Improve your Fox 25BB engine

Project #10 - Improve your Fox .25BB

Fox .25BB Specifications

Bore: .680
Stroke: .680
Disp: .247
Weight: 8oz
RPM: 15,000 w 9-4

User Manual

The .25BB is, perhaps, the most conventional looking Fox engine. If it were not for the MKX style carburetor, most would not recognize it as being a Fox at all. It has an almost European look to it. The .25BB evolved directly from the .19BB and is, naturally, of similar design. An important difference however is the redesigned, lower compression head button.

While the current head button is acceptable in stock form, buttons in early versions resulted in engines that were slightly over-compressed. If you have an earlier .25BB and find the needle settings finicky, or the engine makes a "frying egg" sound at full throttle, we recommend adding a .01" shim between the head button and cylinder, or install the current series button.

Besides a better head button, later versions have improved porting and more power as well. We have found the current .25BB friendly to operate and powerful. Our only complaint with this engine is that it does tend to vibrate a bit, especially at higher rpm. A good, solid firewall and engine mount is recommended. Staying with the larger sizes in the recommended prop. range should also help.