This one was a real puzzle! It's a very well-executed unit which appeared to be a commercial product of some kind. Trouble is, no-one could put a name to it! The only real suggestions came from two distinct individuals, both of whom noted certain features which are characteristic of the earlier work of Hans Drenkhahn of Germany, best known for his later Dremo range. However, no substantive evidence to confirm this connection was presented at the time.
The engine in question is a 2.44 cc plain bearing crankshaft front rotary valve (FRV) diesel featuring cross-flow loop scavenging along the same lines as that introduced in late 1956 by Enya with their groundbreaking D15-I diesel and further developed in the MVVS 25D-1958 from Czechoslovakia (now the Czech Republic). Checked bore and stroke are 14.90 mm (0.587 in.) and 14.00 mm (0.551 in.) respectively for a displacement of 2.441 cc (0.149 cuin.). The engine weighs in at a healthy 174 gm (6.14 ounces) as illustrated, although this includes my own steel prop mounting sleeve nut and washer which were made to replace missing originals. I have no idea whether or not they match the originals - they just look right and do the job.
I also can’t confirm the correctness of the Enya needle valve assembly which I used to replace the missing original on the basis that it was a perfect match. The Enya assembly is a good match for many engines, so this proves nothing. A different assembly could of course have some slight effect upon the engine’s weight.
On test, the engine proved to be a good starter and a great runner. It was found to develop around 0.290 BHP @ 12,900 RPM - a very respectable performance for a (presumably) late 1950's plain bearing diesel.
IDENTIFICATION MADE! - after much digging and delving, this engine was positively identified as a previously-undocumented example of the work of Hans Drenkhahn of East Germany. It is a Dremo 2.5 cc diesel dating from c. 1959. A full description and analysis of this very rare motor may be found elsewhere on this website, along with a summary of Hans Drenkhahn's life and work.