Then you had a strong 051 or a weak 049. Cox designed both those engines with the same power so they could be used on the same airplane in two different events. I know as I flew those events back in the sixties on Goldberg's Blazer and 1/2A Viking free flight airplanes and on a 1/2A Starduster. Holland Hornet and Atwood also made 049's and 051's for the same reason. Atwood also made a .49 and .51 to fly in class B and C with the same plane.

I still have several of each on 1/2A C/L airplanes.

Cox also made the 051 red with a small groove in the piston so they could be easily told apart so they wouldn't be used in the wrong event. Holland Hornet made the drive washers a different color for the same reason.

Any free flighter can tell you this.

You can get a Piston/cylinder and glow head for $9.95 at this link. coxengines.ca/product.php?productid=121&cat=4&page=1 or just go to www.coxengines.ca