Cox Model Engines

Cox Model Engines

Cox model engines are used to power small model airplanes, model cars and model boats. Cox engines were in production for more than 50 years between 1945 and 2006 by L.M. Cox Manufacturing Co. Inc. who later became "Cox Hobbies Inc." and then "Cox Products" before being sold to Estes Industries when it became Cox Models.

Millions of engines were produced and they became the most common 1/2A Class 0.049 cubic inch engine in the world and probably still are today. Although the production of the engines ceased some years ago now, engines made as far back as the 1950's are still sold "as new" and in abundance [] world wide.

Cox history

The Cox range of model engines were the brain child of entrepreneur Leroy M. Cox. [The Engine Collectors Journal, Issue 105, May 1993, History of the Cox Engines Part 1 by Dan Sitter ]

* Cox Manufacturing started out in Leroy's garage in 1945 where he made wooden pop guns for kids and employed local housewives to assemble them. Metal was scarce due to the war which is why the toy guns were made of wood.

* In 1946 metal became readily available again and competitors quickly moved into the market making their metal pop guns cheaper, so Roy moved to something else, making metal tether cars for kids.

* In August 1946 a fire in Cox's garage forced him to move to newer and larger premises at 730 Poinsettia Ave, Santa Ana, California.

* In 1947 Cox developed a racing car which used an engine manufactured by Cameron Brothers. The cars sold for $19.95 and generated $200,000 in sales in their first year of production.

* In 1949 Cox developed their own engine for the racing tether car made from parts from other engines. This engine was called the "O Forty Five" as it was .045 cubic inch displacement.

* In 1950 sales of the car began to fall so Cox moved on to development of a model plane engine. A project which took a year and nearly sent Cox bankrupt, the end result was the Space Bug .049 engine, Cox's first Model Plane engine.

* In 1952 first name change was made to L.M. Cox Manufacturing Company Inc. The Space Bug engine sets the scene for all the Cox engines that followed and it went into full production in 1952. This engine was so popular that it caused problems for other model engine manufacturers.

* In 1953 Cox produced their first Ready To Fly (RTF) airplane the TD1 which was powered by the Space Bug engine.

* In 1956 Cox developed the Babe Bee 049, designed by William (Bill) Selzer, which had an extruded aluminum crankcase, not cast like the others, this engine sold for just $3.95 and stamped the final nail in the coffin of many competitors whose engines were selling for substantially more. The Babe Bee was a high quality high precision engine which started easily and was very reliable, unlike some of the competition.

* In 1957 Cox took over the Flying Circle at Disneyland which was a major coup for the company. The model planes were being flown each day in front of tens of thousands of people and they had a hobby shop right there full of Cox RTF planes. The Cox Flying Circle [] remained in operation until 1965 when it was closed to make way for expansion of Tomorrowland.

* In 1960 Cox hired an engineer named Bill Atwood, (who had already build his own line of engines), to develop a new line of competition engines. Atwood was responsible for the Tee Dee and Medallion line of engines. These engines put Cox on the map as a leading engine in the world for many years to come.

* In 1963 due to continuing growth the company moved to larger (225,000 square feet) facilities which were three times larger than the old site. During this time Cox got into Slot Cars for a short time but this fad died as quickly as it started.

* 1965 Cox International is established in Hong Kong.

* In 1969 Mrs. Cox died and Leroy was having health problems of his own so he retired and sold the company to Leisure Dynamics Inc. Leisure Dynamics continued to expand the range of Cox Model Aircraft as well as adding trains, boats, rockets, kites and radio control boosting sales to 25 million dollars per year. Roy Cox retired with the distinction of being the world's most successful model engine manufacturer.

* In 1970 William H. Selzer was appointed as President of L.M. Cox Manufacturing Inc. (a subsidiary of Leisure Dynamics).

* In 1971 Leisure Dynamics breaks the company in two and moves the model production to Minnesota and leaves the engine production in Santa Ana, California.

* In 1976 Leisure Dynamics changes the company name to "Cox Hobbies Inc."

* In 1980 Leisure Dynamics filed for bankruptcy, taking Cox Hobbies Inc. with them.

* In 1981 Leroy (Roy) M. Cox passed away, age 75.

* In 1983 former Cox engineer and president Bill Selzer (whom Cox hired in 1952) purchased the company out of bankruptcy. All manufacturing was returned to Santa Ana. The company once again flourished into the 1990s with new products being added and another move to even bigger facilities.

* In 1990 Cox Hobbies moved to new facilities at Corona, California.

* In 1993 the company name changed again to "Cox Products". The Pee Wee, Babe Bee, and PT 19 Trainer still in production.

* In 1995 Cox celebrated 50 years and introduced some new engines and RTF models.

* In 1996 Cox sold to Estes Industries / Centuri Corp. and moved to Penrose, Colorado. Things went downhill from here and Cox as the world knew it had gone. One by one each item was withdrawn from sale. Engine parts from different engines were mixed and matched making bastardized versions and quality control was lacking.

* In 2005 an online company calling themselves Cox Hobby Distributors appeared selling RC and electric products and some of the "classic" engines and RTF models. However as each item from the classic era sold out it wasn't replaced.

The engines

Infobox generic
style0 = class="infobox" style="width:22em; font-size:90%;"
color = blue
name = Cox .049 Engine Specifications
sub0 = (All Cox .049 Engines)|
lbl1 = Bore:
row1 = 0.406 inches (10.31 mm)
lbl2 = Stroke:
row2 = 0.386 inches (9.8 mm)
lbl3 = Displacement:
row3 = 0.04997 cubic inches (0.8189 cc)
lbl4 = Bore/Stroke Ratio:
row4 = 1.05:1 (Oversquare)|
hdr2 = Cox .051 Engine Specifications
lbl21 = Bore:
row21 = 0.41 inches (10.41 mm)
lbl22 = Stroke:
row22 = 0.386 inches (9.8 mm)
lbl23 = Displacement:
row23 = 0.0509 cubic inches (0.8341 cc)
lbl24 = Bore/Stroke Ratio:
row24 = 1.06:1 (Oversquare)|
The Cox .049 Engine is a 2-stroke internal combustion glow plug engine. These engines use a glow plug rather than a spark plug for ignition and have no valves or carburetor.

Fuel intake to the engine is controlled by a simple needle valve and venturi system. Fuel/air mixture intake to the crankcase is controlled via a reed valve or rotary valve depending on the engine design.

In a reed valve engine the valve is drawn open by suction as the piston moves upward on the compression stroke. As the piston moves down on the power stoke, the pressure in the crankcase causes the reed valve to close. The fuel air mixture in the crankcase is then forced past the piston via the transfer/bypass ports in the cylinder.

On rotary valve engines the process is similar except instead of a reed, a rotary valve is used, (incorporated in the crankshaft), which opens and closes as the piston moves up and down. The rotary valve is more efficient and adjustable (at design time) as there is a larger and clearer path to the crankcase than in the reed valve setup.

The fuel used to power the engine is called Model Engine Fuel, a mixture of methanol (70%-40%), castor oil (20%) and nitromethane (10%-40%).

The Cox line of reed valve engines designed prior to 1960 used a rear reed valve induction system. In the late 1950s they played around with rear rotary valve induction (as used in the RR1) before moving forward with front rotary valve induction for their Tee Dee and Medallion lines.

Early engines

* 1949 O Forty Five Power Pak (Special Racer Car Engine - Manufactured 1949)This engine was the first designed by Cox but included some major parts (ie piston and cylinder) of the Spitzy .045 engine designed by Mel Anderson. It employed a twin reed valve which was later used for the Space Bug. The engine was a major engineering achievement for its time, by incorporating reduction gears, fuel tank, flywheel and muffler all into one "Power Pak". The air intake was via one of the axles.
* 1952 Space Bug (Cat#349 - Manufactured 1952-1958)The Space Bug was the first engine built entirely by Cox. It was designed for Control Line flying use only and was marketed as a "Competition" engine and sold for $6.95. Back then there was no Radio Control and Free Flight hadn't been considered by Cox at this time. The piston and cylinder were made from mild steel bar stock and the crankcase and fuel tank were cast aluminum.
* 1953 Thermal Hopper (Cat#360 - Manufactured 1953-1958)The Thermal Hopper is basically a Space Bug without the fuel tank. It has a needle valve and venturi mounted on an aluminum plate instead. These were designed for free flight and could also be used for control line flying. It allowed the user to put a fuel tank of their choosing on. The engine output was recorded at 0.066 bhp @ 17,000 rpm with a torque of 4.5 at 10,000 rpm. [Model Aircraft Magazine, August 1955]
* 1953 Space Bug Jnr. (Cat#370 - Manufactured 1953-1958)The Space Bug Junior is a Space Bug with a smaller plastic tank. This cheaper version also only had one intake bypass port and sold for $3.95.
* 1955 Strato Bug (Cat#380 - Manufactured 1955)The same engine as a Space Bug but included a two piece fuel tank which was cheaper to make than the original space bug tank. The later Babe Bee tank was simply a further developed version of this tank. This engine was only produced during 1955 and sold as a mid range sport engine for $5.95. As such not many exist today which makes them very rare and collectible selling for over $300US in 2008.

Standard Bees

* 1956 Babe Bee 049 (Cat#350 - Manufactured Nov 1956-Jan 1996)The classic Babe Bee was the first engine Cox produced with an extruded machined anodized bar stock aluminum crankcase. This crankcase was machine made and was much cheaper and faster to make than the cast aluminum crankcase of the earlier models. This engine was also supplied in thousands of RTF (Ready to Fly) airplanes sold in department stores worldwide. It has an integrated 5cc fuel tank. Max output power was recorded around 0.057 bhp (42 watts) @ 13,500 rpm on 15% nitro. [ Aeromodeller Magazine, May 1961]
* 1957 Pee Wee .020 (Cat#100 - Manufactured Feb 1957-Jan 1996)Buoyed with excitement of the Babe Bee 049 Leroy wanted to make a half size version of the Babe Bee. So he did. And they called it a Pee Wee .020, just like a Babe Bee only half the size.
* 1958 Golden Bee (Cat#120 - Manufactured Oct 1957-Jan 1980)The Golden Bee is a Baby Bee that has a larger (8cc), stunt vented fuel tank and has been anodized gold. The larger tank allowed the planes to fly longer while the stunt vents allowed the airplanes to fly inverted without fuel running out or the engine cutting out. The first versions of the Golden Bee had a single bypass intake port but later versions had two bypass ports making them slightly more powerful.
* 1966 QZ (Cat#450 - Manufactured 1966-1996)QZ stands for Quiet Zone. It's Babe Bee with a muffler, twin bypass port cylinder with no sub piston induction and a high compression (#1702) glow head. An attempt to regain the power loss caused by the muffler. Very similar to the later QRC engine which reportedly worked better. Cox also sold the muffler, cylinder and high comp. glow head components of the QZ as a Muffler Conversion Kit (Cat#495) for $2.98.
* 1976 QRC (Cat#450-1 - Manufactured 1976-1996)The QRC was a modified Babe Bee engine that had a muffler and larger (8cc) fuel tank. In the 70's noise became and issue and the Cox engineers discovered that when adding a muffler the engine would lose significant power. This problem was alleviated by installing a cylinder with no sub piston induction. [049 Collectors Forum, Larry Renger, Former Cox Engineer, Designer of the Black Widow and QRC engines.] The engine was designed for power launching Radio Controlled Gliders and had a red tank with a blue spinner.
* 1976 RC Bee (Cat#360 - Manufactured 1976-1996)This engine was designed for small Radio Controlled model planes. It has a plastic clunk tank and an unusual cast crankcase. The Leisure Dynamics team thought that cast crankcases would be cheaper to produce, however they discovered that there were many manufacturing defects and they were difficult to machine, resulting in a high failure rate, so they returned to the tried and proven machined aluminum bar stock crankcase.
* 1982 Dragon Fly (Cat#4504 - Manufactured 1982-1996)This engine was designed for the Radio Controlled model planes. It's basically a Baby Bee with a clunk tank and a muffler throttle.
* 1989 Texaco (Cat#4506 - Manufactured 1989-1996)The is engine was designed for 1/2A Texaco RC duration competition. The engine has an additional fin on the larger glow plug which disapates heat better allowing the engine to swing a larger propeller. i.e. 7 or 8 inches. This engine has a red 8cc fuel tank and a black crankcase.
* 1989 Texaco Jnr (Cat#4507 - Manufactured 1989-1996)The same engine as the Texaco above except that it has a smaller 5cc fuel tank which is also red.

High Performance Bees

* 1956 RR1 (Cat#390 - Manufactured 1956-1965)The RR1 uses a rear rotary valve intake rather than a reed valve in an attempt to achieve more power. The engine came after the Babe Bee and looks very similar with its anodized, machined extruded aluminum crank case and fuel tank. The power improvement was negligible so Cox reverted to the cheaper easier to build Babe Bee. The engine was made for quite a few years and sold for $6.95. It has become a collectors item due to its uniqueness and pretty colors and is worth around $300-500US in 2008.
* 1959 Space Hopper (Cat#150 - Manufactured Nov 1958-1961)The Space Hopper was Cox's first attempt at a beam mount engine plus the first steps towards their greatest engine, the Tee Dee. The engine was basically like the Thermal Hopper was to a Space Bug. That is a Babe Bee without a tank, but a venturi and needle valve relying on and external fuel tank. It is claimed it has as much power as a Tee Dee but how can this be if the Tee Dee was the most powerful engine? Anyway, this engine was short lived and made way for the Tee Dee in 1960. They look very retro and due to their apparent rareness are worth around $200US in 2008
* 1973 Black Widow (Cat#150 - Manufactured May 1973-Jan 1996)During the 70s a couple of Cox engineers were playing around with different colored Babe Bee and Golden Bee parts and came up with an all black engine with a red spinner. They hopped it up a bit with a dual bypass cylinder from a Medallion, a black Golden Bee tank and a slightly larger (0.062") venturi intake. The Black Widow was born. These engines were marketed as a High Powered Combat Engine. On later Black Widows the red rubber spinner was replaced by a red anodized aluminum Tee Dee style spinner. In the late 1990s some Black Widows were produced with the a dual bypass slit exhaust cylinder. The slit exhaust was to prevent fires. According to an Aeromodeller engine test done in August 1974 the Black Widow on 25% Nitro output power was 0.08 bhp (60 watts) at 15,000 rpm with a max. torque of 6 at 9,000 rpm. [Aeromodeller Magazine, August 1974, UK, Engine Test by Peter Chinn]
* 1995 Killer Bee 049 (Cat#340 - Manufactured 1995-1996)The Killer Bee was a real good attempt at making a fast reed valve 049 engine from information that had been learned over the years of racing and competition. It had a tapered cylinder and lighter piston similar to the Tee Dee, a stronger balanced crankshaft and a new reed valve shape. They had a Yellow plastic needle valve. Later in 2002 Estes produced a Killer Bee that had none of these features but looked like the original Killer Bee except for the needle valve.
* 1996 Killer Bee 051 (Cat#360 - Manufactured 1996)The Killer Bee 051 existed so that modelers could fly the same plane in two competition classes (ie A and 1/2A) simply by changing the engine.
* 1996 Venom (Cat#140 - Manufactured 1996)The Venom was Cox's last attempt at making a really fast 049 mouse racing engine. Again taken from ideas learned from years of competition, this engine put all those ideas into an off the shelf product. It used the Killer Bee crank shaft loosely fitted into the crankcase, and a cylinder with porting very similar Tee Dee cylinder and tapered like the Tee Dee and with a lightened piston like the Tee Dee. The rest of it was like a Black Widow. The problem was that the production engine was not the same as the prototype. A mistake had been made in manufacturing and the piston was lightened too much. This made the engine fast but the piston weak and they would blow the top off the piston after a few runs at high speed. As such only 1000 were made and they never bothered to make any more. They are now worth a lot of money (around $300US in 2008) in the box.

Tee Dees

* 1961 Tee Dee 049 (Cat#170 - Manufactured Nov 1960-Jan 1996)This is without a doubt, Cox's greatest and most famous engine. This engine was "THE" engine to use in competition for many years. It was designed by Bill Atwood who had been hired by Cox specifically to produce the Tee Dee line of competition engines. The important features of the Tee Dee are as follows:
* Tapered cylinder and a lightened and tapered piston
** Result: tighter piston fit at TDC and less piston mass = more performance.
* Two deep bypass ports with two bypass booster grooves on each bypass port, extending slightly above the main bypass groove
** Result: significantly better air fuel mixture induction = more performance
* True peripheral venturi
** Result: more efficient fuel intake, fuel draw and induction = more performance
* Precision balanced and milled crankshaft
** Result: better fuel intake, better balanced engine = more performanceThe Tee Dee was tested by Aeromodeller Magazine in 1962 and the output power was recorded to be .105 bhp (78 watts) @ 22,000 rpm with a max torque of 5.5 at 18,000 rpm on 25% Nitro. [Aeromodeller Magazine, November 1962, Engine Analysis 101 by R.H. Warring] (Note: The modern Norvel AME 049 engine which has a ceramic coated aluminum piston outputs .2 bhp (150 watts)@ 17,000 rpm). In 1973 the bypass porting, crank shaft timing and venturi were modified slightly and a mesh screen was added to the venturi to keep out dirt. This resulted in a minor performance improvement over the earlier versions.
* 1961 Tee Dee 051 (Cat#200 - Manufactured Oct 1960-Jan 1996)The 051 was simply a Class A version of the engine, physically the same on the outside only the bore was different and the piston had a small groove in the skirt to differentiate it from the 049. The 051 also had a RED carb body.

* 1994 Tee Dee .05 RC (Cat#201 - Manufactured 1994)This engine had a proper RC carby and a full sized standard muffler and was designed specifically for RC flying. It had no Sub Piston Induction. Only two production runs of 1000 each were done so there are only 2000 of these in existence, making them the second rarest production engine next to the Venom)

Cox also built Tee Dee's in .010 (Cat#130), .020 (Cat#160), .09 (Cat#210) and .15 (Cat#180) size. All these engines were very successful.

A little known fact was also this: One of the things Cox wanted Atwood to do was make him a .010 engine. Cox had already tried to halve the size of the Pee Wee .020 but couldn't get it to run for some reason. The suspicion surrounded a problem with the tiny reed valve. Atwood found that the front rotary valve worked well on the .010 size hence the .010 was born. Why? Because Leroy Cox wanted one.


* 1962 Medallion 049 (Cat#240 - Manufactured Nov 1961-Jan 1996)Also know as the "Poor man's Tee Dee" these were similar to the Tee Dee in appearance but had cheaper parts on them, making them cheaper to buy. The cylinder was a non tapered twin bypass with no boost ports (like the one used on the Black Widow), the crank shaft was drilled out rather than milled like the Tee Dee, and the carb body was a one piece unit with a conventional needle valve and spray bar. These engines were marketed as a Sport / Stunt engine as they were much tamer and much less cantankerous then the Tee Dee. These are a great reliable easy to use little engine even today. They can be purchased for under $100US in 2008.

The Medallion engine was also produced in (Cat#230).09 and (Cat#220) .15 cubic inch size.

* 1995 Medallion 051 (Manufactured 1995)The .051 came about when a special order for 300 Medallions was placed on Cox by the National Free Flight Society in the USA. It was a gentleman's agreement done on a handshake and it nearly didn't happen when Cox was sold to Estes. But the determination of the NFFS resulted in the deal happening, however Cox only came through with 258 engines. The NFFS engraved each engine with a serial number and kept a record of who purchased each engine. These are probably the rarest of all Cox Engines due to the small size of the production run.

Product engines

These are all variations of the Baby Bee with different backplates designed for different RTF aircraft.
* 1959 Super Bee (Cat# 350-1)Babe Bee with twin bypass cylinder to give more power for the P40 Warhawk RTF model. Early version had 'P40' stamped on the cylinder. It had a standard Babe Bee tank.
* 1964 Silver Bee (Cat# 350-6)Similar to the Super Bee but it had a larger 8cc non-vented fuel tank. It did not have P40 stamped on the cylinder but it did have the twin bypass ports. This engine came with the Spitfire RTF airplane.
* 1962 Series 190 Product Engine (Cat#190)Babe Bee with plastic backplate and brass needle valve. Various shapes were used depending on the model they were fitted to. Some had dual bypass port cylinders while others had single, it depended on the airplane.
* 1963 Series 290 / Spook Product Engine (Cat#290)Came on a blister pack as a "Two Ninety" replacement engine. Basically a replacement 190 engine. An aluminum back plate was used for the "Spook" flying wing combat model kit engine (290-1).
* 2000 Surestart (Cat#191)Another variation of the modern Babe Bee. These were pretty good because they have a choke tube attached to the grey plastic backplate. The choke tube makes the engine even easier to start. They were fitted to the very last RTFs (e.g. PT19 and Hyper Viper) before they went out of production.

Other Cox engines

Other engines made by Cox are:

* 1959 Sportsman .15 (Cat#110 Manufactured 1958-1961)Rear reed valve .15 sized version of the Space Hopper
* 1959 Olympic .15 (Cat#140 Manufactured 1959-1961)Sportsman with twin ball races - designed for FAI Power FF event.
* 1961 Special .15 (Cat#260 Manufactured 1962-1964)Second version of Tee Dee 15 with thicker cylinder and wrist pin conrod instead of ball socket.
* 1964 Special .15 MkII (Cat#270 Manufactured 1964-1968)Third version of the Tee Dee .15 Single exhaust port Schnürle port intake system and gold crankcase.
* 1965 Olympic .15 Drum Valve Prototyped only (50 made by Bill Atwood)
* 1968 Concept II .35 & .40 Prototyped only - Bill Atwood
* 1976 Cox Conquest .15 (Cat#2800 Manufactured 1976-1978)This engine took over from the Tee Dee .15 as "THE" engine for FAI racing, combat and free flight for many more years until the Russian AAC Engines came along.
* 1976 Cox Conquest .40 - prototyped only


There were a wide range of cylinders produced with three different wall thicknesses. Most are interchangeable between all engine types which can create problems when buying a used engine. All early cylinders had a thin wall which was later found to need improvement because they bent easily in a crash or when trying to undo with a Cox wrench. Some people refer to these as Mk1 cylinders. The next type was thickened at the exhaust ports and are also known by some people as Mk2. The third type was thick wall the way down from the cooling fins to the bottom. This one facilitated the exhaust throttle ring and some people refer to this as a Mk3 although Cox never referred to them like this. [Engine Collectors Journal, Issue 127, May 1997, Cox Product Engines Parts Breakdown by Dan Sitter ]

Referring to the Cylinder Cross-sections above:

# depicts a thin wall Tee Dee .049 cylinder with dual booster ports on the bypass port.
# is a late model thick wall cylinder with slit exhaust and a single bypass booster.
# is a Black Widow #1 cylinder with no bypass booster with a stepped wall.
# is a pre-1955 cylinder used on early Space Bug, Space Bug Jr, Thermal Hopper and Strato Bug. Note how the thread diameter for the glow head is much smaller. All engines produced post 1955 did not use this cylinder.

The most powerful cylinder piston combination without a doubt is the number 4 Tee Dee 049 cylinder. This cylinder has a tapered grind and tapered and lightened piston so the piston fit gets tighter as the piston reaches top dead center (TDC). The intake or bypass porting is 2 deep ports with 2 bypass booster ports on each bypass. This setup causes a swirling of the intake fuel air mixture which promotes better combustion. The Tee Dee cylinder was the basis for the design of the Venom and Killer Bee Cylinders. "Notes:"
SPI = Sub Piston Induction
1 The Killer Bee and Venom had a special competition lightweight piston and a heavy duty crank shaft for speeds above 22K rpm.
2 The Tee Dee also had a lightweight tapered piston and the crank was stronger and ported different to the Medallion crank.


The early engines that were produced pre 1957 had a light alloy piston rod which is retained in it's socket by a slotted steel retaining cup which in turn is held in place by a steel circlip located in a shallow groove in the interior piston wall. This arrangement (known as a three piece piston) was abandoned as of 1957 in favour of a hardened steel rod which was swaged into a bearing cup formed integrally in the piston interior. The advantage of this latter set-up was that it simplified assembly and the bearing could be re-set to take up play using a suitable "reset" tool to re-swage the cup. [Keilkraft Cobra 049 and the British 1/2A Revolution by Adrian Duncan]

The engines that used the early style three piece piston are any engines produed prior to 1957. i.e. Space Bug, Space Bug Jnr, Thermal Hopper and Strato Bug. Since the Space Bug, Space Bug Jr and Thermal Hopper where produced up until 1958 you will find versions of these early engines around with the later style pistons. (and cylinders)

The post 1957 piston is coated with copper on the inside and top. This was done to prevent the nitrile hardening process from hardening the ball socket joint area. The outside wall of the piston was then finely machined and polished to produce a chromed appearance. The piston is not coated with chrome. To prove this point, if you can clean a Cox piston up with alcohol then place it in water overnight - it will rust. [Larry Renger - former Cox Engineer]

Piston / Conrod ball socket joint free play

Sometimes, especially with engines that have had a lot of use, the piston / conrod ball socket joint is very loose and will adversely affect performance. A Cox “reset” tool, available for the 020, 049/051 and the 09 series engines, is used to tighten the ball joint back up again. The correct free play is .001 to .003 inches. The .15 engines use a wrist pin so a socket reset tool is not required for these.


On the Bee engines there were 2 types of crankshaft produced. (and they are interchangeable)
# All Bees with the exception of the Killer Bee and Venom had the same crankshaft that was only good for about 20,000 rpm before the conrod pin would break off.
# The Killer Bees and Venom had a heavy duty balanced and lightened crankshaft that improved performance and could withstand speeds in excess of 22,000 rpm.A company named Davis Diesel Development in the USA also make a similar crankshaft called a Killer crank for their diesel conversions. They found under the heavier torque loads caused by running diesel fuel, that the pins would break also; hence they produced their own killer crank.

On the Tee Dees and Medallions the cranks are also interchangeable but quite different. The Tee Dee has a large square hole at the intake end of the crank whereas the Medallion has a smaller round hole. The timing is also different.

Glow heads

Cox Glow Heads

There were 5 types of Cox Glow Heads produced for the Cox 049:

# #302 Pre 1955 Standard Head - Hemispherical shape - Low Compression - Smaller dia. threads - used on Space Bug, Thermal Hopper, Space Bug Jnr and Strato Bug Only. [Cox Dealer Catalogue Feb 1962]
# #302-1 Post 1955 Standard Head - Hemispherical shape - Low Compression - Larger dia. threads - used on all engines produced post 1955 until the #325 head was produced in 1980. [Cox Dealer Catalogue Feb 1962]
# #325 Standard Head - Hemispherical shape - Low Compression - Larger dia. threads - used on all post 1956 engines except Tee Dees, QZ, Killer Bees, Venom and Texaco. Replaced 302-1 head in 1980.
# #1702 Hi Compression Head - Trumpet shaped - High Compression - knurled top - 2 fins - used on Tee Dees, Killer Bees, QZ and Venom
# #315 Texaco Head - Hemispherical shape - Low Compression - 5 fins - used on Texaco Engine only. Allows greater cooling for engine swinging larger propellers.

Aftermarket glow heads

# Standard glow plug head—uses standard conventional glow plug—low compression—standard performance
# Turbo glow plug head—low compression—medium performance
# Norvel Freedom Glow Plug—high compression—high performance—rare and hard to get.
# Galbreath Head with Nelson Plug Combo—high compression—high performance—most popular, gives the engine an immediate performance boost by up to 2,500 rpm depending on the engine.

Reed valves

There were three main types of reed valve produced:

# Early engines—Space Bug, Thermal Hopper, Strato Bug—circular twin brass reeds
# Bee Engines—star shaped single brass reed.
# Later Bees, Killer Bees, Venom—oval shaped stainless steel reed.

Later variants of the oval reed were made of Mylar and Teflon. Some say Mylar is the best while others prefer the stainless steel and then others prefer the Teflon. Claims are that Mylar and Teflon are lighter and make the engine easier to start and go faster, but they don't last as long as the stainless ones.


049 engines go well on a 5x3 to 6x3 prop. A 5.7x3 APC works well. To get any suitable speed for mouse racing a 4 inch pitch prop is required at high revs but to do this plenty of nitro-methane is also required. E.g. 4.75 X 4 prop with 40% Nitro. Texaco engines are designed to use bigger props. e.g. 7x4


The highest performance is achieved with fuel of 30% or more nitro content. At least 20% oil (50/50 castor/synthetic) is required. With high nitro fuel it may be necessary to lower the compression by installing up to six or more additional head gaskets.

When using Castor Oil it is advisable to clean the engine cylinder wall with a Schotch-Brite pad to remove castor oil varnish build up that will occur. Especially after lean running. This build up of varnish will cause the engine to run inconsistently.

To avoid this problem occuring it is advised to use a Synthetic oil or synthetic and castor oil blend. Synthetic oil contains detergents that will keep the cylinder wall clean, however these small engines do rely on some castor oil build up to maintain high compression at higher running temperatures.

Using clean fuel and keeping every thing clean and free from dust and dirt particles is also very important for consistent running in an engine of such small size as these.

Diesel conversion

Davis Diesel Development [] manufactures and sells heavy duty Bee cranks (Killer cranks) and diesel conversion heads. These can be purchased direct from their web site or from eBay. The DD cranks are similar to the original Cox Killer Bee crank.

Cox Ready to Fly Model Airplanes

Over the years, as well as producing millions of model engines Cox also produced a similar number of Ready To Fly (RTF) airplanes, as well as boats, cars, helicopters, trains, etc.

The following is a list of the RTF airplanes produced by Cox between 1953 and 1977:

Part numbers appear to skip and jump, however cars, hellicopters and boats produced around same times had numbers similar to the airplanes, therefore causing gaps.


ee also

*Control line (model aircraft)
*Cox Models
*Free flight (model aircraft)
*Model aircraft

External references

* [ Biography of Leroy M Cox]
* [ The Cox Engines Museum]
* [ Model Engine Collectors Journal]
* [ American Model Engines Encyclopedia]
* [ Yahoo 049 Collectors Forum]
* [ Model Engineering and Model IC Engine Projects]
* "Original Author: Warren Leadbeatter - Model Engine Collector - AUS-14782"

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