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Standard Taper Pin Size Chart (ASME B18.8.2)

Guide · Updated October 28, 2026

A standard taper pin is a solid steel pin ground to a taper of 1/4 in per foot on diameter, with both ends crowned. It is driven into a hole that has been taper reamed with the parts assembled, so it locates and locks them at the same time, and it can be knocked out and reused. Inch taper pins are covered by ASME B18.8.2. They are sized by number and by the large end diameter. The chart below gives sizes 7/0 to 13. The figures match across three published copies of the B18.8.2 taper pin table: the Coburn Myers taper pin sheet, the Almabolt taper pin catalog page, and the ICS Cutting Tools taper pin reamer chart.

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Step by step

  1. Find the size: Read the size number off the print or the old pin. Taper pins are called out by number, from 7/0 (the smallest, .0625 in at the large end) up to 13 (1.241 in).
  2. Pick the length: Choose a length so the large end sits flush or slightly proud when the pin is seated. Lengths step by 1/8 in up to 1 in and by 1/4 in above 1 in.
  3. Work out the small end: Small end = large end − 0.02083 × length. The taper is 1/4 in per foot on diameter, which is .02083 in per inch.
  4. Step drill: Drill just under the small end diameter. Longer and larger pins get two or three step drills so the reamer does not have to remove too much.
  5. Taper ream: Ream with the matching number taper pin reamer, with the parts clamped together, and check depth often with the pin itself.
  6. Seat the pin: Ream until the pin goes in by hand to about 80 to 90% of its length, then tap it home with a soft hammer.

ASME B18.8.2 taper pin large end diameters (inches)

Columns are: size number, basic large end diameter, commercial class large end (max–min), precision class large end (max–min), and the small end of a 1 in long pin, worked out as large end minus .02083. Precision class is not listed for sizes 11 to 13. End crown radius runs about .010 in over and under the basic diameter, for example .260/.240 in on a No. 4. Length tolerance is ±.010 in.

Size 14 is also listed in the standard, but the published copies disagree on its basic diameter, so it is left off here. Check the standard or your supplier if you need it.

The taper math

1/4 in per foot is 0.25 / 12 = .020833 in per inch on diameter. On radius it is half that, .010417 in per inch, so the half angle is atan(.010417) = 0.597°, and the included angle is about 1.19°. To find the small end of any pin, multiply the length by .02083 and take it off the large end. To find how far a pin will move for a change in hole size, divide the change by .02083. A hole that is .001 in big lets the pin sink about .048 in deeper.

Worked example: No. 5 pin through a 1 1/2 in hub

A gear hub is pinned to a shaft with a No. 5 taper pin. The pin goes all the way through the 1 1/2 in hub, so pick a 1 3/4 in pin to leave a little proud on each side. The large end is .2890 in. The small end is .2890 − 1.75 × .02083 = .2890 − .0365 = .2525 in. A 1/4 in drill (.2500) is just under the small end, so it is a good pilot. Over that length the reamer still has to open the top of the hole by almost .039 in, so step drill: run 1/4 through, then a 17/64 (.2656) about halfway down, and ream with a No. 5 taper pin reamer.

Worked example: how deep to ream

You want a No. 2 pin (.1930 large end) to sit flush on a part. You ream until the pin goes in by hand, and it stands .200 in proud. Driving it home closes about .200 × .02083 = .0042 in of diameter, which is too much interference for a pin. Ream deeper until the pin stands about 1/8 in proud by hand, which is about .0026 in, then tap it flush.

Taper pins vs dowels and roll pins

Taper pins are best when parts need to come apart and go back in exactly the same place, such as gears and collars on shafts, or fixtures that are pinned after they are set up. Hardened dowel pins locate more precisely but need a reamed straight hole and a press fit. Roll pins are faster and cheaper but do not locate as well. Metric taper pins (ISO 2339, DIN 1) use a 1:50 taper and are sized by the small end, so they need metric reamers.

Common problems

FAQ

What is the taper on a standard taper pin?

1/4 in per foot on diameter, which is .02083 in per inch of length, or about 1.19° included angle.

What is the diameter of a No. 4 taper pin?

The large end is .2500 in basic. A 2 in long No. 4 pin has a small end of .2500 − 2 × .02083 = .2083 in.

What is the difference between commercial and precision class?

Precision class has a tighter large end tolerance (+.0010/−0 on sizes 7/0 to 10) and is more closely controlled. Commercial class runs +.0013/−.0007 on the basic size. Sizes 11 to 13 are commercial only.

Are inch and metric taper pins the same taper?

No. Metric taper pins (ISO 2339 / DIN 1) use a 1:50 taper, which is .020 per unit length, and are sized by the small end. Inch pins use 1/4 in per foot and are sized by the large end. Do not mix the reamers.

Pin and retaining-ring size charts

Picking a fastener to locate or retain a part? Compare the other pin and ring charts in this set:

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General shop information only — follow your drawing, tool maker's data and your quality system.