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A short history of the Fox Combat Specials and other closely related engines
(as I see it)
by Bill Ives
It all began in 1957 when Duke Fox introduced the new Fox .35 Combat Special. It had the new crankcase with four screw rear cover, round intake with removable insert, new spade type NVA, plain aluminum head, bushing main bearings, with hole rear of crankcase for wristpin removal. "Fox .35" on the bypass.
In 1958 Duke Fox figured a way to get more power from the combat special. He improved the intake port timing, he improved the piston and baffle, and he painted the cylinder head black so that you could tell the difference from the first combat special since they both look the same except for the above mentioned changes. Also, in 1958 Fox introduced the new .29X Racing Engine. It looks just like the black head .35 except it has a new style cylinder head with machined fins and is aluminum color. This engine has only "Fox" and a circle on the bypass and has a bore of .738.
In 1959 Duke Fox introduced a "low price series" of engines and called these engines "Rockets". The first model Fox .35 "Rocket" had a red head and the name "Rocket" and a rocket design on the bypass. The intake venturi was made smaller than the previous combat engines and it did not have a removable insert. Aside from these minor changes, it looks like the first and second Combat Specials.
In 1960 the new Fox .35 Combat Special Series III was introduced. It had a new two-piece crankcase with detachable front housing held on by four screws, it has twin needle bearings on the crankshaft, a new square intake without insert and a pressure fitting in the four screw rear cover. It also said "Rocket" on the bypass and had the wristpin hole in the rear crankcase. It also had a large "picture window" intake port in the cylinder liner. In 1960 Fox started using the same Series III crankcase casting for the "Rocket" .35 so now the crankcase on the second type Red Head "Rocket" .35 has four lugs on the front of the crankcase, otherwise, the second type Red Head "Rocket" .35 looks the same as the first type "Rocket" .35.
The Series III Combat Special was also made with a rocket on the bypass end no wristpin hole in the crankcase rear.
In 1961 Fox introduced the new Fox .40 Rat Race engine, 1st type. It used the same crankcase as the 2nd type .35 Red Head "Rocket" with a rocket on the bypass and four lugs on the crankcase front. It had a larger intake with insert, boss for pressure fitting on the rear cover. It had the same .800" bore as the .35 engines, but it had a longer stroke. The .35 engines have a .700" stroke and the .40 has a .790" stroke which means the cylinder liner on the .40 is .090" longer than the .35 liner. Thus the .40 engine has a .090" gap between the head and the cylinder top fin and it has the wristpin hole in the rear of the crankcase. The Fox .40 R/C engine was also introduced in 1961.
1962 was a banner year for Duke Fox. He introduced several new engines and made several changes in the old engines.
First the changes - by making the crankpin shorter and the con rod big and slightly narrower it was now possible to remove the rear cover and slip the con rod off the crankpin and remove the piston and con rod as a unit. No longer was it necessary to try and fish the wrist pin out thru the hole in the rear of the crankcase, which was sometimes impossible if the wrist pin was badly gummed up. Now the smaller round hole in the crankcase was no longer needed.
The crankcase was redesigned, the name "Rocket" was no longer used on the bypass, and the beam mounting lugs were made wider by 3/16". Lateral mounting space was increased from 1 1/2" to 1 5/8". The above changes were made to the Series III Combat Special, the 3rd type "Rocket" .35 and both .40 Rat Race and .40 R/C engines.
The new improved 1962 "Rocket" .35 3rd type no longer has a "red head", it is now plain aluminum. It now has a larger intake venturi with removable insert, no "Rocket" on the bypass, no hole in the rear, and wider beam mounts.
Now for the "ALL New" 1962 engines. The New Fox .40BB Rat Race engine has a new golden color crankcase and head. The new crankcase is of one-piece design with a square intake like the Series III Combat engine. It has one ball bearing on the rear and one needle bearing on the front of the crankshaft, and no reinforcing webs on the crankcase front. It has no insert in the intake and has a pressure fitting in the rear cover. It has a decal on the bypass that says Fox 40 BB Rat Race. The stroke on the .40 is still .790", and a four screw rear cover.
The .29X Racing engine now looks just like the Series III Combat Special but it only has a .738" bore and Fox only on the bypass. The ALL New .35 "Blue Ribbon" engine was introduced in the fall of 1962.
The new .35 "Blue Ribbon" engine used the same crankcase as the .40BB Rat Race engine but it was not gold color, it was plain aluminum. It used two needle bearings on the crankshaft main, a square intake with insert, no pressure fitting, no webs on the front, "Fox .35" on the bypass, and no hole in the crankcase rear.
In 1963 the .35 "Blue Ribbon" engine was improved and had a name change. It is now called the Fox .35X and looks the same as the old .35 "Blue Ribbon" engine except it now has three webs added to the crankcase nose for strength. It has only one needle bearing main and one bushing on the front. The .35X proved to be such a successful combat engine that the Fox .35 Series III Combat Special was discontinued. Both the Fox .40BB Golden Rat Race engine and the .29X Race Engine stayed the same as they had been since 1962.
In 1964, the Fox .40 Rat Race engine was improved. It now has a new stronger crankshaft and a new stronger solid wrist pin with brass end pads. Combat flyers using the .35X had been blowing up the engine by trying to get too much extra power out of it. They asked Duke Fox to strengthen the crankshaft and conrod. While Duke was redesigning a new stronger crankshaft he decided that, since the combat rules allowed competitors to use up to a .36 size engine, he would increase the size of the .35X to ..36 cubic inch engines for maximum displacement. He increased the stroke to .715" and called the new engine a Fox .36X. It looked just like the .35X except it now has a bulge at the top of the bypass and has Fox .36X on the bypass. The 1964 Fox .36X had the same head and center plug as the .35X.
There were more new engines and changes in 1965. The Fox .36X has a new head with an angled plug. A new custom fitted for combat engine is produced, the Fox .36X BB.
The .36X BB looked just like the plain .36X except the outside crankcase was polished, it has a single ball bearing main, no insert in the venturi, a pressure fitting in the back cover, and a wider milled exhaust port. It still has "Fox .36X" on the bypass.
A new racing engine was introduced - the new Fox .29X BB Rear Rotor Race Engine. The engine has a twin ball bearing main and an adjustable rear rotor intake, a new head with a groove around it and a center glow plug. It only has "Fox" on the bypass and webs on the crankcase front.
Fox .40 BB Rat Race remains the same.
The Fox .40BB for 1966 has a new big 19/32" crankshaft, otherwise, it is the same.
A new .29X racing engine was introduced in 1968. It had a new front rotary valve intake, single needle bearing main, a crankcase like the .36X, a head like the old .29X BB, an insert in the venturi, and "Fox" only on the bypass.
For 1971 the Fox .36X now has a web in the exhaust stack. An all new Fox .40 Stunt Engine was introduced. It was a big, massive, round intake engine with a four screw rear cover, new NVA, "Fox" on the bypass, and a center plug.
Fox had been experimenting with a new design combat engine. In 1974 Duke Fox released the all new schneurle ported combat engine, the so called Mark 1 Combat Special. It looked something like the old .36X BB but yet it was different. It had a solid finless cylinder head with an angled plug, new shorter front crankcase housing with twin ball bearings on the crankshaft, four screw rear cover, big massive NVA with insert in the venturi, "Fox .36X" on the left bypass, and a web in the exhaust stack.
The 1975 .36X schneurle combat engine is called the Mark II. It differs very little from the Mark 1 Combat Special, yet it does not have the web in the exhaust stack. It has an all new NVA which was designed to hold the pressure of a bladder fuel tank for fast combat. A suction type NVA and insert also came with the engine for use in slow combat. The pressure type NVA has an angled intake pipe and it has a two piece needle valve assy. "Fox .36X" is on the bypass, polished case, redesigned thrust washer.
In 1976 Fox introduced the first "Tall Back Door" schneurle ported combat special and called it the Mark III. The Mark III Combat Special is completely redesigned. It has an all new crankcase of dull finish with a six screw tall back door for ease of machining. It has a new small exhaust stack and opening, an all new button head with finned polished clamp, center plug, twin ball bearing main, square slanted intake with a pressure type NVA. A suction NVA and insert was also supplied with the engine. It has "Fox Combat Special" on the left bypass.
From 1976 to 1980, slow combat was gaining in popularity. Competitors found that the Fox Mark III combat engine did not perform well when used with the suction NVA and insert for slow combat. Duke Fox designed a new, tall, square intake stack with two needle valve locations available. The lower position for pressure for use in fast combat with no insert, and the upper position for use in slow combat with suction NVA and the insert. At the same time he designed a new thrust washer so that it would cover the front ball nearing better and keep dirt off the bearing. He anodized the thrust washer a nice bright red color so that you will not lose it in the green grass (just kidding). He decided not to polish the cylinder head but he still put "Fox Combat Special" on the left bypass. He called this engine the Mark IV Combat Special. Now both fast and slow combat flyers had a super performing engine.
In 1983 Fox released the Mark V Compact Engine. The engine was designed for the noncompetitive sport flyer. It has most of the features of the Mark IV Combat Special except it only has a single ball bearing main and is drilled and tapped for a muffler. It also has the button head with clamp. It does not have any markings on the left bypass or anywhere on the engine. It is schneurle ported and is known as the Fox .36BB Control Line Engine.
From 1980 to 1986 the Fox Mark IV Combat Special was the king of both slow and fast. In 1986 Duke Fox decided the combat flyers of the world needed a new engine so he introduced the all-new Mark VI Combat Special. It used the ABC piston and cylinder setup plus a slightly longer nose with twin ball bearings. The diameter of the crank was increased to .590 and it used a 7/32 diameter tubular wrist pin retainer. The intake stack is now cut at a nice angle nut still has the holes for high and low needle valve locations. It has a new pressure type NVA, the thrust washer is a nice polished aluminum color, the finned head clamp is also polished, the exhaust stack has two bosses that are drilled and tapped for a muffler. If the front threaded portion of the crankshaft is damaged, it can be replaced by unscrewing it from the crankshaft and installing a new threaded stub only. The left bypass has a long vertical flat area with the name "Fox" and the letter "C" stamped into it.
Later (when?), the Mark 7 was introduced. Bigger crank, better flow in the crankcase, chrome-plated aluminum liner (AAC), and other changes. About an ounce lighter. (Was it designed by Willy Wiley?) In practice, the chrome peeled from the liner, giving them a disappointingly short life.