Why is TDC labeled as Z1?

I’m sure this is an easy one...

Doing my very first leak down test the other day, while listening to air escaping out my exhaust, and scratching my head while being 360 from the TDC I needed, I started pondering what the Z in Z1 meant?

Quote:

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Originally Posted by Dave Awesome

...what the Z in Z1 meant?

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Zylinder

Jon B.
Vista, CA

Ahhh! Danke.

I never knew that either - thanks Dave and Jon.