Using the "positive stop" method.  You screw a tool into the spark plug hole to stop the piston from reaching top dead center.  This is the way we do it on race car engines:  This is a quote from a post I made some time ago:

(QUOTE)   Eyeball the piston at TDC. You can use a rod or something and feel for it, or use a dial indicator. Then set your degree wheel so that the pointer is pointing at Top Dead Center.

Then install your piston stop. Rotate the crankshaft so the piston touches the stop and note the degree reading.

Rotate the crank in the other direction until it touches the stop and note the reading.

If the readings are exactly the same, you lucked out and set the degree wheel properly by chance.

More likely, the readings will be different by a few degrees. Let's say one reading was 44 degrees before top dead center, and the other was 50 degrees after top dead center. Move your pointer or degree wheel to half that distance. (In other words split the difference and move the wheel or pointer 3 degrees.)

Now rotate the crank in both directions and note the readings again. They should be equal, or very close. If they are off a bit, adjust the wheel or pointer again. You want the reading to be the same each time the piston touches the stop.

When you have achieved this, you can remove your piston stop, and now when the pointer/degree wheel indicates zero, or TDC, the engine will be at it's true Top Dead Center.

You can then set your ignition timing, check your port timing, etc.

The site <font color="#00265e">www.ch-ignitions.com</font> has a printable degree wheel you might find helpful.

To prove to yourself that a dial indicator doesn't show true top dead center, try this. With your degree wheel and pointer in place, install the dial indicator to indicate off the top of the piston. Now very slowly turn the engine over until the dial indicator stops rising. Find the exact point where the indicator stops rising, and then stop turning the engine over and note the degree wheel reading. Now once again, slowly turn the engine over in the same direction until the dial indicator starts back down. You will notice that you can turn the crank several degrees before the dial indicator starts back down. This is called piston dwell, or the time the piston spends literally stopped at TDC while the crank "flops over" and starts pulling the piston back down. This amount varies from engine to engine, and is related to the rod/stroke ratio and other factors, but it is why a dial indicator can't ever show you true TDC unless it is used to measure a certain distance down on either side of TDC, just like the simple and preferred "Positive Stop" method.  (END QUOTE)

The other thing to know, as mentioned, is that you need to use the degree wheel with the ignition system all hooked up.  When you actually see a spark at the plug at 28 degrees on the degree wheel, you're set.  You can't just position the magnet/sensor setup, you have to actually see the spark because the spark doesn't actually occur until the magnet is <u>leaving</u> the sensor.

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