The TDC, or top dead center, is the highest point in the engine's cycle. It's where the pistons are at their highest point in the cylinders. On a 12v Cummins engine, the TDC is located on cylinder #6. This is because the firing order for a 12v Cummins is 1-5-3-6-2-4. So, cylinder #6 is at the top of its stroke when cylinder #1 is on its compression stroke.