(TDC) for number one piston
1 Top Dead Centre (TDC) is the highest point in the cylinder that each piston reaches as it travels up the cylinder bore. Each piston reaches TDC on the compression stroke and again on the exhaust stroke, but TDC generally refers to piston position on the compression stroke.
2 Positioning the piston(s) at TDC is an essential part of many procedures such as camshaft and timing belt/chain/pulley removal, and distributor (where applicable) removal.
Engine in-car repair procedures 2A-5