Engine Builder Unit Converter
The conversions that come up on a build sheet.
| Millimeters | 25.4000 | |
| Centimeters | 2.5400 | |
| Thou (0.001") | 1,000.00 |
- 1 in = 25.4 mm exactly · 1 cu in = 16.387064 cc · 1 liter = 61.0237 cu in
- 1 hp = 0.7457 kW · 1 lb-ft = 1.35582 N·m · 1 bar = 14.5038 psi
When you need it
Engine work runs on mixed units and always has. Bores in inches, chamber volumes in cc, boost in psi or bar depending on who wrote the gauge, and torque specs in whatever the manual felt like that day.
The ones that bite
Chamber volume is always quoted in cc even on an engine measured everywhere else in inches, so compression work is inherently mixed-unit. Piston dish and dome volumes are cc as well. Keep those in cc and convert the linear dimensions, not the other way round.
Boost gauges are sold in both psi and bar, and one bar is 14.5 psi, not 15. Rounding that to 15 puts you three percent out on the pressure ratio, which matters when you are working out effective compression.
The conversions worth memorizing
One inch is 25.4 millimeters exactly — it is a definition, not a measurement. One cubic inch is 16.387064 cc. One liter is 61.02 cubic inches.
Machinists work in thousandths of an inch. A thou is 0.0254 mm, so a metric micrometer reading to 0.01 mm resolves to about four tenths of a thou — near enough for most engine work but not for bearing clearances, where you want a metric instrument reading to 0.001 mm or an imperial one reading in tenths.