Improving your Fox 50BB RC engine

Project #3 - Improve your Fox .50BB

Fox .50BB Specifications:

Bore: .906
Stroke: .790
Disp: .501
Weight: 12oz. (without muffler)
RPM: 14,500 with 10-6

User Manual

Introduced in the mid-80's, the Fox .50BB came about by increasing the bore of the .45 to equal that of the Eagle .60. The .50 even uses the same ring as the .60 although the piston is different. This was generally a good engine and one of our favorites, the only real problem being related to the design of the head button.

It was designed during a period of time when Duke was obsessed with making his engines run well on FAI (no-nitro) fuel. The design of the head button, with its extra-wide squish band and small combustion chamber, reflects this and the compression was often too high for friendly operation on typical nitro sport fuels. Earlier engines in particular tended to run hot, could be finicky to adjust and would sometimes flameout during throttle transitions. Some engines would detonate severely making a noticeable"rattling" noise that was often mistaken for airframe vibration. There were three head button versions, that we know of, and the severity of problems will depend upon the version you have, nitro content, prop. size and the elevation of your flying site.

The head button can be modified to cure these problems and allow the engine to run well on the typical 10 -15% nitro sport fuels most of us (in North America) like to use. To accomplish this you must have access to a lathe, and possess some basic machining skills.

If you would like to try a custom button but don't have the means to do the modifications, we have a recommended source for custom machine work.

The degree of the modifications depends upon which version of head button you have. There are three that we know of. While they all look the same at a quick visual glance, the depth which they extend into the cylinder liner is slightly different. (This is the dimension described as "variable" in the diagram below). On early engines, this dimension was 0.160". These are the engines that would often make the "rattling" noise when run on anything much above 5% nitro. The diagram below shows the "full" modification, which is required if you have such a head button. This modification includes opening the combustion chamber up until the squish band has been reduced to 0.185". You will want to maintain about the same radius on the combustion chamber, and this will require that you make your own cutting tool for the job. The other modifications shown in the diagram should be self-explanitory. Just take .030" off the top of the button, then add a 5 degree angle to the squish band. (The angle is not super critical, so if you are out slightly it will still work ok). Just a note on the radius indicated: We have found that adding a slight radius to the inner edge of the squish band makes the engine a little more friendly and broadens needle settings. This is easily accomplished by wrapping some 400 wet/dry sandpaper around a small dowel and working the inner edge with this while spinning the button up in the lathe. Again, this is not super critical, just round the edge a bit. Do not round the outer edge that contacts the cylinder wall, however, as this will negatively affect engine performance.

Head Button Modifications

Over the years, Fox realized that compression was a problem and began lowering it by removing material from the bottom of the button (ie, increasing the deck clearance but leaving the width of the squish band the same). This was done twice, that we know of. The first buttons extended 0.160" into the liner, then this was reduced to 0.150", and finally on the latest buttons became 0.140". While each of these modifications helped, the engine could still exhibit problems depending upon the nitro content, prop size, elevation of the flying site, etc. We have found that the .0.160: and 0.150" buttons both work best when modified fully as shown in the diagram. The 0.140 button, however, may not require opening up the combustion chamber. However, the other modifications (lowering the plug and adding squish band angle) should still be applied.

If you've been wondering what the deal is with lowering the plug, we've found this to be a contributor to the tendency to flameout during throttle transitions. The plug is a bit higher up in the combustion chamber than other engines of similar design and displacement. We assume Duke did this as part of his original plan for operation on no-nitro fuel. Oh yes, be sure to continue to use a long-reach plug. We "want" the plug to be lower and there will still be adequate threads left after lowering it.

Carburetor Note:
Over the years the .50 has been supplied with three carburetors. Production began with the same MKX-B size carburetor used on the .45, later changed to the larger MKX-C size, which was the same one then used on the Eagle .60, and finally, an EZ B series carburetor. We feel the engine is actually a bit over-carbureted with the MKX-C carburetor when using the most practical size propellers for most applications, a 10-7 or 11-6. This can result in an overly rich mid-range and slow, slobbery, throttle response. We feel the slightly smaller MKX-B size carb. is a better choice for the .50 in most applications. The midrange is much better, and there is no rpm loss except when trying to spin smaller props that are really not practical for the .50 anyway.