Here’s an interesting 6.3 cc diesel from my own collection which was obtained some years ago on eBay. When I first got it, I had assumed that the odd side-mounted carb might have been the result of someone assembling the engine incorrectly. However, on closer examination it turned out that the engine is actually designed to run in that configuration. Indeed, it will only run that way - there are cutaways in the piston and cylinder which have to be in the right places for the rod to clear and the bypass to work.
The sand-castings look a bit rough, but they're perfectly sound. The general standard of machining and fitting is very high – the I-sectioned steel rod in particular is a work of art! Overall, the engine is very well made indeed, and proved on test to be both an easy starter and a very fine runner. It’s clearly of British origin – the cheese-head screws are the first giveaway in that area. The cylinder and head hold-down screws are 4 BA, the backplate screws are 6 BA and the prop nut is of all things 5/16 - 26 BSB (British Standard Brass) - a very odd thread indeed, but it does exist in Jolly Olde, and no-where else. Better not lose that prop nut!! The shaft has a main journal diameter of 3/8 in exactly.
Measured bore and stroke are 0.749 in. and 0.872 in. respectively. It appears that the nominal bore and stroke were intended to be ¾ and 7/8 in. respectively for a nominal displacement of 6.33 cc (0.386 cuin.). Quite a big diesel! The engine is a bit of a lump at all of 418 gm (14.75 oz.).
A noteworthy feature is the odd location for the intake. As I said, the engine will only run with the intake where it is due to the internal geometry of the design, so that’s how it was intended to be. Question is – why?!? There must have been a compelling reason to position it in this rather vulnerable location. The only logical reason that I can think of is the fact that it allows the engine to be mounted right up against the firewall in a model while retaining clear access to the carburetor and tank. It seeems likely that the maker had a specific application in mind when making this engine. The very small fuel tank implies that the engine was intended for use in a free flight context, but that’s about all that can be deduced.
Another weird feature is the fact that the gudgeon (wrist) pin is retained in the piston using a piece of safety wire which passes through a hole drilled through the pin and one of the bosses and is then secured by looping back and twisting around itself! Quaint, but effective! And then there’s the needle valve – tensioned with a simple lock-nut (the spring in the running image was added by myself for convenience during testing). Unusual ........................... Finally, the connection between the carburetor and cylinder strongly suggests a maker with a previous steam background!
As things stand, I’m pretty certain that this engine is the work of a talented model engineer and is probably a one-off. If anyone knows more, please get in touch through the email link provided!