Timing mark

{{Short description|Indicator within an internal-combustion engine}} {{refimprove|date=August 2025}} [[Image:Timing marks.svg|thumb|right|300px|Timing mark on pulley at 6° before [[top dead centre|TDC]].]]

A '''timing mark''' is an indicator used for setting the timing of the [[ignition system]] of an [[engine]], typically found on the [[crankshaft]] pulley (as pictured) or the [[flywheel]].{{cite book |last1=Beever |first1=Colin |title=Ignition and Timing: A Guide to Rebuilding, Repair and Replacement |date=30 June 2015 |publisher=Crowood |isbn=9781847979742 |url=https://books.google.com/books?id=U2JGCgAAQBAJ |access-date=1 May 2023}} 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]] using a [[timing light]], which flashes in time with the ignition system (and hence engine rotation).{{cite news |title=Car Care Guide: The Timing Light |url=https://books.google.com/books?id=-dgDAAAAMBAJ |access-date=1 May 2023 |agency=Popular Mechanics |date=October 1981| publisher=Hearst Magazines }} Shining the light on the timing marks makes them appear stationary due to the [[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]] and advancing with increasing engine speed. The timing can be adjusted by loosening and slightly rotating the [[distributor]] in its seat.

Modern engines often use a [[crank sensor]] directly connected to the [[engine management system]].

The term can also be used to describe the tick marks along the length of an [[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 [[barcode]]s, such as [[PostBar]], [[Data Matrix]], [[Aztec Code]], etc.

==References== {{Reflist}}

==External links== *{{Commonscatinline|Timing marks}}

{{Automotive engine |collapsed}}

[[Category:Ignition systems]] [[Category:Synchronization]] [[Category:Engine technology]]