Engine builders bounce between metric and imperial constantly — service manuals are mm, US tooling is in thousandths, and most spec sheets mix both. This converter handles either direction at machinist precision.
The conversion
1 inch = 25.4 mm exactly (defined, not approximate, since 1959).
- mm → inch: divide by 25.4
- inch → mm: multiply by 25.4
- thousandths to mm: 0.001" = 0.0254 mm. A 0.005" feeler gauge = 0.127 mm.
Where rounding bites you
A 4.000" bore is 101.6 mm exactly — but a 4-inch nominal piston is often 4.0008" (101.62 mm). A 0.001" rounding error matters when you're checking piston-to-wall clearance. Always keep at least 3 decimal places in inch values and 3 in mm.