This extremely well-made 2.75 cc diesel was acquired on eBay in 2018 from an English seller. The engine is clearly of British origin, since all threads are BA, BSF or Whitworth.
Measured bore and stroke are 0.567 in. (14.40 mm) and 0.665 (16.89 mm) respectively for a displacement of 2.75 cc. The engine clearly had a back tank originally, since a 6 BA tapped hole is provided in the centre of the integral backplate. As illustrated without the tank, the engine weighs a checked 185 gm (6.52 ounces)
The engine exhibits some very unusual features. For starters, the use of cross-flow loop scavenging is unique among British sideport diesels of the seeming period. The arrangement of the front of the crankshaft is also highly individualistic. However, perhaps the most odd feature is that weird little vertical fitting on the top of the main bearing. It might be supposed that it’s an oil reservoir, but in fact its open end is stopped by a screw. It is connected to the crankcase through a conventional FRV valve in the main journal. Primary induction is of course sideport, but this seems to be an adjustable secondary FRV induction system to allow a degree of fine tuning. It’s timed for the conventional running direction. The FRV intake can be set anywhere between fully closed and wide open using the screw at the top.
The quality of the engine’s construction is truly superb. Everything is first-class, right down to the hex head screws used to retain the cylinder casting and the front cover. The state of the piston crown and contra-piston underside confirm that the engine has had a lot of use, but all fits remain absolutely beyond reproach. Compression is outstanding.
Normally I would have said that this was more likely than not to be the work of some very talented model engineer. That said, the use of die-castings would be rather unusual for a one-off. Someone went to a lot of trouble to produce those four dies! The high level of interest in the auction for the engine implies that a few bidders knew what this was but weren’t saying – one-off home-builds don’t normally command that level of interest.
Although I've received a number of suggestions, the most persuasive appears to be that put forward by Miles Patience. Miles and his very knowledgeable Dad reckon that this is a previously-unattested 2.75 cc diesel prototype originating from Ten-Sixty-Six Products of Worcester in England. The engine's architectural features certainly support this identification. If any reader knows more, I’d love to hear from you, either directly or through the blog site.