Device that whistles when a piston reaches Top Dead Center.
How to use a TDC Whistle:
Whistle that screws into a spark plug hole to tell when the piston is at top dead center (TDC). Especially useful because it works for all the cylinders not just #1.
Top Dead Center Whistle as seen in Basic Engine Building:
Here we have our TDC Whistle screwed into the spark plug hole of our cylinder #1. As we turn the crank, the air rushing out of the hole will cause the tip to whistle as the piston reaches TDC inside the cylinder.