Thanks Woodduck. Your engine was made on 17 October 1986, in Briggs Plant 11. This plant has been closed for some time, and at this point I can't even tell you where it was located.

Here is the Illustrated Parts List:
http://bsintek.basco.com/BriggsDocumentDisplay/Z6nlyHVJ1DajI.pdf

Here is the Operator's Manual:
http://bsintek.basco.com/BriggsDocumentDisplay/msoBGL-K_ny7tH217FzoPu.pdf

To address your specific question, most pistons have a notch in the piston head. The notch must face toward the magneto.