There is a dimple on the timing cam gear. This dimple needs to be directly pointing down when the piston is TDC. You can check this when it is assembled with the backplate off and the engine has completed its exahust stroke and begining the intake. There is a brief period when both valves are closed. Use your finger accross the lifters and when you feel that both valves are closed look at the piston location from the back of the engine and it should be TDC. Just make sure you are on the right cycle (not on the compression cycle).