There are a bunch of different ways to make a piston stop, or you can buy one pretty cheap too. I had a spark plug and some aluminum rod sitting around so I made my own.

1) Remove the bump on top of the base of the plug - this is holding the porcelain in.

2) Remove the tab on the bottom of the plug.

3) Break the ceramic stuff from around the electrode.

4) Knock the porcelain out the top of the plug by smacking the (now bare) electrode. Use a punch so you don't damage the base - it still needs to thread into the head.

5) Drill out the (now bare) plug base to whatever size your "stop" will be. I used 3/8" aluminum rod. You could also thread the base of the plug, then use a long bolt to make an adjustable stop. I made the fit a light press fit so I could adjust the length to where I wanted it before filling it with JB Weld.

6) Secure the stop in the base. I used JB Weld. The extension shown stops Sport pistons about 19 degrees from TDC.

I drilled a hole through the center of the stop so air could escape as the piston came up on compression. As long as the stop was in the lathe, I also put a couple grooves in it to help the JB Weld hold things in place.