What is the gap between coil and flywheel on a Briggs and straton 17.5 hp motor?

You can check the Briggs and Stratton website for the exact specs for you engine number.

OR

Most people use a standard business card between the flywheel and coil. That works fine. Just line up the coil flanges with the magnet part of the flywheel and the put the business card between them. The magnet will hold it in place. Then tighten the bolts holding the coil and you are ready to go.