Aspin Rotary Valve Engines

 Last update Mar2019
More drawings to come when traced
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Essay on Frank Aspin and his rotary valve engines
Old magazines reproduced
Patents downloadable as PDF
Factory artwork cleaned and full size
Aspin factory floor photos
Live Aspin 1 - 500cc single with Velocette gearbox in trials frame
Live Aspin 2 - 6 cylinder bus engine
Live Aspin 3 - 200cc single on Honda bottom end
Live Aspin 4 - 200cc single on Kawasaki bottom end
Aspin was not alone is pursuing rotary valves. An additional table details material from other engineers regarding their rotary valve developments.

All Aspin engines known to me can be accessed via the table below.
201903 Added Aspin & Meriden factory Triumph 650 drawings
201812 Added V8 drawings
201810 Added traced DOT factory drawings
201809 Added Bob Tait DOT drawing

An introduction to Frank Aspin and his rotary valve engines

After the First World War there was a great deal of innovation concerning the internal combustion engine. This was the era of Ricardo and Cross, of special fuels and sleeve value engines, which were expected to be the dominant design since they could run a higher compression ratio on the available petrol and produce more power.

250cc DOT/Aspin motorcycle intended for 1953 IOMTT. Drawing by Bob Tait.
Some traced drawings @ P53 below

All images and text copyright JohnWood 2000-2019, if you reference please credit. click here to contact John Wood

Here are the engines I know about, some details are sketchy and some only mentioned in dispatches. Some surviving art work indicates that there were at least nine different prototype rotor designs.
The dates are occasionally vague, or not available. Wish I knew!
ProjectID SizeDateDrawing?Photo?
#1 1933NoRotor#1
#2 NoRotor#2
#3 No Rotor#3
#4 Rotor#4
#5 No Rotor#5
#6 No Rotor#6
#7 No Rotor#7
#8 No Rotor#8
#9 No Rotor#9
P51 1321953No Chainsaw with conventional 2stroke engine
P53 2491954No 249cc four-stroke twin for DOT motorcycles
P57 2641956No No 264cc two-stroke twin
P81 No No 2stroke diesel with quarter speed rotor
P13186001944No 8.6L bus engine (200 bhp)
4600 NoLeyland 4.6L truck engine in low-loader for Vineys Transport at Preston
2215 No No Wolesley 6/80 (2215cc)
20881953No No 2.0L Ferguson Tractor engines
2000 No V8 - designed but not built
1800 No Ferguson tractor
18001954 1.8L flat 4 aero engine
1250 Hayes industrial engine
10961944No No Flat 4 aero engine (used for Army tests, and in Morris Minor)
937 No 1.0L Ford-8 (24 bhp Ford, 37 bhp Aspin)
650 Meriden Triumph 650cc air cooled parallel twin [71mm x 82mm]
6001949No Flat 600cc twin for Hardnett car in Australia, and a Lea-Francis designed Aspin car
5001939No No BSA twin (development abandoned by BSA), built at BSA to Aspin drawings.
500 No No 500cc air cooled parallel twin [71.42mm x 61.89mm] split air/fuel rotor
3501939 Velocette in BSA frame for long term test
3501939No No Velocette single in Mk5 KTT frame (development abandoned by Velocette), built at the Aspin works.
2501933No Development engine based on Rudge 500cc Ulster bottom end (67x70.5mm)
30 No No 30cc power generator and 2cc model aero engine
No V10 uniflow 2-stroke (blew up on test bed)
No Aspin factory floor in Bury

Summary of the essay about what did Frank Aspin ever do for the Engine ?

In my view Frank Aspin did quite a lot for the Internal Combustion Engine though, rather like the Romans, you cannot see much now without actively looking. It is difficult today to see the diversity of technical innovation, and the uncertainty of direction that was obvious and evident in the 1920's and 1930's. It was far from certain that the poppet valve that is used today would actually be the mechanism of choice. Indeed, the great Harry Ricardo in his book "The High Speed Internal Combustion Engine" thought that the sleeve valve engine would probably be the victor. It ran higher compression ratios on the same fuel, gave better torque, and was less prone to detonation. So it was in this environment that Aspin (and Cross and Minerva and many others) thought that the rotary valve engine should have their chance, others thought that the sleeve valve engines had already won. During WW2 more horse-power was generated by sleeve valve engines than any other type, examples are the Bristol Hercules engine, and the Napier Sabre 24 cylinder 5,500 hp engine.

This article is about Frank Aspin the development engineer, not about him as a person, and I have few such details to include anyway. The Company Secretary for the Aspin Company was a friend of my mother, and I "inherited" some papers from him. I have tried to get help to build an Aspin engine based on a 600 Yamaha engine, but solving the many problems that plagued him with new materials technology and plastics. I asked for help (just information, not money) from several large concerns involved in the motor trade, they just did not want to know me. I tried several Polytechnics to have a student project started as a joint venture, again no money involved, but they did not want to know either. Why am I surprised when it has happened to so many previous British endeavours before.

Our story starts with Frank Aspin acquiring a Rudge Ulster single cylinder motorcycle engine and converting it to a simple 250cc rotary valve head. This was in 1933, and legend has it that it cost 10/- (that is ten shillings in old UK money, or half of one British pound Sterling). He claimed 18 bhp at 7500 rpm, 31 bhp at 10,000 rpm, with 14:1 compression ratio, and maximum rpm at 14,000. By 1937 it was reported in Motorcycle that the Aspin was running at 12,000 rpm with a compression ratio of 17:1, but both Aspin and Cross reported that their engines ran well at compression ratios over 20:1, by 1960 Aspin claimed that with further work to be able to give 50 bhp. Such things dreams are made of.

Nor was Aspin the only dreamer. Velocette of Hall Green Birmingham were interested to build an Aspin Velocette, and devoted much time to the project. they converted a 350 KTT on the promise that it would produce 35 bhp compared with the existing 25 bhp. After much work, and many seizures it eventually produced 22 bhp, thus less reliable and less powerful than the existing bikes. Early attempts by the Cross rotary were no more successful, their entry in to the 1935 TT races on the Isle of Man ended with one bike not fast enough to qualify and the other stopping after two laps with "plug trouble". Laurie Bond seduced Norton with a similar dream nearly twenty years later, and again the final result was a rotary valve motorcycle engine that was less reliable and less powerful than the best of breed existing engines.

The F.M. Aspin Engines Ltd. Company was eventually drawn down by the closure of the Bury Engineering Company with debts of over twenty thousand pounds (Sterling 0.5M today) and just at a time when the Amal carburetor factory had supply problems and stopped the Aspin chain saw factory for four weeks. The bank (as sympathetic then as they are now) called in the overdraft and closed the Company. I believe that the worry of all these affairs had adversely affected Franks health.

The Aspin engine had its sceptics and its supporters. Mr. Bamford of JCB fame was a supporter and admirer, and I believe did run the Wolesley 6 for several years, but alas there is no trace of it now. In one of several letters to me he said "Unfortunately we don't have any trace now of the famous Aspin rotary valve [engine] which I am sure would have been a wonderful thing had Frank lived to perfect it". Brian Woolley the journalist was a supporter, he wrote to me that "It is quite astonishing that so soon after his death so much has been forgotten about Aspin's work", but he was "not impressed by their [journalists of the era] unquestioning acceptance of everything that Frank Aspin told them". His parting gesture to me was "Best of luck - I think you'll need it!". An article by Brian Woolley concludes "the single sleeve valve could be revived today. With an aluminium alloy sleeve, Nickasil plated inside and out and running in a light alloy cylinder .... ..... who knows what results might be achieved? I believe .. but what am I saying?"

The most famous sceptic was P.E. Irving the legendary motorcycle engineer and designer. His views and those of others are encapsulated in the paragraph below.

It may all have been a mistake. It was certainly not a scam, as Frank was fully committed to the project, and he was funding it with his own money rather than primarily asking for other people's money. I summarise here what is the sceptic view of his work. Look again at the original claim for a modified Rudge, over 30 bhp at 14,000 rpm. this is far and away above any power or revs from a Rudge of the time or indeed any other motorcycle engine of the time. they would literally have blown themselves to pieces. Nor would the main bearings and big-end's have taken kindly to 14:1 compression ratios, but these can be (and were) strengthened in the Aspin development engine. The real difficulty lies in the fact that there was no independent testing of Frank's work, and that all the articles and news reports are adapting press releases made by Frank personally. Any engineer or scientist who does not expose his work for peer testing risks being labelled a sham. Here we have an engine running at 14,000 rpm where all contemporary engines, including the Rudge on which it was based, could only make half that. The sceptic view is that "well, you must have ****ed up the rev counter drive then!". Not such a silly suggestion, actually., though you would think that you could easily tell the difference between 7000 and 14,000 rpm without a rev counter, right or wrong. The maths of bhp means that if you get the revs wrong then the bhp is wrong for any given torque measured on a dynamometer. Some of the real world testing showed the engines to be lack-lustre and prone to seizing when worked hard. The same dyno and rev counter would not have been used on the car and truck engines, so it cannot have been a common error affecting every test. I cannot make up my mind, but the engines seemed to sparkle in the laboratory and be rather unimpressive in daylight.

Now let us discuss the engines. I did read somewhere that the Aspin design was based on earlier work by the Reverend Peacey, but I have not been able to find out much about him. If you (Dear Reader) know anything about the Reverend then send me an e-mail

During the course of this article I hope to discuss the following engines, but at the time of writing I do not have all the details that I would like

The Early Years 1932 - 1937
rotor1 rotor2 rotor3
The 250cc single cylinder test engine started life as a second hand Rudge engine that was given a new barrel and head. the basic design may have drawn on the earlier work by the Reverend Peacey who had similar ideas, but the Aspin design had a steeper cone angle. The Rudge engine was a popular choice in the day, as Cross also selected this engine to build his prototype. In the prototype engine, Aspin made a cylinder head (presumably from solid) with a cylindrical rotor having a port cut in its side driven from the top using helical gears, the dimensions were 67mm bore and 70.5mm stroke. The diagrams with this section show the three stages of rotor development during the formative years, starting with a cylinder then proceeding with a hybrid conical section driven at its mid-point to a full conical shape driven from the top. The first design, the cylinder, rotated at half engine speed and the sparking plug was in the top. Sealing at the bottom is by a small beveled edge, and there is a piston ring around the top. Gas tightness around the port depends on quality of fit, so machining and distortion are important. This design was quickly superceded by a design of part cylinder and part conic, again with the spark plug in the centre. The combustion space in Mk2 is eccentric permitting an increase in the area of the port. This rotor was made of phosphor bronze running in a cast steel insert, which itself was a press fit in the alloy cylinder head. The performance was good and the oil consumption high, and the spark plugs had a bad time of it. Enter design Mk3 with a 60 degree angle cone.. The spark plug is moved to the side and therefore for the most part is not subject to any hot gas, and is only expected to fire a relatively rich mixture, the larger fuel droplets are flung to the outside though overall the fuel:air ratio was quite lean. the rotor is now case-hardened nickel steel running in an aluminium bronze housing. Performance was again very good but the motor needed careful warming through since aggressive use when cold would be certain to cause a seizure of the rotor.

During this time, and indeed for many years after, Aspin was able to build an engine with good reliability but at the expense of burning oil, if the design or clearances of the rotor were such to stop any oil being burned and clean up the exhaust then the reliability dropped out of sight. By detail design Aspin tried to find a middle way including special metal coatings on the rotor, nothing was very successful, the whole situation is too marginal. It needs to be simply better by design (pun intended), it needs to be more obviously correct and less marginal and better able to take abuse. Later developments used fabricated rotors and even added water cooling without bringing the hoped-for breakthrough. The original engine with its cylinder rotor running "vertically" in its air cooled head was running on a test rig by 1933. The early development is undocumented, and the workshop notebooks are lost in the 1970's clear out along with any remaining hardware.

The Early Years period ends with the Aspin modified Velocette 350cc being tested without success by the Velocette factory in Hall Green Birmingham. The only report that I have came from a sceptic so the view is slanted, but it is clear that despite several suggestions from Frank Aspin, the factory was not able to have the engine make better power and reliability then the current best of breed. The engine was given to Aspin to make of it what he could, at his own expense. Encouraged by press reports, and persuaded by claims of 35 bhp expected by Aspin, the factory has spent considerable time and money on this experiment. The engine was to have been tested to destruction on a test rig but the Aspin works never really got around to it. It was lying in a corner some years later when spotted by a Jack Ashworth, who was loaned the engine for a 20,000 mile extended road test provided that the engine was not opened up or stripped, and that the engine was returned to the works at the end of the test. Together with the journalist "Wharfedale" the engine was mounted in a BSA M20 chassis and featured in several articles. When the test was completed the bike was ridden back to the Aspin works, but the staff were too busy to be bothered with it, and it was left to Wharfedale and Jack to dismantle it. At least with their journalistic background they thought to take a photo! The engine was in excellent order (no mechanical problems since this engine burned oil!) and in the rebuild it was decided to fit a higher compression piston at 10:1. That came as a surprise to me as the original development engine ran at over 12:1, so why build the Velocette engine at only 8:1 ? No wonder the engine was "soft", but it could pull high gears up hill from practically zero revs. To my knowledge the rebuilt engine never features in any further report of Motorcycle so I guess it was either wrecked or skipped.

The War Years 1937 - 1950
There were reports in the press of a flat-4 aero engine and a 650cc twin engine. In 1937 Wharfedale wrote about the aero-engine flat-4 as being complete, there is a drawing but again no photograph. It was an all alloy engine with steel cylinder liners, 1.8L, 80 bhp @ 5000 rpm, 130lb weight and 83mm bore. A number of these engines were made, and most were exported to the USA where interest and purchasing power seemed to be higher. In a much later article by LJK Setright, there is a photo of the 650cc twin and a short description, but I can find no reference to such an engine in any other report. For the war. Aspin produced a 30cc 2hp motor that could be carried in a back-pack by a soldier to run a portable radio transmitter. These were available in volume production, but I can find no photo or drawing of this engine.

After the war some 2cc model aircraft engines were made, and as I understand, were made after the War by a Swiss company under licence. The original 2cc model engine was literally screwed together. The head threaded onto the barrel and the barrel screwed into the crankcase mouth, and a vertical rotary shaft geared directly to the crankshaft drove the cylinder rotor. In 1944 there was another flurry of journalistic activity. Autocar had tested a family car with a four cylinder engine. They reported 64mpg at 30mph, 50mpg at 40mph, and 45mpg at 50mph. Also ready in 1944 was a 4.6L truck engine

The Later Years 1950 -1980
The Aspin engine was certainly much more refined now, and so it should be after 30 years of development, but the interest in the engine was no self-sustaining and Frank seems to have to fight for ever piece of publicity. Ford UK were interested enough to send two or three Cortina engines to be converted for evaluation, there was a Wolesley 6 car and a small fleet of buses in Northern Ireland. In some unpublished lecture notes from the late 1960's, Frank Aspin claimed that his engines with "suitable investment" would be able to produce 300 bhp/litre. Exaggeration or what! 300bhp/l would do quite well in a 2001 motorcycle GrandPrix so to claim this before 1970 is just unbelievable, and I guess that no-one did.

Ready just before the start of this "Later Years" section, a converted 8.6 litre Leyland truck engine was fitted to a single decker bus belonging to the Northern Ireland transport Services company. The vehicle ran successfully for many thousands of miles until oil burning problems started. The cause was the breakup of the special metal coating on the rotors, and it were withdrawn from service. The Company did not demand its repair or replacement so the test could not have been convincing. Better, yes, but maybe not worth the additional effort or special consideration during servicing. The engine was repaired and used as a electrical generator for the Aspin works for many years. A flat-4 aero engine had been tested by the Royal Engineers. It was reported as a 1.0 litre engine and may have been the prototype for the 1.8 litre flat-4 aero engine made later. This 1.0L engine was tested for 2000 hours on a rig and then for 52,000 miles in a Morris Minor car, before being tested for fuel economy and showed itself to be significantly better than the control engine.

In a letter dated 1966, he is planning to sell conversion kits as an after-market item though a dealer in Knutsford who had shown some interest. It seems a bit late in the day for this, and I don't think it ever took off. By this time, the original engine company had lost its patent rights due to the passage of time, and Frank was worried that someone might steal his ideas. He describes how he has now a new and simplified design with a "completely different design of rotor, a different combustion space, and operates at higher pressure". The new design was to be the subject of new patents and was "therefore very valuable". Although it was difficult to see from the inside, after 30 years of hard effort to get the engines accepted with very little success, I doubt that there were many (or any) persons waiting to take over and make their fortune.

If you want to read more about what Frank Aspin did, then read my essay