Concept II .35 Front Rotary R/C

Concept II .35 R/C (6.0 cm³)

Prototyped in 1968 (#2500)

A prototype of the front intake, Schnürle ported .35 Concept II R/C engine. This engine is similar to the Concept II .35 Sport and the Concept II .40 R/C. The main differences to the Sport engine are:

  • the plain venturi carburetor has been replaced by an R/C carburetor,
  • the crankshaft is supported by two ball bearings, and
  • the crank web is covered by an aluminum cover reducing crankcase volume.

All  Concept engines were again developed by Bill Atwood and resemble some of his earlier designs. They never made it into production because the expected sales volume did not relate well with the production costs.

Measuring my sample engine gave the following results:


bore 20.2 mm
stroke 18.6 mm
swept volume 5.961 ccm
0.364 c.i.


Exhaust port 145°
transfer ports 125°
inlet timing 178°

The exhaust timing is what one would describe it as a nice sport engine. So I would expect that the Concept would have been a quite powerful engine suitable for typical R/C models. 

The engine was probably intended to be run with an expansion chamber muffler similar to the one found later on the Conquest .15

This exploded view of a Concept II .35 Front Rotary R/C shows its main components.
Note the aluminum cover on the crank web and the propeller driver fixed by a cotter pin driven through the crankshaft. The main ball bearing is also centering the front casting inside the main crankcase.

The view on the left shows how the bottom crankcase shell fits to the upper part by using a small grove on each side . The castings are of perfect quality, thin walled and lightweight. Note the integral cylinder head.


The piston crown is slightly roof shaped and carries an L ring. The lower end of the forged (or pressed) connecting rod is bushed and has a lubrication hole. The piston pin seems to be pressed in, I did not try to remove it.

Peeking into the cylinder casting from below reveals the transfer channels: the main transfer (at right center) and two pairs of boost channels visible at top and bottom. These channels are separated by a web which does not extend fully down into the crankcase. As there is no sleeve, the channels are open towards the piston and the the contact surfaces seem to be chrome plated.  My measurements show that all ports have the same timing.

The cylinder head shows a bell shaped combustion chamber, similar to a Tee Dee glow head.

The cutout at the left center provides clearance for the conrod - it is not a transfer port.

Concept II .40 Front Rotary R/C



Concept II .40 R/C (6.0 cm³)

Prototyped in 1968 (#2500)

A prototype of the front intake, Schnürle ported .40 Concept II R/C engine. This engine is very similar to the Concept II .35 R/C shown above and has the following differences:

  • increased bore (21.4 versus 20.2 mm)
  • different connecting rod
  • modified crank shaft,
  • different carburetor.
Measuring my sample engine gave the following results:


bore 21.4 mm
stroke 18.6 mm
swept volume 6.69 ccm
0.408 c.i.


Exhaust port 144°
transfer ports 125°
inlet timing 178°

The timing seems to be identical to the .35, indicating that indeed the only logical difference is enlarged cylinder bore. The carburetor is a metal design with a plastic ratchet clip, maybe from a 3rd party manufacturer (do you recognize it?). I have not seen a muffler for the Concept engines, only what is drawn in the part list.

The crankshaft and the aluminum web cover has been modified to accommodate the new conrod.

This view shows the forged conrod which has two lubrication holes at the lower lug. This conrod lug is bushed and externally filed into shape so that it fits the crankpin and its recess in the crank web as well as the crankcase without interference. As one would expect from an experimental prototype, this does not yet look like a possible solution for production.

Last modification of this page: 21.05.18

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