This is on a williams 3 coins machine but should work on many others. Find the credit unit, then locate the zero switch. This is the switch that is always closed until the credits reach 0. I used a small hitch pin (I bought an assorted case at menards around 7 dollars) and wedge it into the zero switch terminals where the wires are soldered. (See picture) This allows you to have free play on your machine without any damage or needing to do any soldering or wiring. And if you ever want to go back to coin play all you need to do is pull the pin out. I hope this helps someone.

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Yes like dr_nybble said it will still make the credits go up if you achieve a special while playing (or at least that's what the special does in this game)