 – 12.018 (+7/+18 µm) | 12.000 – 12.018 (0/+18) | 18 µm int. to 11 µm cl. | 12.016 – 12.034 (+16/+34) | 2 µm int. to 27 µm cl. | 12.006 – 12.024 (+6/+24) | 11.8 mm (0.20 mm stock) |
| Ø16 mm | 16.007 – 16.018 (+7/+18 µm) | 16.000 – 16.018 (0/+18) | 18 µm int. to 11 µm cl. | 16.016 – 16.034 (+16/+34) | 2 µm int. to 27 µm cl. | 16.006 – 16.024 (+6/+24) | 15.7 mm (0.30 mm stock) |
| Ø20 mm | 20.008 – 20.021 (+8/+21 µm) | 20.000 – 20.021 (0/+21) | 21 µm int. to 13 µm cl. | 20.020 – 20.041 (+20/+41) | 1 µm int. to 33 µm cl. | 20.007 – 20.028 (+7/+28) | 19.7 mm (0.30 mm stock) |
| Ø25 mm | 25.008 – 25.021 (+8/+21 µm) | 25.000 – 25.021 (0/+21) | 21 µm int. to 13 µm cl. | 25.020 – 25.041 (+20/+41) | 1 µm int. to 33 µm cl. | 25.007 – 25.028 (+7/+28) | 24.7 mm (0.30 mm stock) |
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Limits from ISO 286-1 (the same tables as the ISO limits and fits calculator). Deviations in brackets are in micrometres. "int." = interference (press), "cl." = clearance. The fit range runs from the tightest to the loosest combination of pin and hole.
Which metric hole: H7, F7 or G7?
H7 for the fixed part. ISO 8734 and DIN 6325 dowels are ground to m6 so they can be pressed into an H7 reamed hole. Strictly H7/m6 is a transition fit: at Ø10 mm it runs from 15 µm interference to 9 µm clearance. In practice a sharp H7 reamer cuts in the lower half of the band and the pin holds, but if the pin must never be loose, gauge the hole and keep it near the bottom of H7, or ream the fixed part a few microns under.
F7 for the part that comes off. An F7 hole gives a sliding fit on an m6 dowel: at Ø10 mm, 2 µm interference in the very worst case to 22 µm clearance. Use it in the plate that is lifted off the pins, such as a fixture lid or a die half.
G7 for close location. G7 sits lower than F7 and gives a close push fit on m6 (10 µm interference to 14 µm clearance at Ø10 mm). It locates more accurately than F7 but can need a tap to assemble, so it suits parts that are assembled rarely.
How to use this chart
- Find the dowel size and decide which part holds the pin (press fit) and which part slides over it (slip fit).
- Drill with the pre-drill size shown. Spot drill first and drill straight; a reamer follows the drilled hole and will not correct position.
- Ream to the press or slip size. Decimal under-size and over-size reamers (for example .2495 and .2505 in) are stock items. Reamers often cut .0001–.0003 in over their own size, so check the first hole with a pin gauge before running the batch.
- Press the pin in with an arbor press, not a hammer, and give blind holes a vent (a flat on the pin, a cross-drilled hole or a through hole) so trapped air does not push the pin back out.
Worked example: 1/4 in dowel locating a fixture plate
Standard 1/4 in pin: .2501–.2503 in. Base plate (press): drill letter C (.242 in), then ream .2495–.2499 in, giving .0002–.0008 in interference. Top plate (slip): drill C, then ream .2504–.2507 in, giving .0001–.0006 in clearance.
Worked example: Ø8 mm m6 dowel
Pin 8.006–8.015 mm. Base: drill 7.8 mm, ream 8 H7 (8.000–8.015 mm). Lid: drill 7.8 mm, ream 8 F7 (8.013–8.028 mm), which slides on the pin with up to 22 µm clearance.
Shop tips
- In aluminium and other soft materials, work at the light end of the press range; a heavy press fit galls the bore and the pin can walk out after a few heat cycles. In cast iron, which does not stretch, use the light end too, or the casting can crack.
- Some charts quote nominal −.0005 to −.001 in for a press fit. That heavy interference suits large pins in steel hubs; it is too tight for small pins and thin walls.
- Pull-out (tapped-end) dowels and through holes let you remove pins from blind holes later.
- Use oversize (+.001 in) dowels to repair a worn or over-reamed press hole: ream the hole to suit the oversize pin.
- For holes that must stay accurate after heat treatment, ream or grind after hardening, or jig grind.
Related calculators and charts
Sources: ASME B18.8.2 (hardened ground machine dowel pins), ISO 8734 and DIN 6325 (m6 dowel pins), ISO 286-1 (limits and fits). Inch hole sizes and pre-drill sizes are typical shop practice. Check critical fits against the standard, the pin maker's data and your drawing.