Fox .35 "Stunt"

Name Fox .35 Designer Duke Fox
Bore 0.8" (20.32mm) Stroke 0.7" (17.78mm)
Type Glow Ignition Capacity 0.352 cuin (5.768cc)
Production run Thousands Country of Origin USA
Photo by Ron C Year of manufacture 1948 to current



This engine was was the second design put into production by Duke Fox. The initial versions, which were introduced in 1948, were die cast using a gravity die casting process for the crankcase, backplate, and cylinder head. Initially, the backplate was retained by two 2-56 screws. This was changed to a three bolt pattern with 4-40 screws in 1951, and has remained that way ever since. Likewise the cylinder head was initially retained by four 4-40 screws. This was increased to six with the 1952 model. In that year, the gravity die casting (aka "permold" or "permanent mold") was replaced by pressure die casting which results in more precise parts with a better finish and detail. This is probably why the permold engines have been called "sand cast", even though they never were [1].

Today, the engine is popularly referred to as the "Fox 35 Stunt" due to the great success it achieved in AMA Stunt competition in the hands of George Aldrich and a host of others. This success in the USA was not initially repeated in other countries, such as Australia, due the near unobtainability of nitromethane in the 1950's and '60s. The Fox 35 has a rather low compression ratio (about 8.5:1) and needs at least 5% nitro in the fuel to reach anywhere near its potential, or even start, for that matter. Downunder, with no nitro and no knowledge of the need for it, the Fox had a poor reputation until the problem was understood and the additive gradually became available to the common modeling public in the later 1960's.

As detailed by Bill Mhorbacher, the engine has continued with only small alterations, year after year. In 1988, Fox issued a special "40th Anniversary" edition with a "blue ribbon" emblem cast in high relief on the bypass. This tradition continued after the designer's death in 1991, with 50th, and now 60th Anniversary specials. The 50th Anniversary models were produced with a black finish and gold plated plug, screws, NVA, and prop driver components. Only 500 of these were made, each with a Certificate of Authority signed by Betty Fox, with Sharon Shawkey as Notary [2].

The first engines used a Universal Needle Valve Assembly (NVA) sourced from Austin Craft. This changed in 1953 to the unique Fox flat-taper design bemoaned and replaced by many users. The design does give fine metering, but only over part of a full rotation. If the desired mixture is not achieved, the needle must be rotated 360° to bring the flat around to the jet once again. A rather loose fit of the externally threaded needle in the spraybar makes it prone to air leakage, which does not help matters. This rather psychotic design remained until 1998 when a more conventional needle was used, even if it had a very light taper and a blunt end [3]. The air leak problems remained until the 2011 model when Fox changed to a new design incorporating a "O" ring to both replace the spring ratchet, and prevent air leaks.

As countless control line stunt fliers have discovered, a 35 will hold a steady "four-two" break setting throughout a pattern, provided the tank setup and fuel are up to the task. Even today, the venerable Fox 35 is favored in classic stunt events and sport flying. Starting is generally very easy—I've even seen one start without being attached to a starting battery! Various user modifications have been applied to the engine over the years, such as "hemi" heads, backplate-stuffing, and additional crankshaft balancing—the latter having the opposite effect to backplate-stuffing! The improvements range between marginal and negative. Although it does require a lengthy break-in period, and ignoring the tricky needle management, a correctly setup engine will perform beautifully out of the box. Altogether a fine tribute to Duke Fox, testified to by its amazing longevity.


[1] Mohrbacher, Bill: Fox 35 (part 1), The Engine Collectors' Journal, The Model Museum, Buena Vista, USA Volume 36, Issue 206.
[2] ibid (part 5), Volume 37, Issue 211, p11.
[3] Mohrbacher, Bill: Foxology by Mohrbacher: Fox Needle Valves, The Engine Collectors' Journal, The Model Museum, Buena Vista, USA Volume 33, Issue 192.




  Model Engine News Home

Please submit all questions and comments to