Need to line up marks on timing belt replacement?

My question is this, when doing a timing belt replacement what is the point of lining the marks up on the pulleys and getting top dead center? Why can’t you just replace the belt and be done?

My thinking is that if the engine shut off in that position and you just replace the belt, the pulleys are still in the same position.

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[Richard KirshyParticipant]

Your thinking is correct except you have to assume that:
1- the engine is in time and has not perhaps slip a tooth prior to replacing the timing belt.
2- what happens if you remove the old and a cam sprocket moves, what point of reference do you have to put it back into position?
Lining the marks up at TDC will give you a absolute point of reference.

• This reply was modified 5 days, 8 hours ago by  Richard Kirshy.

[Tuan vanParticipant]

I still don’t quite understand why it needs to be lined up. I think I may not be remembering how other parts work in relation to each other. Is there reading material or pictures that can help me understand better? As words are harder to help understand a concept.

[Richard KirshyParticipant]

There are two timings that are required.
One being ignition timing which involves when the spark is introduced into the cylinder during the combustion stroke.
The other is cam / crank timing which is the one controlled by the timing belt.
Here is an explanation:
https://www.liveabout.com/engine-ignition-and-cam-timing-281468

Another with pictures:
https://www.carthrottle.com/post/the-ins-and-outs-of-engine-timing-and-what-happens-when-it-goes-wrong/

[Author]

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