Workshop manuals contain extensive sections on engine specifications expressed in metric units (millimetres, Newton-metres, litres, etc.). Older manuals and many American manuals use imperial units (inches, foot-pounds, gallons). Knowing how to convert between the two and understanding common unit confusions saves time and prevents costly mistakes.
Length and Distance
1 inch equals 25.4 millimetres exactly. 1 foot equals 304.8 mm. 1 thousandth of an inch (often written as “thou” or “mil”) equals 0.0254 mm or 25.4 microns. Engine clearances are usually specified to thousandths of an inch in imperial manuals or hundredths of a millimetre in metric manuals. A piston-to-bore clearance of 0.002 inch is the same as 0.05 mm. A spark plug gap of 0.040 inch is the same as 1.02 mm.
Torque
1 foot-pound (ft-lb) equals approximately 1.356 Newton-metres (Nm). To convert from ft-lb to Nm, multiply by 1.356. To convert from Nm to ft-lb, multiply by 0.738. A wheel nut spec of 80 ft-lb equals approximately 108 Nm. A spark plug spec of 18 ft-lb equals approximately 24 Nm. Many torque wrenches read in both units, eliminating the need for conversion. Inch-pounds (in-lb) are used for smaller fasteners – 12 in-lb equals 1 ft-lb.
Pressure
1 psi (pound per square inch) equals approximately 6.89 kPa (kilopascals) or 0.069 bar. Most workshop manuals list pressures in kPa or bar. A tyre pressure of 32 psi equals 220 kPa or 2.2 bar. A fuel pressure of 50 psi equals approximately 345 kPa. Engine compression of 150 psi equals approximately 1034 kPa or 10.3 bar.
Volume and Capacity
1 US gallon equals 3.785 litres. 1 imperial (UK) gallon equals 4.546 litres – this is significantly different and a common source of error when reading older British manuals or comparing fuel economy figures from US versus UK sources. 1 US quart equals approximately 0.946 litre. 1 fluid ounce equals approximately 29.6 ml. Engine oil capacity in older American manuals is typically given in US quarts.
Engine Displacement
Engine displacement in metric is litres or cubic centimetres (cc). 1 litre equals 1000 cc. In imperial it is cubic inches (CID or CI). 1 cubic inch equals approximately 16.39 cc. A 350 cubic inch V8 equals approximately 5.7 litres. A 2.0 litre engine equals approximately 122 cubic inches.
Temperature
To convert Fahrenheit to Celsius, subtract 32 and multiply by 5/9. To convert Celsius to Fahrenheit, multiply by 9/5 and add 32. Engine operating temperature of around 90 degrees Celsius equals 194 degrees Fahrenheit. A thermostat opening temperature of 82 degrees Celsius equals approximately 180 degrees Fahrenheit.
All specifications in MechanicMate workshop manuals use the original units from the factory service publication (typically metric for vehicles sold in Australia). Browse our workshop manual catalogue at mechanicmate.net/shop.
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