# The pulley spins and the reading still holds still > Timing marks are cut into the part that spins, not the part that stays still, so the engine measures itself while running. A dab of white paint is what makes the scale legible at all. August 19, 2026 · https://justog.club/stories/ignition-timing-marks A pulley rim, drawn as an arc. Graduations run 20, 15, 10, 5, down to a zero and back out to 5 on the other side — BTDC to the left of it, ATDC to the right. A pointer sits fixed above the arc. One graduation is picked out in white. Beneath it all, large and in red: 10 BTDC. The scale is on the part that moves. The pointer, the one fixed thing in the drawing, is what it's read against. ## Why is the scale cut into the pulley instead of printed on something fixed? Because the thing being measured is a moment in a spinning cycle, and only the spinning part can show you where in that cycle you are. Ignition has to happen at a precise point before or after the piston reaches the top of its stroke — before, in most cases, so the mixture has time to start burning as the piston arrives. That point exists on the crankshaft's rotation, not anywhere else, so the scale that reads it has to rotate with the crankshaft. A fixed pointer above it does the only job a fixed point needs to do: mark where the moving scale is, right now, this instant. ## What does BTDC to ATDC either side of zero actually mean? Zero is the exact top of the stroke, and everything on the scale is a distance from it in either direction. BTDC — before top dead centre — is graduated to the left of zero; ATDC — after — to the right. Most petrol engines fire somewhere in the BTDC range, which is why 10 BTDC sits large at the foot: it's the setting this particular scale is built to confirm, not just one point among many on it. ## Why is only one graduation painted? Because a bare cast or machined scale is close to unreadable at the moment it's actually used — under a strobe light, with the engine running. A timing light fires in sync with the ignition and freezes the pulley's apparent position for the eye. Against a plain metal rim, every graduation looks the same in that flash; against one dabbed in white paint, the eye finds the mark instantly. The scale was always there. The paint is what turns it into something a person can actually read at speed, in poor light, and it's applied by hand by whoever last set the timing — not stamped at the factory with the rest of the rim. ## What makes a good version of this design? Keeping the arc honest to what a strobe actually shows: a scale, a pointer, and exactly one highlighted line, not a full clock face of equal marks. The temptation is to make every graduation the same weight, which reads as tidier but loses the point — the white mark is doing something the others aren't, and treating it the same as its neighbours erases the one detail the whole object exists to carry. It's the same discipline as [trusting the one figure that's actually meant to be checked, not all of them equally](/stories/spark-plug-gap-card). ## Who owns a timing scale like this? Nobody. Degrees before and after top dead centre are a description of a rotation, not a design anybody holds. What's owned is a specific manufacturer's pulley casting, its part number and any branding stamped into it — licensable if a real one is the point. Left generic, the scale is free to be exactly this figure, this arc, without borrowing anyone's casting to say it. ## Does this only apply to engines set up this way? Any mechanism where a moving reference has to be read against a fixed point works on the same principle — a fuel pump's static timing mark, a distributor's rotor position, a cam's degree wheel. It's a close cousin of [a valve clearance table](/stories/valve-clearance-table), which solves the opposite problem: that one measures a gap with the engine deliberately stopped and cold, because the moment it's checking can't be caught in motion at all. Timing marks exist because this moment can only be caught in motion. ## Does it print well? Yes — an arc, straight graduation lines and one filled mark are clean, flat artwork with nothing that needs a gradient. Black ground with white line work keeps the scale legible at a glance; red is worth reserving entirely for the 10 BTDC figure and the single painted graduation, so the two things that matter most stay the only colour in the design. Keep the graduation numerals fewer and larger than a real pulley would carry them — the weave loses fine numerals long before it loses a clean arc. ## How do I get one made? Describe your scale at [JustOG](/) — the range, which mark is painted, and the setting underneath it. Pick a direction, drag the crop frame, see it composited on the real garment, and it's made to order and shipped. Designs other people have published are in the [shop](/shop). One graduation in white paint, on a rim that never stops moving while anyone reads it.