Improving the Fox 45BB RC

Project # 9 - Improving the Fox .45BB

Fox .45 Specifications

Stroke: .790
Disp: .449
RPM: 13,500 w 10-6
Weight: 12oz

User Manual - for engines with EZ Carb
User Manual - for engines with MKX Carb
User Manual - for engines with 2-jet Carb
Evolution History of the .Fox .45

The Fox .45 has always been relatively friendly to operate having escaped the over-compression problems that plagued so many Fox engines in later years. Over its long production life there have been several head button variations, some very unusual, but all seemed to function quite well. The current button is still useable in stock form, being only slightly over-compressed, in our opinion. Over the years, problems tended to be mostly carburetor related, especially when the old 2-jet carburetor was used. The introduction of the MK-X series improved things a lot, but the current EZ series has, again, resulted in some carburetor related problems. Check out our Carburetors section for more information on specific carburetors and recommended fixes.

Although the .45BB has been a pretty good engine over the years, recently we stumbled upon a way to improve the current version. A friend had stripped the glow plug threads in his .45 head button and needed a replacement right away to compete in a fun fly event that weekend. Unfortunately, we did not have a spare button for the .45 on hand just at that moment, and before removing one from one of our own engines to get this fellow going, we decided to try an experiment. We did have spare buttons for the .46ABC, and it only took a few minutes to turn the button down on a lathe so it would fit into the slightly smaller bore of the .45. When the engine was test run, we were surprised to find it ran about 500rpm faster than a typical current .45. Now curiosity had the better of us and we just had to verify this by taking the button from one of our own .45's and and swapping it with the modified .46 button. We swapped back and forth several times, and the results were consistent. The engine had, indeed, picked up at least 500rpm. Not only that, the handling qualities improved somewhat as well in that the needle setting became broader and less critical. So, it would seem the .45 prefers the somewhat more open combustion chamber shape provided by the .46 button.

Looking through several decades of Fox advertising for the .45 it is interesting to note the large variation in claimed performance over the years. When it was first introduced Fox claimed 14,000rpm with a 10-6. A year later 14,500 was claimed, and by the early 1980's, an amazing 15,000rpm. Then a gradual decline again, finally ending at the current claim of 13,500rpm with a 10-6. Although we never achieved 15,000 rpm on any of our 45's, we did reach into the 14's. Our experiments with the .46 button lead us to believe that head button changes over the years are at least partly responsible for the lower rating of the current .45.

If you are inclined to experiment, the .46 button is easy to modify, and a small hobby lathe can handle the job. Simply machine off enough material from the skirt of the button so it will fit into the .45 liner. Only one caution here, the tighter the fit the better the performance. You don't want a lot of clearance between the button and liner, just a light press fit, so take off a little at a time and test fit.

For more information also see the Evolution History of the .45.