# Timing mark

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Indicator within an internal-combustion engine

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Timing mark on pulley at 6° before [TDC](/source/Top_dead_centre).

A **timing mark** is an indicator used for setting the timing of the [ignition system](/source/Ignition_system) of an [engine](/source/Engine), typically found on the [crankshaft](/source/Crankshaft) pulley (as pictured) or the [flywheel](/source/Flywheel).[1] These have the largest radius rotating at crankshaft speed and therefore are the place where marks at one degree intervals will be furthest apart.

On older engines, it is common to set the [ignition timing](/source/Ignition_timing) using a [timing light](/source/Timing_light), which flashes in time with the ignition system (and hence engine rotation).[2] Shining the light on the timing marks makes them appear stationary due to the [stroboscopic effect](/source/Stroboscopic_effect). The ignition timing can then be adjusted to fire at the correct point in the engine's rotation, typically a few degrees before [top dead centre](/source/Top_dead_centre) and advancing with increasing engine speed. The timing can be adjusted by loosening and slightly rotating the [distributor](/source/Distributor) in its seat.

Modern engines often use a [crank sensor](/source/Crank_sensor) directly connected to the [engine management system](/source/Engine_management_system).

The term can also be used to describe the tick marks along the length of an [optical mark recognition](/source/Optical_mark_recognition) sheet in order to confirm the location of the sheet as it passes through the reader. See, for example, U.S. Patent 3,218,439 (filed 1964, granted 1965), which refers to a timing track down the left side of the form, and U.S. Patent 3,267,258 (filed 1963, granted 1966), which refers to a column of timing marks on the right side of the form.

The term can also be used to describe the timing patterns used in some [barcodes](/source/Barcode), such as [PostBar](/source/PostBar), [Data Matrix](/source/Data_Matrix), [Aztec Code](/source/Aztec_Code), etc.

## References

1. **[^](#cite_ref-beever_1-0)** Beever, Colin (30 June 2015). [*Ignition and Timing: A Guide to Rebuilding, Repair and Replacement*](https://books.google.com/books?id=U2JGCgAAQBAJ). Crowood. [ISBN](/source/ISBN_(identifier)) [9781847979742](https://en.wikipedia.org/wiki/Special:BookSources/9781847979742). Retrieved 1 May 2023.

1. **[^](#cite_ref-popmech_2-0)** ["Car Care Guide: The Timing Light"](https://books.google.com/books?id=-dgDAAAAMBAJ). Hearst Magazines. Popular Mechanics. October 1981. Retrieved 1 May 2023.

## External links

- Media related to [Timing marks](https://commons.wikimedia.org/wiki/Category:Timing_marks) at Wikimedia Commons

v t e Internal combustion engine Part of the Automobile series Engine block and rotating assembly Balance shaft Block heater Bore Connecting rod Crankcase Crankcase ventilation system (PCV valve) Crankpin Crankshaft Core plug (freeze plug) Cylinder (bank, layout) Displacement Flywheel Firing order Stroke Main bearing Piston Piston ring Starter ring gear Valvetrain and Cylinder head Flathead layout Overhead camshaft layout Overhead valve (pushrod) layout Tappet / lifter Camshaft Chest Combustion chamber Compression ratio Head gasket Rocker arm Timing belt Valve Forced induction Blowoff valve Boost controller Intercooler Supercharger Turbocharger Fuel system Diesel engine Petrol engine Carburetor Fuel filter Fuel injection Fuel pump Fuel tank Ignition Magneto Compression ignition Coil-on-plug Distributor Glow plug Ignition coil Spark plug Spark plug wires Capacitor discharge ignition Engine management Engine control unit (ECU) Electrical system Alternator Battery Dynamo Starter motor Intake system Airbox Air filter Idle air control actuator Inlet manifold MAP sensor MAF sensor Throttle Throttle position sensor Exhaust system Catalytic converter Diesel particulate filter Gasoline particulate filter EGT sensor Exhaust manifold Muffler Oxygen sensor Cooling system Air cooling Water cooling Electric fan Radiator Thermostat Viscous fan (fan clutch) Lubrication Oil Oil filter Oil pump Sump (wet, dry) Other Knocking / pinging Power band Redline Stratified charge Top dead centre Portal Category

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Adapted from the Wikipedia article [Timing mark](https://en.wikipedia.org/wiki/Timing_mark) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Timing_mark?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.