Beating the Drum—

The FROG 349

by Adrian Duncan

---

Click on images to view larger picture.

This time we take a look at a British classic—the FROG 349 diesel. I have to confess that this engine has always been a great favourite with me—I've been a continuous FROG 349 BB user since the mid 1960's and still fly them regularly. So if any of what follows reads like a bit of a plug, I'm OK with that!

My reasons for liking this engine so much are threefold. Firstly, it displays a great deal of technical interest in its design, being well out of the rut in terms of "classic" British diesel design. Secondly, it's very well made indeed and appears to run forever if properly used and maintained. And finally, it handles extremely well for a relatively large diesel and produces a good helping of very useable power at moderate rpm—extremely user-friendly and pulls a pretty substantial model. Let's go back to the beginning...

Genesis Of A New Design

The manufacturers of the FROG 349 for much of its production life were International Model Aircraft (IMA) Ltd. of Morden Road in the London suburb of Merton in Surrey. IMA was one of the oldest-established model aircraft manufacturers in the world, having been founded in 1931 as a subsidiary of the very prolific Lines Brothers organization, who had been major players in the toy industry since 1919 following a family involvement in that industry which had begun in around 1850. The trade-name "FROG" ("Flies Right Off Ground") was used by IMA from the outset for their kits and was carried over to their engines. Lines Brothers also owned the famous Triang toy brand, the triangle emblem of which represented the three Lines brothers who owned the company.

During the Second World War Lines Brothers not surprisingly stopped making toys and model products altogether and turned all their efforts to war production. 7,000 people were employed at the Merton factory, eventually making over 1,000,000 machine guns and 14,000,000 magazines for Hurricane and Spitfire aircraft, as well as shell cases, land mine cases and special optical apparatus to help troops see in the dark. In 1940 the Merton factory was partially destroyed by bombing, but production was maintained.

Following the cessation of hostilities, Lines Brothers resumed their former activities, including the manufacture of model products by their IMA subsidiary. Their refurbished factory at Morden Road was then one of the largest model production facilities in the world, and production of model engines soon commenced at the factory, the first such product being the 1.75 cc spark ignition motor which appeared in early 1946.

The Morden Road location was incidentally quite close to that of IMA's rival engine producers ED, who were located in nearby Kingston-on-Thames and began producing model engines in late 1946. The presence nearby of several scrap metal yards from which raw materials could readily be procured was doubtless of benefit to both firms.

IMA were notable for their interesting and often innovative engine designs. Their "bicycle spoke" series was well out of the design rut and gave many post-war modellers an excellent introduction to power flying. They were early in the glow-plug ignition field in Britain with their 160 Red Glow model, and went on to produce perhaps the best general-purpose British glow motor of the 1950's, the sturdy and dependable FROG 500 Red Glow. In the mid 1950's they introduced the diaphragm-induction FROG 149 Vibramatic, which was certainly a one-of-a-kind and worked surprisingly well. That model too appeared in a glow version in addition to its more common diesel form.

First details of the prototype FROG 349 were "leaked" to British aeromodelers by Peter Chinn in his regular Latest Engine News column in the November 1958 issue of Model Aircraft. The prototype appears to have made use of the rounded cooling muff of FROG's front induction 249. Chinn followed up in the March issue of his column with a picture of a production engine, now fitted with the straight sided cooling fins.

At the time, IMA and Keil Kraft advertisements alternated on the back cover of Model Aviation and June was Keil Kraft's turn, so although the June issue featured Chinn's full review of the engine, the first advertising announcement appeared in the June '59 issue of rival publication, Aeromodeller, whose review of the engine would not appear until December of that year. The following month, IMA's regular rear cover Model Aviation placement contained a distinctly low key entry for the 349, taking second place to their new kit for the Hawker Tempest. Regardless, the release represented the continuation of a FROG tradition which by then had become well-established, namely the introduction of model engine designs exhibiting a significant degree of outside-the-box design thinking.

Much of the credit for the 349 BB must go to George Fletcher, who was IMA's chief engine designer throughout the period under discussion. At this point, we should note that one of the individuals who worked with Fletcher at IMA in the 1950's was Gordon Cornell, who was to go on to become the chief engine designer at the rival ED company and was later to produce engines in his own right.

As of 1957, IMA had models in the popular 0.8 cc, 1 cc, 1.5 cc. 2.5 cc and 5 cc displacement categories. This left a gap in their coverage of the marketplace—the 3.5 cc category. It may be asked why this was important. There are really two reasons—firstly, a 3.5 cc engine that did not weigh substantially more than the 2.5 cc opposition could be used in a sport-flying context to give more zip to models designed originally for 2.5 cc engines, particularly in a control line flying and to many British modellers, this was an attractive proposition. Secondly, the rules for S.M.A.E. combat, which was extremely popular at the time, permitted engines of up to 3.5 cc. So there was a definite niche for engines of this displacement, particularly given the fact that control line was then still the most popular form of aeromodelling in Britain.

It's worth noting that at .21 cuin, the FROG 349 was just over the US displacement limit of .20 cuin for Class A. So although FROG engines were marketed in the USA, the American market clearly had minimal influence on the decision to develop the new model.

However they came to their decision, it must have been in mid 1957 that the IMA management asked George Fletcher to design a new model that would give the FROG line a presence in the 3.5 cc category, and to do so with a product that was clearly a FROG original in design terms. Radio control was just beginning to find its feet in the British modelling spectrum, and it seems clear that the new engine was to be designed to cater to both control line/free flight and R/C applications.

Fletcher must have relished this challenge, and set to work immediately. A prototype of the new design was soon in existence. It looked generally similar to the later production version, although it utilized a head and cooling jacket seemingly borrowed from the contemporary FROG 249 BB and undeniably needed a little "cleaning-up".

This prototype was sent to Peter Chinn, who was recognized at the time as Britain's leading tester of model engines as well as being an objective and knowledgeable critic. It appears that Chinn was frequently called upon to test and critique prototype engine designs by a number of British and even overseas manufacturers.

Following the completion of his test of the FROG 349 prototype, the results of which were not made public but were presumably communicated to Fletcher and IMA, Chinn published a photograph of the tested engine in the February 1958 issue of "Model Aircraft", noting the fact that it had been just "one of the manufacturers' experimental models tested during the (previous) year". At the time of publication of this photograph, the decision to put the new design into production had not yet been taken—indeed, Chinn specifically commented that the engine was "not likely to go into production".

However, towards the end of 1958 the IMA management had a change of heart and decided to proceed with the production of the new model. With this decision as a green light, Chinn published further details of the engine in the November 1958 issue of "Model Aircraft". The engine finally saw the light of day in production form in early 1959, some 18 months after its initial inception.

The entry of the new model into the marketplace was reported in the March 1959 issue of "Aeromodeller" in the regular "Motor Mart" feature. By May of 1959 examples of the engine were appearing regularly on British flying fields, and "Aeromodeller" reportedly had their own example running-in on their test bench. This being the case, it's a little odd that the "Aeromodeller" test of the engine (see below) was not published until December 1959!

It was stated in the "Motor Mart" feature for May 1958 that the engine was proving itself to be most impressive and had "already ...raised a few eyebrows in combat circles". The quality of the main casting was highly praised, being described as "the best crankcase casting yet seen on a British engine". So what made this seemingly-impressive engine tick? Let's find out...

Description

When Peter Chinn described the FROG 349 as "one of the most technically interesting diesels to appear from a British manufacturer for a long time", he was certainly by no means overstating the case. The new design fairly bristled with features which were new to the British model diesel design scene.

For a start, the cylinder design was refreshingly different. Cross-flow loop scavenging was employed along with a side exhaust stack, giving the engine a rather "glow motor" appearance. The only previous British diesels to adopt this style of scavenging were the ED Mk. IV Hunter (which dispensed with a stack) and the Series 2 ED Bee.

However, the porting on the FROG 349 was markedly different from that of the two ED models. The exhaust was more or less conventional, consisting of an adjacent pair of milled exhaust ports opening into a stack cast integral with the main crankcase on the left side of the engine (facing forward). However, these ports were cut through an extremely thick flange machined into the cylinder at port level.

Transfer was accomplished through five substantial holes drilled upwards through this flange at an angle. These overlapped the exhaust ports to a significant extent and provided good direction to the incoming mixture. The drilled transfer holes were fed through a bypass passage cast into the main crankcase in the conventional "glow motor" manner, albeit on the right hand side in K&B style. The result was a very free-breathing transfer system with a relatively long transfer period.

Somewhat unusually for an engine using this style of porting, the FROG was provided with a significant amount of sub-piston induction. One might expect that much of what was drawn in from the confines of the stack would be spent exhaust gasses! However, other engines such as the ETA 29 Mk. VIc used the same approach successfully.

The cylinder was conventionally located on an internal flange formed in the crankcase below the exhaust port level, with a gasket to provide a good seal. The cooling jacket was a separate turning in light alloy which fitted over the plain cylinder liner. An innovation here was the use of a thick nylon ring between the cooling jacket and the upper surface of the cylinder flange. The reason for the use of this nylon ring has always been given as the provision of a good oil seal between the jacket and the crankcase in order to confine oil residues within the exhaust stack, but this has always seemed a little thin to me. For one thing, it's somewhat less than effective in practise!

I've always suspected (but cannot prove) that the real reason for the inclusion of this component may have been rather more subtle and was possibly related to internal stress management. Internal stresses in large diesels are considerable, and Fletcher may have chosen this approach to help deal with this issue in the new 349. The nylon ring created some built-in resilience to the system and gave the cylinder the freedom to move microscopically against the hold-down system under the stresses of diesel combustion. It may thus have functioned to some extent as a "shock absorber" rather akin to the metal item used for the same purpose in the OK Cub diesels. There is no doubt that George Fletcher was aware of the OK diesels—he had used the O-ring contra-piston and the compression screw locking insert from those engines in the earlier Mk. I version of the FROG 80.

Nylon was also to be found in the cylinder head, which featured an internally-fitted threaded nylon insert which formed part of the compression screw thread and served as a friction lock to discourage movement of the compression screw during airborne operation. This was very effective in practise. The entire cylinder assembly was held down by four long screws which engaged with tapped holes in the main casting. A nice touch was the use of locking washers in conjunction with the assembly screws to encourage them to remain snugly tightened.

The other really innovative feature of the FROG 349 was the form of rear drum valve used. This was of the "inside out" variety in that the internal gas passage in the steel drum was open to the atmosphere via the downdraft venturi rather than to the crankcase as with the more usual crankshaft rotary valve or rear drum valve set-up. The inner end of the gas passage in the drum was sealed by an integrally-turned disc which was slotted at the perimeter to engage with a small spigot on the end of the crankpin and thus drive the drum. The induction port fed directly from the interior of the drum into the crankcase through a very short up-draft passage formed in the backplate unit. In effect, a crankshaft rotary valve in reverse!

This arrangement provides both a very direct induction passage and a smaller-than-usual crankcase volume since the interior of the drum valve does not form part of the crankcase volume during the engine's base compression cycle. In addition, the fact that the drum valve is un-stressed by the con-rod and does not have to transmit the engine's torque to the propeller allows the designer to take as large a "bite" as necessary for the internal gas passage and associated induction port to maximize efficiency without worrying about mechanical strength. Gordon Cornell was later to use the same concept in his fabulous and extremely rare little Dynamic .049 ball race diesels.

A slightly odd feature of the drum valve was the fact that while the port in the bearing itself was square, the corresponding port in the drum was round. This resulted in opening and closing being considerably less rapid than it might have been. Presumably this was related to the need to keep costs down. Another somewhat questionable feature was the use of only two screws to secure the backplate unit. It's true that this unit was unstressed, but the maintenance of a good seal could be problematic even with the use of a gasket. Like the cylinder screws, these two screws were equipped with locking washers.

The cast iron piston in the FROG had a very steeply conical crown, with the mild steel contra-piston being shaped to match. The piston was relatively heavy and the crankshaft was unbalanced. As a result, vibration levels were perhaps a little higher than they could have been, although the nylon ring mentioned earlier may have helped a bit in this respect and the engine was not a major offender in this regard. Presumably the fact that the piston was not lightened by internal milling and the crankweb was left in its plain disc configuration were cost-related design decisions.

At the outset, the FROG 349 was made available in two distinct versions. One used a conventional plain bearing of the Vandervell type, while the other used a single ball race at the rear in combination with a plain Vandervell bearing for the balance of the shaft. The use of a single ball race for the BB version represented a sound design decision—front races really do little for an engine's performance and longevity and add both weight and cost to the motor for a relatively limited return. The use of a rear ball race however has much to commend it in terms of providing stable low-friction radial support to the shaft at its loaded end. Bearing life should be greatly enhanced by this measure. It was likely for this reason that the plain bearing model of the FROG 349 apparently found few takers and was phased out fairly early on. This model appears to be very rare today.

The two variants of the FROG 349 were visually distinguished by the fact that the ball bearing version had a very attractive matte grey finish applied by vapor blasting, while the plain bearing version had a shiny tumbled finish. After the phasing-out of the plain bearing model, the tumbled finish was applied to the ball bearing version as well and the matte finish cases were no longer produced.

All of the castings used in the FROG 349 were of the highest quality and represented a striking demonstration of IMA's (or perhaps their contractor's) expertise in producing high-quality die-castings of quite complex components. The crankcase and backplate in particular were very impressive examples of the precision die-casting process applied to quite complex components.

The use of a downdraft intake kept the length of the engine quite reasonable and also placed the needle in a very convenient and safe location from the operator's standpoint. Another good feature was the use of a light alloy bolt to mount the propeller, in place of the more usual threaded extension of the shaft. In a hard crash, the prop bolt would bend or break rather than the shaft—a far less daunting replacement! Advantage was taken of this fitting to carry the necessary tapped central hole in the shaft back to a point just short of the plain crankweb, thus minimizing the weight of the crankshaft while maintaining the smallest possible crankcase volume.

Bore and stroke of the FROG 349 were a slightly over-square 16.93 mm and 15.24 mm respectively for an actual displacement of 3.43 cc (.209 cuin). Weight of the ball bearing model was 6-5/8 ounces, with the plain bearing model weighing in at a slightly lighter 6-1/2 ounces. These were actually quite reasonable figures considering the displacement and specification of the engine. Some 2.5 cc engines weighed only half an ounce less.

The cost-cutting measures noted above were clearly effective, since the price of the engines was by no means unreasonable considering the value for money that they represented. Price of the ball bearing model in 1959 was £3 19s. 2d. including purchase tax, while the plain bearing model sold for £3 13s. 3d including tax.

Overall, the new FROG presented the market with a relatively large-displacement diesel displaying a wide range of unusual features. It was also very well made indeed in addition to being competitively priced. Small wonder then that it quickly became a familiar sight on flying fields in Britain and elsewhere!

The FROG 349 On Test

The first published test of the FROG 349 was that published in the June 1959 issue of "Model Aircraft". Although unattributed, there's almost no doubt at all that it was conducted by Peter Chinn. The test focused on the ball bearing version of the engine, although the simultaneous availability of the plain bearing model was noted.

In the test report, Chinn commented upon the design originality of the engine and praised its compact and robust construction. He noted that it was the most powerful British 3.5 cc diesel then available, and was quite complimentary regarding its handling qualities, particularly with respect to its control response. He stated that there had been "more poor large diesels than good ones" but unhesitatingly placed the new FROG in the latter class.

One point upon which Chinn dwelled at some length was the fact that the FROG 349 needed a lot of running-in time to reach its optimum performance levels. He found it necessary to give his own example some four hours of running-in prior to taking his test readings, and noted that it was only after the third hour that the engine really began to show its true potential.

On test, Chinn was able to measure an output of 0.318 BHP @ 13,000 rpm. It may be true that this figure was matched or even exceeded by several contemporary 2.5 cc competition diesels, but those engines produced their peak power at significantly higher revs and were relatively costly in addition. The higher torque of the FROG 349 allowed it to produce a power output which matched the contemporary 2.5 cc competition diesels (and beat some), but it did so at significantly lower rpm. Hence it produced a lot of very useable power and could swing a large and hence efficient prop very comfortably. It was also priced very competitively by comparison.

Overall, Chinn's test was a highly positive endorsement of the new model, and it must have done much to encourage more modellers to give it a try.

Although the rival "Aeromodeller" magazine had reportedly had their own example of the FROG 349 running-in as of May 1959 (as noted earlier), it was not until December 1959 that Ron Warring's test report was finally published. To make up for this, Warring elected to test both the plain bearing and ball bearing versions of the 349.

Once again, this test was very favourable. Warring described the engine much as Chinn had done, and noted that the 349 appeared to be built to rather closer tolerances than previous FROG engines. He cited this as the reason for the engine requiring an extended running-in period, again just as Chinn had done. He described the handling characteristics as "very good" and repeated Chinn's comment regarding the non-critical control response.

One issue on which Warring focused in particular was the potential of the 349 for radio control use. He conducted a number of experiments with speed control and found that the sub-piston induction represented a considerable impediment to effective throttling, as one would expect. He decided that the best results could be achieved with a combination of an exhaust restrictor fitting and an intake throttle. He drew attention to the fact that the intake of the engine was equipped with an oversized boss at the needle valve location to allow for drilling-out and installation of a throttle, and the exhaust stack was dimpled to make provision for a bolt-on exhaust fitting.

It seems possible that Warring did not run his examples in for as long as Chinn had done, because he was unable to match Chinn's performance figures. He found a power output of 0.3025 BHP @ 12,200 rpm for the ball bearing model, with the plain bearing version achieving 0.28 BHP @ 12,000 rpm.

Later History

It appears that the plain bearing version of the FROG 349 did not attract much sales attention and was dropped relatively quickly. As a result, this version of the 349 is quite rare today—in all my years of flying and collecting, I have personally yet to encounter one in the metal.

It's not hard to understand why this was so. The plain bearing model was only some 6 shillings cheaper than the ball bearing version. It was also a little less powerful while being only fractionally lighter in weight. Finally, it would never have stood up to long hard use as well as the ball bearing version. The outlay of an extra six shillings must have appeared more than worthwhile to most purchasers.

At some point after the plain bearing model was discontinued, the matte finish formerly used on the castings of the ball bearing version ceased to be applied. Instead, the same kind of shiny tumbled finish which had previously been reserved for the plain bearing model was now applied to the ball bearing version. This change has led quite a few people to believe that they have a rare plain bearing model, when in fact they have a later ball bearing example.

In 1960 an R/C version of the 349 was introduced. This had a neat bolt-on exhaust collector which rendered the sub-piston induction more or less ineffective and thus improved the throttle response. It also allowed for easy connection to an exhaust pipe for getting exhaust residues away from the model. A simple barrel throttle was installed in the oversized boss which had been present all along on the intake venturi. Weight crept up to 7.1 ounces as a result of these additions.

Despite FROG's efforts with this model, the R/C glow engine had by this time reached a relatively high level of effectiveness and user acceptance and was generally considered superior to the diesel for applications requiring speed control. Consequently, the diesel FROG R/C model appears to have had trouble drawing much sales attention—examples are relatively uncommon today. It actually seems a little odd that IMA never tried a glow version of this engine for R/C—it might have worked out quite well.

The FROG 349 made an excellent marine engine thanks to the very convenient location of its intake relative to the flywheel. It was quite popular in this form, and I recall seeing a number of them in use at the local model boat pond. The exhaust collector noted above was particularly useful in this application and kept the interior of the boat far less messy than it would otherwise have been.

In 1962, IMA ceased all model engine and kit production. Naturally, this included the FROG 349 BB and also (sadly) the outstanding Viper 149 diesel which had been introduced only a year previously and was one of the best FROG engines ever made. At the time when production ceased there were significant numbers of completed engines in existence, and these continued to be sold off by the AA Hales organization of Potters Bar in Hertfordshire, just north of London. Hales were one of IMA's major distributors and also produced their own range of model kits under the "Yeoman" label (who remembers the "Dixilander"?).

The cessation of engine production by IMA left George Fletcher looking for fresh employment, which he soon found with the nearby ED company. Interestingly enough, he replaced his former colleague Gordon Cornell as chief designer for ED, the latter having left ED following some philosophical differences of opinion with the ED management at the time.

Returning to the FROG 349 story, the Hales organization must have detected some level of continuing demand for the FROG range, because in 1964 they made arrangements with Lines Brothers to have all of the FROG engine tooling transferred to Davies-Charlton Ltd (DC) on the Isle of Man. Thenceforth the FROG engines were manufactured by DC Ltd. under contract to A. A. Hales. The box style was revised from the former IMA "standard" box to a revised and slightly larger box made of slightly less sturdy material. A. A. Hales name replaced IMA on the box and, as we shall see later, the serial number sequences were restarted with a code letter preceding the digits, thus making it easy to tell the difference between all IMA and DC made engines.

Not all of the FROG engine range went into reproduction. The abandoned designs included the outstanding 1.5 cc Viper diesel and its glow relative the Venom, as well as the Vibramatic 149 diesel and the rather unsuccessful FROG .049 glow model.

It must be said that the quality of the FROG engines appeared to suffer somewhat by the change of manufacturer. It's my personal impression that the 349's (and other FROGs) made by DC never quite matched the IMA originals for consistency in terms of quality. However, they generally worked OK and at least spare parts continued to be available for a few more years.

As far as we can determine, the final consumer market advertisement for the AA Hales produced FROG engines appeared in the April 1966 issue of "Aeromodeller" magazine. Although IMA were still producing the plastic kit range in great volume, they ceased advertising in the "Aeromodeller" after June 1966. The April '66 ad included the 349, so it seems to have survived to the very end of production of the mainstream FROG engine range, being in production for over seven years. There was an engine known as the FROG Venom .049 marketed for a few years in the early 1970's, but this was just a red-head radial mount version of the DC Wasp, but this was simply a case of badge engineering—a DC Wasp with the FROG name added.

How many examples of the 349 were made? Like all FROG engines, the 349 was numbered sequentially, the serial number being hand-engraved rather than stamped. The lowest 349 serial number in my collection bears the number 471, while my highest-numbered example carries the number that I'd thought to be 27230, suggesting that about 30,000 engines had been made. As it turns out, that leading "2" is actually the letter Z! The fact that these numbers were engraved by hand makes it easy to mistake one for the other, but this is highly significant as the leading letter indicates that it was manufactured by Davies Charlton Ltd, not IMA! Let me explain and digress a bit...

One of the most rewarding outcomes arising from the publication of articles like this is the receipt of further information from interested and knowledgeable readers. In the case of this page, after the first version appeared in December 2008, we were contacted by Kevin Richards (England), who is best known as an expert on the ED range, but also knows his Frogs pretty well. Kevin set us straight on the serial numbering and production period, requiring some serious editing of the page.

The first issue that required attention was the startling correction to the serial numbering. It appears that in the beginning of 1964 when AA Hales arranged for DC to continue manufacture of certain models in the Frog range, they introduced the use of a letter prefix to identify the model in question. This was a very sensible idea as IMA had simply numbered the engines of each displacement sequentially starting and consequently generated complete sets of duplicate serial numbers! The approach taken by AA Hales eliminated the duplicates and made each number unique. The following table shows the system used:

One of the remaining question is whether they continued the sequence started by IMA, or whether they restarted the sequence using the letter prefix to differentiate between the older and new models. There is strong evidence to suggest that they restarted the sequence. For starters, Kevin is aware of an IMA Frog 150R bearing the number 28031 and a later, DC-made Frog 150 Mk III with the lower number of F19920.

Of course, it's possible that the 150 Mk III was seen as a different engine, justifying a sequence restart, but the 349 numbers support the global restart hypothesis. The highest serial number known to us without the Z prefix is 13225 (owned by an Australian collector friend of mine). The lowest numbered Z prefix example of the engine known to us is Z7223. This is pretty hard evidence that the count was restarted when DC Ltd took over the manufacturing. The highest Z-prefix serial number known to us is Z8189.

The last question is whether they restarted at 00001, or some higher number. If we assume they and IMA started at one, it now appears that IMA made at least 13,225 examples of the 349 between January 1959 and some time in 1962 when they ceased engine manufacture. Less than 4,500 examples a year; not all that great a production record, but remember that the engine was in a displacement category that was a bit off the mainstream. If we assume that DC Ltd restarted the count in 1964 when they commenced manufacture and that there were no gaps in the sequence, then we appear to have evidence that at least another 8,189 examples of the engine were made. So the total produced appears to be around the 20,000 mark.

Kevin provided three other valuable points which are well worth recording. Firstly, the engine number Z8189 had a red-anodized cooling jacket and Kevin has seen at least one other engine that was similarly equipped. It appears that this represented a last-ditch attempt by AA Hales to tart the engines up a little and promote more sales. Secondly, Kevin confirms from personal experience that some of the later Hales examples of the 349 came in boxes which were orange rather than green. Finally, Kevin is able to confirm that although the engines ceased to be advertised as of 1966 (as stated in the original article) they remained available through dealers until at least 1970. Kevin bought new examples of both the 349 BB and the 249 BB from his local hobby shop in that year, and he still has the 249 BB new in the box complete with 1970 date-stamped factory guarantee!

This cannot of course be taken as hard evidence of continued manufacture as of that date—it’s possible that they were simply selling off old stock by that time. But it does confirm that the engine remained available from dealers for a considerably longer period than we had supposed.

Our sincere thanks to Kevin Richards for his generous help in setting the record straight!

User Impressions

Here I can speak with some authority because I've been using the FROG 349 BB continuously for over 40 years now! I have two examples which receive semi-regular use, and I wouldn't like to guess how many running hours they now have on them. I've had to rebore one of them, but the other is still going strong on the original bore. These engines really last if you fuel them and use them correctly!

The FROG likes a fuel with at least 2% amyl nitrate or equivalent, but seems relatively unresponsive to greater percentages of nitrate. I suppose this is because it is a moderate speed engine. On un-doped fuel it runs rather harshly—the nitrate really smooths it out. The engine also likes at least 25% oil and has a marked preference for the oil being of the castor variety, probably because it tends to run very hot by diesel standards—another reason not to overdo the nitrate! The plain bearing at the front of the shaft wears relatively quickly unless you apply such lubrication. It's also very important to balance the prop very carefully if you want that bearing to last.

I find the FROG very easy to start as long as you don't flood the crankcase. If you do that, the thing tends to become quite vicious and can go into hydraulic lock mode very easily. And being a largish diesel, it can clobber your flicking finger hard! I usually just choke to fill the fuel line, give it one choked flick (and no more) and then go at it with a good hard flick to start (wearing a rubber hose finger shield!). A light prime helps sometimes, but keep it light—the 349 starts on just a smell of fuel. This is especially true of hot re-starts—just a whiff of fuel is enough.

Once running, the engine delivers on its reputation. Controls are very responsive without being overly sensitive, and there's ample feedback to help you to get the setting right. Once set correctly, I find that a well run-in example like both of my "flyers" will start and run at the same settings—very user-friendly! Vibration is present, but is less than one would expect—perhaps the nylon cylinder hold-down ring noted earlier helps here?

Suction is very good, and the engine runs very consistently in the air. It does tend to get very hot, though, and seems to do best on a slightly rich mixture.

Prop choice is a bit of a puzzle. An 8x6 prop under-props the engine, while a full 9 x 6 seems to be just a little too much prop for optimum results. For control line, I use a "fat" 9x6 nylon cut down to 8-1/2 x 6, which gives a nice wide-bladed prop which really moves some air and will survive a crash with no problems. The engine turns this at a little over 11,000 rpm on the ground, which should allow it to reach its peak in the air. Prop it for higher speeds on the ground and you'll go past the peak in the air for sure. Remember that this is not a high-speed engine!

But it can be; at one time (long, long ago!) I specialised in tuning FROG 349's for S.M.A.E. combat, and one of my two flying examples is such a modified motor. The 349 was actually a good motor for combat because it had plenty of useable power and its stack and rear intake meant that it didn't fill up with dirt every time it hit the ground.

I used to mill out the piston interior to lighten, drill out the comp screw and prop bolt interiors for the same reason, apply a little counterbalance to the crankweb, replace the standard contra-piston with a cast-iron item having a squish bowl, grind the transfer ports upwards a little to increase their overlap with the exhaust and thus reduce the blow-down period slightly and square off the induction port in the drum valve to match the port in the casting. If all of this is done right, you end up with an engine that is slightly lighter than stock, has very little vibration and gives a far later PAW 19D CT a really good run for its money!

Conclusion

The FROG 349 was a very popular engine, and deservedly so. It remains one of my own favourite engines for flying purposes, and I expect to keep using mine until I can no longer fly! I also consider it to be among the most interesting and innovative British diesel designs of the 1950's.

Because of the engine's popularity over a considerable period of time, examples in fine condition remain relatively obtainable and affordable today. Anyone who wants to try one should be able to find a reasonable example without too much trouble. And I recommend that you do so! This one is a real British classic which is sure to be appreciated by anyone who likes model diesel engines.