BMP 3.5

by Adrian Duncan

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Unfortunately there's not much to tell about this motor. The B.M.P. 3.5 was manufactured by Bijou Mechanical Productions (Bournemouth) Ltd., who were located, as their name suggests, on the English south coast in Bournemouth, Dorset, a little east of Poole. The engine was introduced at the start of 1947 with distribution handled by JW Kenworthy of Bournemouth. By May, distribution had shifted to Henry J Nicholls Ltd of London. John Goodall, commercial model engine dealer and one-time editor/publisher of Model Engine World (MEW) claims that the engine was distributed by Model Aircraft (Bournemouth) Ltd—better known as "Veron", but no evidence has been found that supports this assertion. In Issue #1 of MEW, he relates how the engine introduced him to engine collecting and collectors, having such an influence that he named his company BMP (Barton Model Products). Regardless, BMP engines did not remain on the market for very long. HJN advertised it infrequently during the period 1947-48, but by 1949 it had disappeared. It does however enjoy the distinction of being one of the first, if not the first English diesel to feature a twin ball-race crankshaft.

Bore and stroke of the engine are 14.99mm x 19.40mm for a calculated capacity of 3.44cc. The engine is a bit of a lump at all of 10.125 ounces including tank—heavier than some much larger engines! The addition of the four extra holes at the front of the very substantial engine bearers (which also doubled as webs for the main bearing) did little to improve this situation and served merely to add some individuality to the engine's appearance! The engine is undeniably very sturdy, but the cost in terms of weight seems a little excessive.

According to Fisher's book, the early B.M.P. 3.5 cc diesels displayed no serial numbers, while the later examples did have such numbers. My own illustrated example bears the serial number 34, stamped in rather small figures on the forward end of the left-hand exhaust "stack" (viewed from the rear) just to the side of the stamped initials "B.M.P" which appear on the front of the transfer port casting along with the stamped designation "3.5 cc" just below. The serial number can be seen in the left front view photograph, and the other engraving may easily be seen in the right front view.

Another set of letters and figures appears on the crankcase below the right engine bearer (viewed from the rear). This series is cast into the crankcase in relief in very small letters and reads PM 909. It must be some kind of identifying mark for the casting rather than the specific engine, and may indicate that the production of the raw un-machined castings was contracted to an outside firm who thus required some means of differentiating the castings made for B.M.P. from those made for others.

The engine is of the conventional side-port pattern. There are two exhaust ports set slightly towards the rear of the cylinder rather like those of the Series I ED Bee. These feed through substantial stub exhaust port "stacks" cast into the aluminum alloy cylinder barrel, which is separate from the lower crankcase. This cast alloy barrel also incorporates a "bulge" at the front to accommodate the single bypass passage. Mixture is transferred from the bypass into the cylinder by way of a single quite large transfer port cut through the cylinder liner at the front. This transfer port overlaps the exhaust port to a significant extent. Port durations are in fact unusually long for an engine of this type—clearly the makers were hoping for some reasonable operating speeds and were prepared to sacrifice a little low-end torque to get there.

Overall, the engine is very well made, especially where it counts. All fits are exceptionally good and the main castings are pleasingly polished. A nice touch is the fact that the engine bearers are milled on their undersides to ensure that they are both level and parallel, a refinement employed by ETA and others to help ensure a rigid mounting. Another feature is the provision of a cover plate over the front bearing. This is mounted just behind the prop driver and is held in place against the open end of the front bearing housing by three screws. It is clearly visible in the photographs. The very laudable intent was clearly to exclude grit from the bearing. Frog were to emulate this feature years later with their 249 twin ball-race diesel, albeit with a non-metallic form of seal.

The fuel tank is also a casting which is held in place by the same 4 screws which retain the crankcase backplate, thin paper gaskets being used to ensure a seal between the machined surfaces. The backplate thus serves as the front wall of the fuel tank. The engine can readily be run without the tank—one simply removes the four screws, sets the tank aside and secures the backplate once more by re-installing the four screws.

The filler cap is interesting—it does not screw into its hole, but snaps in, being retained by the fact that its hollow spigot is split at four points on its circumference and has a small protruding ring of material surrounding its inner end. The splits allow this ring to contract to pass through the hole and then to spring open again to retain the cap in position.

One unusual feature for a British diesel is the fact that, on this example at least, an aluminum contra-piston was employed. This was quite common practise on the Continent, but is a very rarely encountered feature in a British diesel. When the engine is cold, the fit is very easy, but when the engine really warms up the contra-piston sticks in place and cannot be shifted until the engine is allowed to cool down for a while. That said, it is true that once a setting is established the engine will readily start and run on the same setting, so this is perhaps less of a problem than it might otherwise have been. Nonetheless, a cast-iron contra-piston gives a far better measure of control while the engine is running. Perhaps this was a somewhat futile weight-saving measure!?

This example was equipped with a spring-loaded choke flap, which is now missing although its mounting remains brazed in place. Other examples of this engine, including the one already illustrated on your site as well as the one illustrated by Fisher, are missing this flap, but it appears to be an original feature of the illustrated example. The example illustrated in Peter Scott's little book has the flap in place. The spring apparently held it in the open position, and a timer could presumably snap it shut to stop the engine when desired.

My example has a conventional compression screw with a tommy bar, as does the example which was illustrated by Scott. Others, including the example featured in issue #1 of MEW, and in Peter Fisher's book have a slotted head compression screw to accommodate the ubiquitous penny for compression adjustment (shades of ED!). I'd hazard an initial guess that the early models had no serial numbers, were missing the choke flap mounting and used the slotted compression screw, while the later ones had serial numbers, featured the choke flap and used a conventional tommy bar compression screw. We'd need more data to confirm this, and these engines don't exactly fall out of trees.

The company also made a radially-mounted 0.9cc twin ball-race diesel. The only advertisement for this engine appeared in JW Kenworthy's Aeromodeller placement of January, 1947, although it is mentioned in the British Diesel Summary by "Artifex" that appeared in the Aeromodeller of May, 1947. These are definitely the proverbial rocking horse droppings—I've never so much as heard of anyone having an example! But they did exist and a few doubtless still do. You can see the family resemblance in several features, notably the exhaust porting, in this photo from page 22 Laidlaw-Dickson's book, Model Diesels (second editon, 1947).

I've run my B.M.P. and have found it to be a fine starter and runner. It feels positively silky when turned over after a finger-choke—really nice! Fits are generally beyond reproach, although this example has been used and there is some very slight play just starting to develop in the con rod bearings. This is only detectable when the engine is very "dry".

The motor is dead easy to get going using standard procedures, but the aluminum contra-piston means that you need to be quick with the compression setting to get it right before the engine gets too hot! Under-compression won't harm anything—the tank can safely be run out in that condition. However, if you find out too late that you've set the thing over-compressed, it's necessary to stop it before it lugs itself to a grinding halt (potentially doing itself all sorts of damage in the process), allow it to cool, reduce compression as much as you judge to be required and then try again, noting the desired setting prior to starting and making sure that you get the compression screw to that setting before the engine warms up fully. Once a good setting is established, you can leave it alone—the motor will start and run on the same setting with no trouble at all. In effect, you can treat it as a fixed-compression diesel at this point!

A slightly cut-down 10 x 6 prop seems to be a good match for the engine, which seems very happy in the 8,000—9,000 rpm range. Running is very smooth and torque appears to be very good in this speed range. Generally speaking, the engine is quite a sturdy performer for its type, but its power to weight ratio is still rather less than stellar. By comparison with the soon-to-be-released Amco and ED 3.5 cc diesels, it was strictly no contest; the B.M.P. could never have survived their introduction for long.