Kemp 1 cc FRV diesel

The Kemp 1 cc FRV Diesel

Here I’ll present the first-ever published test of an almost-forgotten British 1 cc diesel from the post-WW2 pioneering era of over 70 years ago now – the original 1947 Kemp 1 cc model from Gravesend in Kent, England. This neat little unit was among the first products of the Kemp Engines company which was established in late 1946 by Harold Kemp and was succeeded in mid 1948 by the K Model Engineering Co., also of Gravesend. 

In an earlier article to be found on this website, I evaluated the first attributed product of the new company, the 4.4 cc Kemp K4 diesel of early 1947. I say “first attributed” because there’s a slim but completely unproven possibility that Harold Kemp’s first model engine design was in fact the beautiful but fragile 6.4 cc Keil K6 spark ignition engine of 1946.

A close connection between Kemp and Eddie Keil is attested by the fact that Keil was an early distributor of the Kemp engines. That said, the identity of the designer and manufacturer of the K6 has never been authoritatively confirmed. It must also be admitted that the K6 shows little evidence of Harold Kemp's later design and manufacturing styles.

Whether or not Kemp designed and perhaps manufactured the K6, there’s no doubt at all that the first model engine to be marketed under his own name was the Kemp K4 sideport diesel design. This was a very well-made and fine handling unit, albeit sadly lacking in the performance department. In that context, it must be recalled that Harold Kemp was very much on the learning curve, as were all of his fellow British model diesel designers at the time. At least Kemp had the courage and conviction to pursue his own design path rather than following the lead of OwatLeesil and Airstar in effectively cloning existing Continental designs.

The K4 was soon joined by the 1 cc crankshaft front rotary valve (FRV) model which is the main subject of the present article. This model seems to have appeared in around April 1947, a few months after the K4.

The present article was prompted to a certain extent by the need to correct some mis-information that has got out there. On page 35 of his very useful and entertaining but occasionally misleading “Collector’s Guide to Model Aero Engines”, the late O. F. W. “Peter” Fisher referred to the existence of “the rare K 1.9”, which was the subject of Fisher’s illustration no. 81 on page 102. “Rare” is hardly an adequate word – such an engine never existed! Don’t spend too much time looking …………

The engine which appears in Fisher’s illustration is undoubtedly an example of the 1 cc model which is the subject of the present article. It bore the serial number 762, which fits well with the known sequence.

Another reason to take an interest in the 1947 Kemp 1 cc diesel is the rather odd fact that it was replaced in March 1948 by the Kemp (later K) Eagle 1 cc model, which was a sideport engine. This reversed the usual trend, which generally saw manufacturers of sideport engines making the move to rotary valve designs rather than the reverse. AMCO, AllbonDavies-Charlton and E.D. all come immediately to mind. The circumstances surrounding Harold Kemp’s strange "reverse" design progression with his 1 cc models merit investigation.

I won’t go any further into the background to the appearance of the Kemp 1 cc diesel, since I’ve already covered the full history of the Kemp venture in other articles, including the one about the K4 diesel mentioned earlier. In addition to that article, I’ve also written about the Kemp Hawk, the Kemp Eagle, the K Vulture and the K 2 cc models in other articles which may still be accessed on the late Ron Chernich’s “Model Engine News” (MEN) website. The full story of Kemp Engines and its successor company, the K Model Engineering Co., may be found in those articles.


The concept of utilizing compression ignition in miniature aero engines originated in pre-WW2 Europe. Although it became well established on the Continent during the war years, the British focus on the conduct of the war resulted in the principle not becoming widely known in Britain until the conclusion of the conflict. As a result, early post-war British developers of model diesels were playing catch-up behind a significant number of Continental designers, many of whom already had four or five years of experience as of 1945.

It took some time for British constructors to get to the point at which they were ready to release their first commercial designs. The first commercially-advertised British diesel was the Leesil 2.41 cc model from Bradford in Yorkshire. This was a fairly close copy of the Swiss Dyno model from 1941 with a slightly increased displacement. An advertisement for the Leesil was published in the May 1946 issue of “Aeromodeller” magazine. Despite its advertising precedence over the others, it never really got off the ground, although a few were in fact produced. 

The first British diesels to reach the market in any numbers were the Mills 1.3, initially from Sheffield and later manufactured in Woking, and the Owat 5 cc fixed compression diesel from Bradford. These two models appeared more or less simultaneously in July 1946. They were followed later in the year by the B.M.P. 3.5 cc model from Bournemouth and the Majesco 2.2 cc diesel from Parkstone in Dorset. E.D. joined the party in early 1947 with their 2 cc Mk. II sideport model, as did Harold Kemp with his previously-mentioned  4.4 cc Kemp K4 diesel. The FROG 100 diesel did not actually reach the market until mid 1947, although prototypes had appeared at the Model Engineer Exhibition of December 1946. The AMCO .87 diesel made its debut in August 1947, with the 5 cc Davies-Charlton Wildcat Mk. I and the 2 cc Aerol Gremlin appearing at about the same time.

The Kemp 1 cc diesel was thus released into a British marketplace which was just getting off the ground and was very much in expansion mode at the time. Despite their relatively high prices in the context of the cash-starved early post WW2 British economy, model engines were in such high demand that any engine which would start and run reasonably well most of the time and stay together while doing so was pretty much guaranteed a sale.

Although Harold Kemp’s first engine, the 4.4 cc K4 sideport model, was evidently selling quite well, reference to the list previously presented here indicates that it was one of the larger diesels of the early 1947 period – only the Owat 5 cc model was larger at the time. There was clearly a strong potential market for smaller engines, which would be cheaper to run in terms of fuel costs and would also suit smaller models which were both less costly to build and easier to transport. It was to tap into this promising market that the Kemp 1 cc diesel was introduced, probably in about April 1947.

The Kemp 1 cc offering was preceded into the British marketplace by the B.M.P. 0.9 cc diesel from Bournemouth. However, this sparsely-advertised unit did not make much of a mark on the national scene, evidently being marketed locally for the most part and falling by the wayside quite quickly. It is a mega-rare engine today.

Kemp's main opposition initially came from the excellent Mills 1.3, which quickly came to dominate the small-displacement diesel market despite its rather steep asking price of £5 5s 6d (£5.27). Neither the AMCO .87 nor the FROG 100 diesel reached the market until August 1947. The mid 1947 Clan 0.9 cc diesel from Scotland never really made significant inroads into the British market south of the Border. Accordingly, at the time of its release the Kemp 1 cc diesel effectively had no domestic competitors in its displacement category.

Partially for this reason, Kemp felt able to charge a price of £5 even for the 1 cc model, thus undercutting the Mills 1.3 by a small amount. This was still a very steep price indeed at a time when a man earning £8 a week before taxes would have been considered relatively well off. However, the novelty appeal of such small engines encouraged people to save up and buy them regardless, even at such a price. Kemp was evidently able to sell all that he could manufacture.

Now that we’ve seen the Kemp 1 cc diesel launched onto the market, let’s have a look at the engine itself.

Kemp 1 cc Diesel – Description

The 1947 Kemp 1 cc diesel was a basically conventional crankshaft front rotary valve (FRV) long-stroke diesel. It was produced during the era in which British precision engineers preferred to work in fractions of an inch rather than their decimal equivalents. Nominal bore and stroke dimensions were 13/32 (0.406) in. and ½ (0.500) in. respectively for an actual displacement of 0.0647 cuin. (1.06 cc). The manufacturers were clearly not too worried about exactly achieving the engine’s nominal 1 cc displacement – after all, they were not facing any competition displacement limitations. All-up weight as illustrated with fuel tank and spinner was a fairly healthy 110 gm (3.88 ounces).

The engine was built up around a sturdy main casting which incorporated the crankcase, beam mounting lugs, bypass passages, exhaust ducts and backplate as a single unit. The upper portion of the casting above the exhaust ducts was externally threaded to accommodate the internally-threaded screw-on cooling jacket. The early examples of this engine used cooling jackets made from magnesium alloy, while later examples like that illustrated here featured aluminium alloy components.

The steel cylinder was provided with two rectangular exhaust ports, one on each side, with two transfer ports located between them fore and aft. The transfer ports were supplied by bypass ducts fore and aft which were formed in the main casting, while the exhausts discharged through openings formed in the upper casting at the appropriate locations. The transfers overlapped the exhausts to a significant extent.

The lower cylinder was a drop-in fit in the upper crankcase casting. The cylinder was vertically located by a flange machined onto its outer surface a little way above the exhaust ports. This bore upon the upper surface of the crankcase casting, the joint being sealed with a paper gasket. The screw-on cooling jacket was provided with an internal shelf which bore upon the upper surface of the cylinder flange, securing it in place. This was a good arrangement in that it eliminated any assembly stresses which might cause distortion of the cylinder bore. The same system was later used by Mills Brothers in their Mills .75 Mk. II of 1949.

The steel piston featured an aluminium alloy carrier for the conrod small end.  This was swaged in place by means of a location hole in the piston crown. Since I haven’t dismantled my example, I can’t comment on the rod material. If Harold Kemp ran true to form, the rod was probably steel. There is no sub-piston induction.

The main bearing housing was a separate bolt-on component which was attached to the crankcase with four small countersunk machine screws. The steel prop driver had a plain surface with no serrations, presumably to allow for prop slippage in the event of a crash. The hexagonal spinner nuts on most examples are made from magnesium alloy, although my example was missing this component as received, hence having a replica made from aluminium alloy. It’s possible that some of the later examples were so equipped when new, but I can’t confirm this.

A notable feature of the front housing is the seemingly minute size of the intake. This has an internal diameter of only 0.125 in. (3.175 mm). A surface jet needle valve system is employed, meaning that the only component traversing the venturi throat is the tapered end of the needle. Estimating the average diameter of that tapered section in running position as around 1 mm, reference to Maris Dislers’ invaluable Choke Area Calculator yields an effective area of 4.796 mm2. The worksheet goes on to tell us that a 1.06 cc engine using a venturi of that area needs to be run at a minimum speed of 9,275 RPM to provide even borderline suction. Surprisingly enough, this implies that the intake could actually have been made even smaller with some advantage.

The needle valve assembly is of most unusual configuration. The intake venturi structure at the top of the main bearing is laterally drilled through at a nominal diameter of 3/16 (0.1875) in.  The “spraybar” is a substantial aluminium alloy tubular component of the same diameter which is a press fit in this transverse hole. It is cross-drilled 0.125 in. dia. to match the intake diameter so that when it is installed and correctly positioned the intake and the cross-drilled hole in the “spraybar” are in alignment. 

The “spraybar” is centrally drilled and tapped 10 BA from the longer “needle” side to accommodate the externally threaded needle. The fuel delivery side is however drilled out to 0.125 in. as far as the cross-drilled hole which aligns with the intake.  A separate brass fitting comprising a fuel jet, 0.125 in. dia. installation barrel and fuel tubing spigot is pressed into this hole as far as the venturi throat. 

The entire assembly thus relies on press fits to remain in correct alignment. The use of a press fit is important in that one has to remove the “spraybar” to gain access to the upper front housing retaining screws should servicing or tightening become necessary. The downside is that it wouldn’t take too many removal and replacement cycles to wear the fit to a state of excessive looseness. My example had reached that stage as received, forcing me to use a dab of JB Weld to hold it for running purposes. If servicing became necessary, the JB Weld joint could easily be broken. That said, a bolt-up assembly would be far more serviceable.

This isn’t the end of the complexities either! Needle tension was arranged in a manner which is unique in my personal experience. A small vertical slot was cut into the “spraybar” at the rear to face the upper front housing retaining screw on the needle side. This slot extended all the way into the threaded hole for the needle, exposing the needle at that point. A wire hairpin spring resembling half a safety pin had a bent end inserted into a small forward-facing hole in the front of the main bearing housing’s mounting plate, while the other end aligned with the slot in the “spraybar”, thus bearing directly upon the needle at that point. This spring is seen in the accomanying illustration extracted from Mike Clanford's very useful but often misleading "A-Z" book. 

A hole for the spring installation is provided on each side of the mounting plate, allowing the needle valve assembly to be fitted from either side. These holes are clearly visible in the previously-attached front view.

All of this complexity must have added considerably to the cost of producing the engine. In addition, the friction of the hairpin spring against the needle would do the thread at that point no favours at all. Frankly, one wonders why this arrangement was felt to be worth the trouble! My example has long ago lost its original hairpin spring, now employing a conventional coil spring for needle tension. This works perfectly.

The original needle had a spring connecting the working needle with its control arm, allowing for adjustment with the control disc held well back from the prop disc. This arrangement is visible in the above illustration taken from Mike Clanford's book. My example has lost its spring, instead having the control disc mounted directly on the end of the working needle. Frankly, I prefer this arrangement myself - those long spring-equipped control arms quite often spring their way into the prop disc. 

At the rear, the nicely made fuel tank was retained by a brass stud and nut which engaged with a tapped central hole in the integral backplate. The earlier tanks were made of magnesium alloy, while the later examples were of aluminium alloy. The brass filler cap was of the snap-in variety, having a segmented spigot which compresses to allow the cap to be fitted into its retaining ring. A neat brass fuel supply spigot at the bottom completes the component.

Like all of Harold Kemp's products, the quality of construction of my example of this engine is beyond reproach. It may look a little unimpressive on the outside, but inside where it counts there's nothing at all about which to complain. All fits remain absolutely first class despite the considerable use which this example has clearly received.

Now, having described the engine in sufficient detail, it’s time to find out how it runs!

The Kemp 1 cc Diesel on Test

My illustrated example of the Kemp 1 c model bears the serial number 0852, which incidentally is the highest serial number of my personal acquaintance for one of these engines. It has clearly seen considerable use, but remains in perfect mechanical condition in all respects. Despite the changes to the needle valve control, the needle itself appears to be original. Consequently, those control changes would have no bearing upon the results obtained.

I elected to use the standard back tank for all the test runs.  I actually like testing engines which have their own built-in tanks – there’s a satisfying sense of self-contained compactness in operating in this manner.

I began with an APC 8x6, which I felt would probably be the largest load that this engine would comfortably accommodate. This proved to be a good guess! The initial start turned out to be a bit of a chore – a lot of flicking with essentially no results. No doubt the presence of a considerable amount of storage oil contibuted to this. 

Once the engine eventually began to come to life, I quickly learned that it was unusually sensitive to the amount of fuel in the cylinder - either too much fuel or too little, and it wouldn’t even fire. The amount of fuel required wasn’t large, but it had to be somewhere near right.

The important thing turned out to be to avoid getting too much fuel into the crankcase during starting. If that happened, the engine would bog down and either oscillate or stop altogether. I found the best approach to be a single choked flick to fill the fuel line followed by a “dry” prime with the exhaust port closed. This generally produced a start in quite short order.  Even so, I would not classify this engine as an easy starter – while it's straightforward enough once you have the required approach down pat, it does take a bit of knowing along with some experience.

Once running, things improved a lot. The engine was extremely responsive to the controls without either the compression or needle setting being unduly critical. Both controls held their settings perfectly. Running was quite smooth, although there was the occasional misfire which couldn’t be completely eliminated. Perhaps this was related to the marginal suction predicted by Maris Dislers’ calculator, as reported earlier. 

I have to say that the results achieved fell somewhat short of expectations. The following prop/rpm figures were recorded.




8x6 APC

7x6 APC WB

7½x4 APC WB

8x4 APC

7x6 APC

7x5 APC

7x4 APC















These figures imply a peak output of around 0.028 BHP @ 6,200 rpm, with an unusually sharp drop-off thereafter. The engine appeared to be extremely reluctant to operate above 6,500 rpm. By no means a stellar performance, although we must remember that this unit was designed during the previously-noted period when British designers were just cutting their teeth in terms of diesel design. Moreover, the Kemp 1 cc diesel was one of the first, if not the first, such models to enter production in Britain. It’s always the pioneers who make the mistakes from which they and others learn!

On the plus side, the engine was a very well-made and sturdy unit which should give many hours of good service. With an 8x4 airscrew fitted, it would have had plenty of power to fly a free-flight sport model, although a control line application might be asking a little too much of it. I’m sure that a good number of these engines gave full satisfaction to their owners once the trick of starting them was mastered!

The Aftermath

The Kemp 1 cc diesel seems to have sold reasonably well during its relatively limited production life of perhaps ten months or so. It was still being advertised by Henry J. Nicholls in February of 1948, still at the same price of £5 exactly. Unfortunately, HJN’s advertisement confirms that as of early 1948 the competition from other manufacturers had increased to the point where this price was no longer competitive. Neither was the engine's performance. A watershed had clearly been reached.

Despite its relatively high price, it would appear that Harold Kemp had succeeded in making and selling a fair number of these models up to early 1948. The lowest serial number of which I'm aware is 030, while the highest such number is found on my own engine no. 0852. This implies that the numbering sequence started at 001 and that perhaps as many as 900 examples were made in total. Not a bad record for a small 3½ man company that was also producing the Kemp K4 and (as of late 1947) the 0.2 cc Kemp Hawk.

However, by early 1948 it was obvious that if Kemp was to remain in the 1 cc market, a new model having a better performance coupled with a lower price would have to be developed. Such a replacement design duly appeared in March 1948 in the form of the Kemp Eagle, which was priced at a somewhat more competitive £3 18s 6d. (£3.92).

This was a rare instance in which a manufacturer who had previously been employing FRV induction switched back to the old tried and tested sideport system. In this case, however, it must be recalled that the original FRV 1 cc design had not proved to be a stellar performer. Harold Kemp decided to try what could be done with a radically redesigned engine using sideport induction allied to the emerging “high performance” diesel technology of 360 degree radial porting. At the time of writing, this was the only example of which I was aware of an engine of this period which combined these two features.

Kemp also switched to short-stroke internal geometry when planning this model. The Eagle featured bore and stroke measurements of 0.437 in. (11.10 mm) and 0.400 in. (10.16 mm) respectively for a displacement of 0.98 cc. The idea here was presumably to keep the overall height down - the long-stroke sideport designs of the day tended to be taller than ideal for mounting purposes, particularly when cowling was involved. The design certainly achieved this objective, although the rather cumbersome oversized fuel tank spoiled the effect to some degree.

Despite its sideport induction, the resulting engine was a considerably stronger performer than the original 1 cc FRV design. My own testing of a typical early example of the Eagle (serial number 76) revealed a peak output of 0.048 BHP @ 6,700 RPM. Clearly the new design produced significantly greater torque than the earlier model, since the peaking speed is only 500 RPM higher but the measured peak output is increased by over 70%.

This notable improvement in performance was undoubtedly due to Harold Kemp’s very clever application of 360 degree radial porting to a sideport design coupled with the switch to short-stroke internal geometry. Even so, the Eagle was quickly rendered uncompetitive both in price and performance terms by such 1 cc competitors as the FROG 100 Mk. II and the E.D. Bee, to say nothing of the Mills .75. After the take-over of Kemp Engines by Len "Stoo" Steward and his K Model Engineering Co. in mid 1948, work was put in hand to develop a replacement. 

The result was the December 1948 appearance of the K Eagle Mk. II. This reversed Harold Kemp's earlier transition by making the more usual switch back from sideport to FRV induction. The K Eagle Mk. II featured Steward's "trademark" combination of updraft FRV induction allied to radial porting and the use of a "dog collar" for cylinder retention. It sold for the rather startlingly low price of only £1 17s 6d (£1.87 today), thus undercutting all of the contemporary opposition by a considerable margin. Despite this, it only survived for some 5 months, disappearing from the market by May 1949.  

The engine was also available as a fully machined kit requiring assembly at an even lower price of only £1 7s 6d (£1.37). This kit included all major components fully machined and ready to assemble, including a factory-fitted piston/cylinder/contra piston set. The only custom fitting required from the constructor was the lapping of the crankshaft journal to fit the main bearing.

I’ve covered the full story of the Kemp Eagle and its later FRV Eagle Mk. II successor in a separate article which may still be accessed on Ron Chernich’s “Model Engine News” (MEN) website.


The original Kemp 1 cc diesel was a very worthy effort by a fledgling designer/manufacturer who was still very much “learning the ropes” at the time of its introduction. Being among the first British entries into the 1 cc field, the engine undoubtedly attracted considerable buyer attention and doubtless served many of its users well. However, for the same reason it also fell somewhat short in the design department. It’s greatly to Harry Kemp’s credit that he recognized the limitations of this model as quickly as he did and moved so creatively to come up with a suitable replacement in the shape of the Mk. I Eagle with its unique combination of sideport induction allied to radial porting and short-stroke working geometry.

I hope that you’ve enjoyed this look at another of the pioneering products of what was to become Britain’s world-leading model diesel manufacturing industry during the “classic” era! It was from engines such as this that Britain’s model engine designers learned their most important lessons ….… and learned well!


Article © Adrian C. Duncan, Coquitlam, British Columbia, Canada

First published September 2018