I did some trading and picked up what is either a U.S. or Quadra engine. It is identical in outside dimensions and appearance to the U.S.-41 that I already own. Is there something in the serial # or anything else that will identify both the make and displacement. The guy I got it from thought it might be either a Quadra 35 or 42 but wasn't sure.

Quadra had a 35, 40, and a 42. Can you post a pic? Early 35's and 40's had points type igniton and a steel prop hub. The 42's I think had a magneto and an aluminum prop hub. Look into the exhuast port, how many piston rings. Early 35's had one ring, early 40's had two. I have an early 35 and 40. Run good but kinda heavy. Go to this site and look at their parts diagrams. They are good people to deal with. http://www.bennettbuilt.com