I have had several over the years. They started out as Quadra Engines, made in Canada, then bought by U.S. Engines in New York I believe and the name was changed from Quadra to US engines. Although Quadra made several sizes of engines, all magneto powered, U.S. Engines only got involved with the 25cc (basically a Homelite 25cc string trimmer engine converted to RC use) the 35cc and 40/42cc which U.S. Engines marketed as their 41cc.

The 41 is a nice engine, easy starting and extremely durable. Unless you deliberately try to damage it, the engine will outlast you. The 25 is a side exhaust and the 35-40/41/42 are rear exhaust. To tell the 35 from the 40/42, when looking at the rear of the engine, there are four bolts that usually attach the cup/motor mount. On the 35cc engine, the top two bolts are spaced closer together than the bottom two. On the 40/41/42 engines, the four bolts are all equally spaced.

I have attached a copy of the Quadra Engine Operation Manual covering from the 35cc, 42cc. 52cc and 100cc Quadra engines.