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Reamer Stock Allowance Chart

Typical material left for machine reaming by reamer diameter, with example pre-ream drill sizes in metric and inch.

Ref CT-0060ASME B94.2

Draft, open for review. Check every value against the governing standard and the manufacturer’s data before use.

What it is

A reamer finishes a drilled or bored hole to an accurate size and good finish. It needs a small, even amount of material to cut, called stock allowance. Too little and the reamer rubs and glazes the hole. Too much and it chatters, wears quickly or cuts oversize. Allowance is measured on the diameter.

How to use it

Find the reamer diameter. Subtract the allowance to get the pre-ream hole size, then pick the nearest drill. Check that the drill size falls inside the allowance range, not outside it.

Typical machine reaming allowance (on diameter)

Reamer diameter	Allowance	Example pre-ream drill
Up to 5 mm	0.1 to 0.2 mm (.004 to .008 in)	4 mm reamer: 3.8 mm
Over 5 to 20 mm	0.2 to 0.3 mm (.008 to .012 in)	10 mm reamer: 9.8 mm
Over 20 to 32 mm	about 0.3 mm (.012 in)	25 mm reamer: 24.7 mm
Over 32 to 50 mm	about 0.5 mm (.020 in)	40 mm reamer: 39.5 mm
1/4 in	.008 in	letter C (.242)
3/8 in	.007 in	letter U (.368)
1/2 in	.008 in	12.5 mm (.4921)
3/4 in	.010 in	18.8 mm (.7402)
1 in	.012 in	25.1 mm (.9882)

Key points

Hand reamers take far less stock than machine reamers, often only a few thousandths of an inch.
Drilled holes can run oversize. Check a test hole before committing to a drill size.
For best accuracy, drill, then bore or core drill, then ream.
Reaming speed is usually about half to two-thirds of the drilling speed for the same material, with a higher feed than drilling.
A reamer follows the existing hole. It does not correct position or straightness.

Common mistakes

Pre-drilling with the "next size down" fraction, which often leaves too much stock.
Leaving too little stock, so the reamer rubs and the hole comes out undersize or glazed.
Running the reamer backward out of the hole, which dulls the edges.

Governing standards: ASME B94.2 (reamers). Reamer manufacturer's allowance and speed recommendations govern.

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.

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