Setting up a belt-driven oil pump? This free calculator works out the belt length, the oil pump pulley tooth count and the belt tension figures for 3/8″ pitch L-section drives - whether you run an AT Power Cordis pump or another manufacturer’s pump. Get a recommendation in seconds and add the parts straight to your basket.
Pulley & Belt Calculator
Works for AT Power oil pumps and other manufacturers’ pumps alike. Enter your engine details and we’ll recommend the pulley and belt.
How the pulley calculation works
Oil pumps have an ideal operating speed - AT Power Cordis pumps should spin at roughly 4,500 rpm at maximum engine speed (other makes publish their own figure). The maths is simple:
Maximum engine RPM ÷ target pump speed = drive ratio
Drive ratio × crank pulley tooth count = required oil pump pulley tooth count
Examples: a 9,000 rpm engine with a 16-tooth crank pulley needs a 32-tooth pump pulley (9,000 ÷ 4,500 = 2.0 × 16). An 8,500 rpm engine with the same crank pulley works out at 30 teeth. The calculator rounds to the nearest pulley we manufacture - 22 to 32 teeth, in 1/2″ and 5/8″ bores - and warns you if your combination falls outside the range or lands the pump too far from its target speed.
How belt length is calculated
For a two-pulley drive, belt pitch length follows from the centre distance and the pulleys’ pitch diameters:
L = 2C + π(D₁ + D₂)/2 + (D₂ − D₁)² / 4C
where C is the shaft centre distance and D₁, D₂ are the pitch diameters (tooth count × 9.525 mm pitch ÷ π). The calculator does this for you, matches the result to a stock belt, and tells you the exact centre distance that belt needs. No measurements to hand? Wrap a piece of string around both pulleys, lay it flat, measure it - that measurement is your belt length.
L-section belt sizes (3/8″ pitch)
| Belt | Pitch length | Teeth | Widths available |
|---|---|---|---|
| 210L | 533 mm | 56 | 3/4″ |
| 225L | 571 mm | 60 | 3/4″ & 1″ |
| 232L | 589 mm | 62 | 3/4″ & 1″ |
| 240L | 609 mm | 64 | 3/4″ & 1″ |
| 255L | 647 mm | 68 | 3/4″ |
| 263L | 668 mm | 70 | 3/4″ |
| 270L | 685.5 mm | 72 | 3/4″ |
All sizes are stocked as L-section timing belts. The calculator only recommends a belt that genuinely fits: if the nearest size needs centres more than 6 mm from yours, it says so rather than selling you the wrong belt.
Belt tension, the numerical way
The calculator also gives you the tensioning figures from our installation guide: deflect the belt at mid-span by 1/64 of the span length under a known force - 3.06 N (11 oz) for 3/4″ belts, 4.45 N (1 lb) for 1″ belts - and adjust the centre distance until the deflection is right. Insufficient tension lets the belt ratchet; excessive tension wears bearings and shafts. Full procedure in our Cordis oil pump & timing belt installation guide.
Frequently asked questions
How do I calculate the belt length for two pulleys?
Use L = 2C + π(D₁+D₂)/2 + (D₂−D₁)²/4C, or skip the maths: wrap string around both pulleys, measure it, and match it to the nearest belt pitch length. Our calculator does both from either measurement.
What size oil pump pulley do I need?
Divide your maximum engine RPM by the pump’s target speed (AT Power pumps: 4,500 rpm), then multiply by your crank pulley’s tooth count. Round to the nearest available pulley - ours run 22 to 32 teeth.
How tight should an oil pump drive belt be?
Deflect the belt 1/64 of the span length at mid-span under 3.06 N for 3/4″ L-section belts (4.45 N for 1″). The ideal tension is the lowest that transmits the power without the belt skipping teeth.
What pitch are L-section timing belts?
3/8″ (9.525 mm) tooth pitch, trapezoidal profile. Belt codes give the pitch length in tenths of an inch - a 225L is 22.5″ (571 mm) long, i.e. 60 teeth.
Questions about an unusual drive layout? Talk to our team - we design and manufacture the complete dry sump system in-house in the UK.