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I always get confused with TDC - especially in the sense that there are two TDC's, one on compression stroke (that's the one we want to set ignition timing) and one on 'the other' stroke. It's very basic, and I always figure it out in the end, but I always have to be busy with it for a while, every time I need to 'time' an engine or ignition. So if it seems like you're on TDC, you may not be on the compression stroke and you have to rotate the engine again to what looks like the same point, but it's not, it's 'the other TDC'.