Concept II .35 Sport Engine

I am still looking for samples of the other engines.

Concept II .35 Sport (6.5 cm³)

Prototyped in 1968 (#2510)

A prototype of the front intake, Schnürle ported .35 Concept II Sport.

The Concept engines show an advanced engine design which, unfortunately, never made it into production.

Crankcase, cylinder and head are one single integral part which means that the ringed piston fits into the hard chromed bore of the aluminum casting. A similar design is used in many small piston engines for industrial applications, like chain saws. For model engines this concept was quite extraordinary in 1968. Even today only a few high performance engines use a similar design, usually with a ringless piston, though.
The crankcase is split horizontally and the lower cap is attached with four screws to the upper cylinder part. The exhaust port has two thin webs to restrain the piston ring from expanding into the port.

The engine is very lightweight (218 g) and, probably more important, separate parts for sleeve and head are not needed. This could reduce assembly costs considerably, but on the other hand the crankcase may be more difficult to manufacture than a conventional model engine. Also in case of piston/sleeve wear, the complete upper crankcase part has to be replaced.

Similar engines had been developed with an R/C carburetor and ball bearings as well as with a larger displacement of .40 cubic inch (featuring front or rear rotary inlet).

The crankcase of the Concept engines look remarkably similar to earlier Atwood designs like the Atwood Triumph .49 and .51 (these were sold in 1948 with glow and a spark plug ignition).

The split bottom requires a precise fit to the upper crankcase. Note the slim, integral "cylinder head".

The bottom cap removed reveals some details like the open crank web or ...

... the transfer channels with center  web, the cutout for the connecting rod and the piston. There is no sleeve.