Clanford Clan 0.24 cc
The Clanford Clan 0.24 cc Diesel Revisited
By Maris Dislers
Back in the August 1992 issue of “AeroModeller” magazine, you’ll find a review by Richard Herbert of the then-new 0.24 cc Clanford Clan diesel. An attractive little piece of work that lacked spunk when tested, hence drawing a rather unfavourable review overall which has dogged the engine’s reputation ever since. Examples pop up now and again on the second-hand market, often New In Box because the engine’s reputation led many people to view them as collectible curiosities rather than practical model engines.
The Clan 0.24 cc diesel is not to be confused with the far earlier Clan 0.9 cc model from Scotland which appeared in early 1947, passing through several variants before vanishing from the scene in mid 1948. The story of this entirely distinct earlier engine has been covered in a separate article on this website.
The original Clanford Clan 0.24 cc diesel which is the subject of the present article was commissioned in 1991 by the well-known model engine enthusiast and writer Mike Clanford. Mike is best known today for his authorship of the very interesting but at times misleading 1988 book entitled “A Pictorial A to Z of Vintage and Classic Model Airplane Engines”. The original Clan 0.24 cc diesels were made in England by a firm called Prototype and Precision Engineering. The availability of the engine was first announced in the February 1992 issue of “AeroModeller”.
Bore and stroke of this neat little unit are 6.40 mm (0.252 in.) and 7.50 mm (0.295 in.) respectively for a displacement of 0.24 cc (0.015 cuin.). The engine weighs in at a measly 24 gm (0.846 oz.) complete with tank. The engines were supplied with a supposedly matching KeilKraft 5x3 nylon propeller.
The original batch of some 500 or so engines sold out fairly quickly. Some years later, Mike Clanford arranged for the production of a number of further examples by the CS company of Shanghai, China. These were manufactured in both 0.24 cc and 0.48 cc displacements. More of these below.
After spending time with a New In Box Clan 0.24 diesel from the original 1992 batch that recently came our way, we were “disappointed” that we could find nothing wrong to justify the Clan’s not so lustrous reputation. Actually, all fits were excellent and it proved to be a very capable performer. We formed the impression that Richard’s example was indeed below par. Accordingly, in order to balance the record we decided to present our test results for engine no. 0297 as representing one of the best of its type.
The Clan’s design is so simple that the late Ron Chernich drew up a set of Motor Boys International plans for home constructors. The attached General Arrangement drawing from Ron's plan set includes the engine's basic timing parameters for those interested.
Moreover, Ron went even further than this by publishing a detailed article describing the construction of a Clan 0.24 cc diesel from the Motor Boys plans. In his article, Ron mentioned the periodic occurrence of transfer port errors, which I think might be confined to some of the previously-mentioned Clan engines made by CS. I can only report my own observations of one engine from the original 1992 batch, which was found upon inspection to be free from any such errors. Adrian Duncan advises that his two examples nos. 0170 and 0316 are also correctly configured, both running well.
After 25 years in the box since its factory test run, our Clan 0.24 no. 0297 had to be disassembled and thoroughly degummed. Had we read Richard’s report more carefully, we would not have wasted time trying to start an engine with crankcase compression leaks via the cylinder to crankcase joint and the two cylinder retaining screw threads that go right through to the inside. They were originally sealed by the makers. An important point not mentioned in the very brief instruction sheet.
We fixed this potential problem with a smear of Loctite 243 thread locker on the cylinder seat and balsa cement on the screw threads. These were left to cure overnight. Balsa cement has the advantage of letting go easily when required, especially if soaked in acetone beforehand. We also added a paper backplate gasket, just to be sure.
We have often emphasised the key role of the piston/cylinder fit in determining an engine’s performance. Not just that little extra, but with the miniatures it can be the dividing factor between a good performer and a real disappointment. Achieving the perfect fit is not an easy task at the Clan 0.24 cc diesel’s size, and its dead parallel cylinder bore leaves very little margin for error.
Mike Clanford advised owners that the presumably usual “tightish” fit takes lots of gentle running in. Probably true, but we’d rate piston fit on our example as excellent, hence having no such problem. An excessively loose fit would be a problem, however. In such a case, starting will become difficult and the engine won’t tolerate a decent propeller load. If your example feels “soft”, try upping the oil content to 40%. Use castor oil, of course.
Precise priming is the key to easy starts with tiny diesels. You just can’t expect finger choking or dousing it from a filler bottle not to flood the engine. It appears that Jose Maria González-Calatayud Saiz has fallen for the old trap of over priming, as seen in this clip We’ve all done it at some time, so I find life easier if I get the priming technique and initial compression setting for short bursts down pat before even fuelling up the tank or finding the mixture setting. I use a disposable syringe with blunted 18G needle. Syringes with one-piece plastic plungers are better than the regular type with rubber pistons.
Mike Clanford’s recommended fuel mix of “1/3 each ether, kerosene and castor oil with a touch more ether” (to around 40%) proved to be bang on for the Clan. That extra ether isn’t mandatory, but helps a little when starting. The idea is to introduce a little fuel into the crankcase and get the engine going more or less on the ether fumes alone with a bit of flicking. The very wide explosive limits of ether vapour in air greatly facilitate this.
Our routine ended up being pretty much like Richard Herbert’s. Needle around ½ turn open, but no finger choking. One or two drops in the venturi from a syringe was far less likely to lead to flooding. Compression set a bit below peak, and a few flicks had it going, sometimes after backing off compression to clear any excess prime.
The real surprise on test was that the included KeilKraft 5x3 propeller is really too small for the job! As our performance curves show, power output rises sharply to near peak level by 6,500 RPM. Thereafter, torque is exchanged for more RPM, but power output remains essentially flat at 0.013 BHP all the way up into the 10,000 RPM range. We saw no point in squeezing the engine much past 10,000 RPM, as it becomes less pleasant to operate and the wear rate will inevitably go up.
Actually, the best flight propellers are likely somewhere in the 6x4 to 6x3 bracket for decent torque with a reasonable capability of throttling back with reduced compression to around 5,000 RPM for trim flights. We found a 7x3 prop to be about the largest practical option. Nice flywheel effect from that, but could be too much with a tighter piston fit.
The CS Clan Models
As mentioned earlier, Mike Clanford arranged for the manufacture of an extra batch of engines by the CS company of Shanghai, China, which has since abandoned the model engine manufacturing field. These examples were manufactured in two displacements – the “standard” 0.24 cc model and a double-sized variant of 0.48 cc displacement. These engines appear to have been manufactured in comparable numbers to the originals.
Fortunately it’s very easy to distinguish between the “original” 1992 Clan 0.24 cc engines and their latter-day CS counterparts. For starters, the CS versions have carburettor bodies made from aluminium alloy rather than the moulded high-density black plastic employed in the British-made originals. In addition, the backplates of the CS renditions are sealed with an O-ring as opposed to a paper gasket or no gasket at all.
Another defining indication is the style of identification markings. The markings on the original British-made models are confined to a serial number which is hand-engraved on the outer end of the left-hand mounting lug (looking forward in the direction of flight). By contrast, the CS examples have the name “MIKE CLANFORD” stamped onto the top of the cooling jacket. They also have the designation “Clan 0.24” (or “Clan 0.48”) stamped onto the upper left-hand side of the crankcase, along with the serial number stamped (as opposed to hand-engraved) on the outer end of the left-hand mounting lug.
These CS renditions generated a great deal of discussion on some of the better-known on-line model diesel forums like RC Universe, much of it unfavourable. It appears that a number of CS Clan 0.48 cc owners couldn’t get them running, or if they did, they experienced considerable changes in mixture strength throughout the run, with the engine getting very hot (lean?) near the end of the tank. Several experimenters tried fitting a 3 mm O/D tube inside the standard venturi, burring it at either end to keep it in place to act as a sleeve for the original tube. This reportedly improved suction to the point that the engine became easier to start and would run the tank out reasonably well. However, it still tended to get very hot near the end.
One very knowledgeable owner of the 0.24 cc version of the CS Clan was the late and much missed David Owen of Woolongong, Australia. He reported having run his example and experiencing similar problems. In his case, the piston/cylinder fit was not exceptional and the engine lost power as it warmed up. In addition to this, the venturi/tank assembly left some room for improvement. David sleeved the venturi bore slightly, but though there was an improvement, the result was still not satisfactory.
The effective choke area on the tested example of the original made-in-England Clan (no. 0297) is 0.84 sq. mm, which according to my calculator should work fine at around 7,000 RPM. As a comparison, a Mills P.75 has 3.5 sq. mm effective choke area, which by proportion to swept volume (three times larger) is a good deal more generous. Of course, the Mills runs better at chugging RPM if choked down, but seems OK as delivered as far as most users are concerned. So I'd say that choke area is not a problem with the original Clan 0.24. If CS duplicated those dimensions, then any suction problems must be down to some other cause. However, I have no first-hand data for the CS variants.
Regardless, it does appear to be entirely possible that much of the less-than-stellar reputation of the Clan 0.24 cc diesel stems from negative experiences with the CS renditions of the engine. Neither I nor website editor Adrian Duncan have found any issues about which to complain with the original British-made units from 1992.
The reputation of the Clan 0.24 cc diesel has never really recovered from the rather negative review published by Richard Herbert in “AeroModeller”. Ever since the appearance of that test report, the engine has been widely seen as a cute collectible and nothing more. Ron Chernich’s later comments regarding problems with the cylinder porting have done nothing to alleviate this perception of the engine. Finally, negative experiences with the later CS versions seem to have done their part to further undermine the engine’s reputation.
We hope that the present test has demonstrated that a good example of the original engine (which do exist – I have two of them! A.D.) was actually a very capable performer for its size. There’s no guarantee when you buy an example on eBay or similar, but if you have the opportunity to examine the engine before buying it and are satisfied that its piston/cylinder fit is OK, you’ll be getting a well-made and very useful miniature diesel for small free flight sport or scale models.
Article © Maris Dislers, Adelaide, South Australia
First published July 2017