Over and under metric reamers

Is there such thing as an over and under size reamer set for metric sizes, if so where can I buy one, if not what do people do to get over and under sized hole quickly?


Reply to
Jack Lai
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I never heard of such a thing. I suppose you would just use the next mm size under and the next mm size over. Assume in the smaller sizes they come in 1/10th mm graduations just like the drill sizes. Look at the sizes of mm bits in a standard 118 piece mm drill bit set and figure there is probably a reamer to match each size of drill bit. Just my unqualified opinion -- check in Machinery's Handbook for the straight skinny.

Bob Swinney

Reply to
Robert Swinney

Yes, but how much over/under size? Usualy you get them +/- 1/100 mm to



Reply to
Nick Müller

I ordered them from

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Real happy with the price and quality. They weren't listed and I called to place the order.

Reply to

Hello Jack,

Yes, such sets are available. I have one from Twenty-First Century Manufacturing (often known by their trademark: "21CM"); it is Set No. 160 and contains under-sized, nominal, and over-sized reamers for 3mm, 4mm, 5mm,

6mm, 8mm, 10mm, & 12mm. Interestingly, the reamers are sized by their Imperial equivalents, so the set contains: 3mm (0.1181"), 3mm Under (0.1171"), 3mm Over (0.1191") 4mm (0.1575"), 4mm Under (0.1565"), 4mm Over (0.1585") 5mm (0.1968"), 5mm Under (0.1958"), 5mm Over (0.1978") 6mm (0.2362"), 6mm Under (0.2352"), 6mm Over (0.2372") 8mm (0.3150"), 8mm Under (0.3140"), 8mm Over (0.3160") 10mm (0.3937"), 10mm Under (0.3927"), 10mm Over (0.3947") 12mm (0.4724"), 12mm Under (0.4714"), 12mm Over (0.4734")

As you can see, each "under" reamer is 0.001" (0.0254mm) smaller, and each "over" reamer is 0.001" (0.0254mm) larger, than the nominal size. This doesn't consistently follow ANSI standards for metric clearance or interference fits, but generally falls into the tolerance range for a "Close Running" clearance fit (over) or a "Medium Drive" interference fit (under).

As for suppliers, I know that KBC Tools

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sells the

21CM line of products. If they don't carry this set, they can probably get it.

But even if you can't (or don't want) to buy metric over/under reamers, there is a lesson to be learned from the above: you don't necessarily have to buy specific metric over/under reamers; instead you can buy Imperial equivalents. If you go through the table of ANSI Preferred Metric Fits in Machinery's Handbook, you can select the fit you need, look up the metric size, divide that size by 25.4, and then order the closest matching Imperial reamer. As long as the Imperial reamer falls into the tolerance range of that metric fit, you'll be fine.

- Michael

Reply to


That is exactly what I wanted.

Thanks, Jack

Reply to
Jack Lai

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