Finding TDC: how critical to be precise?

[rigspelt]

Here's how I think it works:

Compression stroke up, both valves remain closed and compression builds.
TDC: both valves closed, spark plug fires.
Power stroke down, both valves closed.
BDC: both valves closed.
Exhaust stroke up: the exhaust valve opens after BDC and then closes before TDC.
TDC: both valves closed.
Intake stroke down: intake valve opens after TDC and then closes before BDC.
BDC: both valves closed again.

To find TDC, remove the spark plugs after cleaning away debris around them.
Put a finger over the first spark plug hole after the flywheel (cylinder #1).
Crank the shaft counterclockwise until feeling a puff on the finger, signifying start of the compression stroke (Image 1 shows where the crank pins are when I do that).
Gently crank-check-crank counterclockwise until the crank pin points to the mark on the flywheel cover (Image 3).

The first piston should be at or near its TDC. But, is it near enough, judging by eye? How precise does one have to be in finding TDC for:

1. Replacing a distributor that was removed for cleaning, when the crankshaft was later turned for other reasons?
2. Setting the valve tappet clearance gaps?

[Kurt]

Sounds like you have the technique down. The pin is vertical when it is aligned with the timing mark you see on the flywheel cover. A related check I like to do to make this less of a guessing game is to perform this procedure with the distributor cap off. If you are at (or very close to) TDC the rotor (tip) should be pointing directly away from the block - on late model engines. On early model engines, the rotor will be pointing towards the aft end of the engine (parallel to the block). Once you get this right, you should be able to fire the engine up and then fine tune your timing by running the engine under load and turning the distributor back and forth a bit and locking it down when you find the point of highest rpm. I have timed my engine many times and I find that the timing is usually perfect before I even test the motor under load. This method will work fine for replacing your distributor - I've done this several times. Not sure about the valve clearances - I've never done that.

[rigspelt]

On closer inspection of the photo of the flywheel cover hole (below), I think I see that the cover mark actually is aligned pointing straight at the center of the crankshaft. The whole engine must be tiltled a bit off what my mind thinks is vertical, in which case Image 3 (above) is the right alignment to find TDC after all (blue arrow in Image 3), and the yellow arrow in Image 3 is a red herring (no pun intended). In any case, I am reassurred from the comments that there is no harm in being off a couple of degrees either side.

[rigspelt]

Here's a sketch that I think shows how the crankshaft pin, piston position, valve state and distributor rotor position relate to each other during the compression-power-exhaust phases of the A4's four-stroke cycle for cylinder #1. I'll need to fix the camshaft image: its profile cannot be oval, otherwise the valves could not stay closed during two consecutive strokes of the piston. Did I make any other errors?

[rigspelt]

Gently crank-check-crank counterclockwise until the crank pin points to the mark on the flywheel cover (Image 3). The first piston should be at or near its TDC. But, is it near enough, judging by eye? How precise does one have to be in finding TDC for:
1. Replacing a distributor that was removed for cleaning, when the crankshaft was later turned for other reasons?
2. Setting the valve tappet clearance gaps?

Answer, based on messages here and elsewhere: The eyeball method works fine.
1. Distributor: The timing will be close or right on with this method. Easy to fine tune it by using a timing light, or running up to cruise power and turning the distributor to find maximum RPM.
2. Valve-tappet clearance: close enough.

The simplicity of these A4's is brilliant.