First of all, keep in mind that in a a 4-stroke engine (which virtually all cars are) each piston will hit TDC twice in the course of a full combustion cycle, once between the exhaust and intake strokes, and once between compression and combustion. When a procedure calls for putting the engine at TDC they're usually referring to piston #1 being at TDC before the intake stroke.
Rotating the engine to TDC really involves two things, manually rotating the crankshaft, and determining when you've reached TDC.
Rotating the crank is most easily done with a ratchet attached to the crank pulley. Things to consider before doing this:
- Make sure the transmission is in neutral, or if you're doing major work, completely disconnected from the engine.
- If the engine is fully assembled (head(s) on and spark plugs in) it can be difficult to turn by hand due to the engine compression. Even with the heads off the friction between the pistons and the cylinder walls will prevent the crank from spinning easily. The amount of force required to turn the crank will depend on how many cylinders you have as well as the compression ratio etc. You shouldn't need a 4 ft. breaker bar, but don't be surprised if you have to put your body into it a little.
- It's generally a good idea to turn the motor in the same direction as when it runs. If you need to back up a little bit, that's ok, but generally avoid freely rotating an assembled engine backwards.
Determining when the engine is at TDC can be done a couple of ways:
- Assuming it was assembled properly there is usually a mark on the crank pulley that will match up to another mark on one of the front covers when the engine is at TDC.
- You can remove the #1 spark plug and put a straw down the hole such that it's resting on the top of the piston. As you turn the crank the straw will move up and down. When it's at its apex just before it begins to fall the engine is at TDC.
Finally, as I mentioned at the beginning you will often need to know if the engine is at the beginning of the intake or combustion stroke in addition to knowing if it's at TDC. Keep in mind, if the head is off or the timing belt/chain is removed there is no difference between TDC before intake and combustion, this is about the valve train's position as opposed to the piston's position.
- If the valve covers are off you can watch the motion of the valves for the #1 cylinder. At the start of the intake stroke the intake valve(s) will be opening and the exhaust side will be closing. At the start of combustion all valves will be closed and not moving.
- You can also look at the cam pulley(s). They will have some sort of markings to indicate when the valves are in TDC alignment. This varies greatly by engine so I can't give any more specifics.