Bearing bore size: matching a metric bearing to an inch-machined legacy shaft
I was replacing a bearing on decades-old American machinery with a metric bearing from a modern supplier, and had to convert carefully to confirm the bore would actually seat correctly on the inch-machined shaft.
We write the way we argue on the bench: exact factor, one sharp trap, one worked example you can re-check.
I was replacing a bearing on decades-old American machinery with a metric bearing from a modern supplier, and had to convert carefully to confirm the bore would actually seat correctly on the inch-machined shaft.
I still keep a screenshot of this relationship because memory is where the wrong cousin of a unit sneaks in.
The factor that settles arguments
This is the relationship our converters use. Screenshot it if you have to — just stop re-deriving it from a half-remembered blog post.
1 mm = 0.0393701 in
Write that line once at the top of your notes. Every later arithmetic step should point back to it instead of inventing a friendlier number mid-stream.
The mistake that keeps showing up
Assuming a metric bearing bore 'close to' an inch shaft diameter will press-fit correctly without converting exactly — even a fraction of a millimeter mismatch causes a loose or overly tight fit that damages the bearing.
I still write the unit next to every intermediate value. It looks pedantic until it catches the one swap that would have shipped wrong.
- Write down the known value in mm (bearing bore).
- Confirm the target unit is really in, not a similar-looking one.
- Apply the factor: 1 mm = 0.0393701 in.
- Round only at the end, to the precision your tool or spec supports.
Worked example
A replacement bearing has a 20 mm bore, and the legacy shaft it needs to fit was machined to an inch dimension.
20 mm (bearing bore) × 0.0393701 = 0.787402 in
That's 0.787402 in — round only to the precision your tool or spec actually needs.
Flip the factor once as a sanity check. If you do not land near the starting number, you mixed definitions or inverted the direction.
Quick reference table
| mm (bearing bore) | Calculation | in |
|---|---|---|
| 1 | 1 × 0.0393701 | 0.0393701 |
| 2 | 2 × 0.0393701 | 0.0787402 |
| 5 | 5 × 0.0393701 | 0.19685 |
| 10 | 10 × 0.0393701 | 0.393701 |
Use the table for speed once you trust the factor. Do not use it as a substitute for understanding which definition of the unit you are on.
Why the exact number matters
A poorly matched bearing bore fit leads to premature bearing failure through excess play or induced stress — this is a precision fit, not a 'close enough' one.
Precision is not about showing off decimals. It is about making sure the shopping list, the machine setting, and the acceptance check are all describing the same physical quantity.
Pocket recap
Multiply mm by 0.0394 to get inches — a 20 mm bore is about 0.787 in, worth checking to the thousandth against the actual shaft diameter.
Keep the converter link nearby for the next time the same debate shows up in a group chat. Re-typing the factor from memory is how the wrong cousin of a unit sneaks back in.
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FAQ
What is the exact mm (bearing bore) to in factor here?+
1 mm = 0.0393701 in
Should I round partway through the calculation?+
No — carry full precision through the multiplication and round only the final answer, to the precision your tool, gauge, or spec actually supports.
What's the single biggest mistake people make with this conversion?+
Assuming a metric bearing bore 'close to' an inch shaft diameter will press-fit correctly without converting exactly — even a fraction of a millimeter mismatch causes a loose or overly tight fit that damages the bearing.
This article was written by The Turbo Unit Converter engineering desk (Applied units & field metrology) and last reviewed on 2026-07-23 against NIST SP 811 and the BIPM SI Brochure. Read our full editorial policy.