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1936 Gwin-Aero .421

The very first of the famous Bunch line of engines.  These were built by Dan Bunch's father-in-law, Joseph Gwin, in Indianapolis, Indiana.  Built mainly as a part time activity to see if he could build an engine for the growing fad of flying model airplanes. 


The engine had a bore and stroke of 13/16" for a displacement of .421 cu. in.  The crankcase, back cover and timer frame were sand cast aluminum. The cylinder was brazed up and the bore was lapped for the piston. Note the exhaust port was two rectangular slots. The cylinder fins on the earliest engine was a muff as pictured and screwed down over the cylinder.  The last of this type had two holes in the top, either side of the spark plug for a spanner wrench.

1936 Gwin-Aero .421

Still built in Indianapolis, this design began to incorporate features that would be found in all future Bunch engines.

The engine featured the same bore and stroke of 13/16" for a displacement of .421 cu. in.  The crankcase, back cover and timer frame were sand cast aluminum and was lighter and more compact. The cylinder was brazed up and now included the fins. It still had the lapped piston. 

A nice brass nameplate was added to the front of the cylinder and was the only Bunch engine to have this feature.

Rather than the whole head and fin assembly screwing down over the cylinder, this model now had a removable head that screwed into the bore.  Spanner wrench holes are visible on the top.

After a time, Joseph Gwin turned the project over to Danner Bunch, who had a model shop in Los Angeles.  Shortly after, the first Mighty Midget was offered to sale

Bunch Warrior

January, 1938 - .488 cu. in.

At the beginning of 1938, Bunch introduced the most unique engine of their entire line. The cylinder assembly was very much like their earlier Mighty Midget engines except a large "W" was engraved on the front bypass.

The main difference was a new sand cast crank- case with only a three point radial mounting system.

The fuel tank was supported only by the fuel line being soldered into the needle valve body and one tab on the bottom that attached to the bottom mounting screw.

It also featured a bar stock timer assembly rather than the cast version of all other Bunch engines.

Aside from the first two Indianapolis engines pictured here, this is probably the rarest of the Bunch line.  An unconfirmed story has it that the majority of these engines were exported to Europe and South Africa.

1938 GWIN-AERO Inverted .488

July, 1938

A unique feature of some of the Bunch engines is the inverted models.  While most engine manufacturers allowed in their instructions that their engine could be mounted and run inverted, Bunch actually stamped the name "up side down" on the bypass.

I wonder if it would run if you mounted it upright?

1945 Bunch Tiger Aero .45

This is the last of the engines produced under the Bunch name.

.45 cu. in. displacement and a real powerhouse for its time. One of the first engines to use a "tuned intake". The extra long venturi tube was a source of irritation to many modelers, but to their horror, when they cut it down they ended up losing as much as 1000 RPM!  It was long for a reason.

This straight finned model is quite rare as most all Bunch engines featured a tapered fin profile


Following the death of Danner Bunch, the project was taken over by Ray Accord who operated the Air-O-Model Supply Co.  With changes, the engines were continued in production under the Air-O name.  Engines were also produced for Berkeley Models who sold them under the Berkeley name.

The first model without the strapped on exhaust is pictured here.

Several models of a .278 cu in diesel were also produced under the Air-O name using the large Bunch crankcase with a small bore cylinder.


Just prior to Danner Bunch's death his last design was produced under the Lucas & Smith name.

These were .596 cu in displacement engines.  While they resembled the Bunch engines in basic styling, they were much larger and more powerful.

Built in two different styles, the D-60-R - rotary valve (actually rear drum valve) pictured and a much rarer version, the D-60-S - sideport version.

For more information on the Bunch series of engines consult the Bunch, Air-O and Contestor section in the Encyclopedia, click here.


More to come, tune in regularly.





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