The (very large) B-51 Marine Diesel

Name B-51 Designer unknown
Bore 19.9 mm Stroke 29.50 mm
Type Compression Ignition Capacity 9.2cc
Production run over 300 Country of Origin USSR
Photo by David Owen Year of manufacture circa 1950


The B-51 is a little-know Russian made marine diesel. The photos and following text come to us from fellow Motor Boy, David Owen, designer of the Mate, and others.


I believe this engine was made in Russia in the 50s. It has a bore and stroke of 19.90 x 29.50, giving a swept volume of 9.20cc. Weight with bare shaft is 750gm. This engine is numbered 311, so one would expect that other examples must still exist. Further specific information on the B-51 is most welcome.

This engine was given to me by a friend, following a recent eBay purchase. It appeared to be in good condition, though dirty and missing the flywheel and coupling and the needle valve assembly. One of the brass nipples on the water jacket was broken off.


Following disassembly and cleaning, a suitably vintage looking nva was fitted and new brass water-cooling nipples were made, using the one remaining original as a pattern. A flywheel and coupling will be made when the size and shape of the original part are known. In the meantime, anxious to see the engine running, a propdrive assembly was made and fitted to the tapered seat on the crankshaft.

All working parts are well-designed and built to a high standard. The steel cylinder and crankshaft are hardened and ground; the latter running in a bronze bush. The piston and probably the contra piston are cast-iron. The steel conrod is fitted with a bronze big-end bush, whilst the little end appears to be a tight fit in the steel wrist pin, the latter running in the piston bosses.

The cylinder is ported like any typical side-port design and the piston has a milled baffle in the crown. The gravity die-cast aluminium crankcase is very similar to the original Dyno, having a separate belt section carrying the inlet and dual exhaust stubs. The cylinder itself is secured through a threaded flange by long M4 bolts which pass through this belt into the lower crankcase. The alloy water jacket screws over the cylinder flange and is sealed with a thick gasket. The venturi inlet has a rotating choke, common on early ignition engines.

Running and Conclusions

Fitted with an 18x6 Punctilio prop which it turned at 4700rpm, the engine started easily and ran very well. Without proper cooling, prolonged running was not possible, but the contra piston moved as it should and the exhaust residue was very clean throughout the runs.

At first glance, I thought the B-51 was a clunky, Soviet era product of typical mediocre quality. However, that is far from the case and the engine would have been a most satisfactory unit in a large-ish model boat, giving sterling service for Russian hobbyists.




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