Dead Centre

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The engine in every E30 is based around pistons going up and down inside the engine block. Knowing where the pistons are is a crucial part of the engine management; if we don't know where the pistons are, we don't know when to add the fuel or fire the sparks.

As a reference, we therefore look to Piston #1. Because all the pistons are connected in a known order, if we know where Piston #1 is, then we know where all the others are at the same time. And because all the valves are set to function relative to the pistons, we'll know what the valvetrain is doing as well. So by putting Piston #1 into a known position, we know the full state of the engine at that moment. Having everything lined up the way the manufacturer intended is known as timing; if the components become unsynchronised, it means your timing is out.

Therefore, every engine has marks to show you where every component should be when Piston #1 is at TDC. Making them all line up is essential when changing the cambelt or chain on any engine, whether it's the M40, M20 or M10.


TDC, or Top Dead Centre, means that the piston is at the absolute top of the engine, at the furthest distance from the crankshaft. This can also be considered Position 0.

Because the piston is at its closest to the head, it is essential that both the intake and exhaust valves are closed, otherwise the piston will smash into them and ruin the engine.


BDC, or Bottom Dead Centre, is when the piston is at the very bottom of its stroke, at the closest distance to the crankshaft. This can also be considered Position 180.


BTDC stands for Below Top Dead Centre. For purposes of Ignition, the spark is always delivered before the piston gets to TDC, to give the flame front time to grow, detonate and push the piston back down. On older engines, especially the Jetronic engines, the distributor was mechanically adjusted to deliver the spark closer or further away from TDC, or in other words, a few degrees BTDC. This is known as advancing or retarding the spark.