The DOHC 250 Velo Engine

Founders Day – a major event in the VMCCs diary of events, was held on the 20^th July this year. I have been going for many years to meet up with old (literally! these days) friends and this is one of the best autojumbles for vintage, veteran and classic bikes. There is much more than that – club stands, riding demos, a Wall of Death etc… but I go for the camaraderie and the possibility of finding some long sought after part.

The weather is usually pretty good as the event is in the middle of summer but this year it was a damp, rainy day and it is held in a very large field; needless to say, my family thinks I’m mad – driving 150 miles each way to wander around a muddy field looking at old motorbike bits. My expectations are never too high – that way you don’t get disappointed, but there is always the possibility of finding something useful.

I stopped off in a tent, not just to get out of the rain but because it was the stall of a dealer that I know (and, incidentally, have a great deal of respect for) and stumbled (literally!) across this.

I recognized it instantly as the cambox for a DOHC Velo engine. The rest of the engine was also there. We shook hands on a deal and with the marvels of modern technology, I could pay him instantly on my phone app and bring the engine home.

Now, DOHC Velo engines are pretty rare. In fact, very rare. I am aware of the 350cc DOHC engines that were made by Velocette and raced in 1936  for which details are given in Ivan Rhodes’ books or on DennisQuinlan’s blog and a number of post WW2 250cc bikes with DOHC engines – the Beasley, the Eldee plus others and which are summarized by Chris Pereira in his excellent book “British 250cc Racing Motorcycles, 1949 to 1959”.

So, what exactly is this engine? Here are a few more pictures of the engine back in my workshop.

 

 

 

And so, a few observations:

-    I have assembled the parts loosely to be able to take a picture that shows what the complete engine would have looked like

-    The 3 main components of the cambox, cylinder head and barrel are aluminium. The design has been well thought out (for example, the detail in the cambox casting, the oil feed slot and drilling from the cambox to lubricate the pushers etc), the castings are all good quality and have been machined to a high standard.

-    The internals of the cambox are all steel – the gears and the support plate - and this results in the cambox being quite heavy.

-    The bevel drive indicates that it was based on a Mk 1 OHC Velo engine rather than the later Mk 2.

-     I don’t know why it has been painted black (it’s not magnesium!) but it has at least prevented it from oxidising.

-    The entire cambox mechanism rotates exactly as one would hope – very smoothly - like a Swiss watch, as the saying goes – by turning the bevel gear from the Oldham coupling end with a large screwdriver or either of the (camshaft) gears at the end. It is in first-class condition.

-    The piston, made by H & G (Hepworth and Grandage), is steeply domed on both inlet and exhaust sides to avoid interference with the valves – clearly intended as a high-performance engine that would have had a high compression ratio.

-    The cylinder bore is 68.15mm so I would presume that the original capacity (with a 68mm stroke) was nominally 250cc.

-    The bottom-end – crankcases and crankshaft are nothing to do with the original engine; the barrel/head/cambox have just been “plonked on top” of a 350 bottom-end to make the appearance of a complete engine. Although the crankshaft turns it can’t be rotated past either BDC (because the piston skirt hits the flywheels) or TDC (because the rings pop out of the top of the barrel). I have an MOV crankshaft in my workshop, which has the same flywheel diameter as the 350 engine but has a shorter (68mm) stroke, and this would allow rotation of the crankshaft without interference with the piston.

-    There is evidence (carbon deposits) that the original engine was run ...but probably not for long; there is no discernible wear in the bore or anywhere else.

And so, what are its origins?

Well, I have asked a number of people and nobody actually knows where or when this engine originated. It has been suggested to me that it is “home-made”, made by an engineer by the name of Woods, designed post WW2 by Bertie Goodman etc.

I don’t believe it is any of these and I will state what I believe it actually is ….but first what it is not.

There are 2 features of the engine that distinguish it from every other DOHC Velo engine that I can find: firstly it is clearly based on a Mk 1 OHC engine whereas all of the others are based on the Mk 2 (with the exception of those that use a gear train to take the drive from the crankshaft to the cambox that are derived from the pushrod engine); secondly, I can find no other engine for which the cambox contains 7 gears – all the other DOHC engines use 5.

A few pictures below that illustrate this point.

The 1936 Velocette DOHC (picture courtesy of Ivan Rhodes – see also page 78 in his most recent book)

(pictures courtesy of Ivan Rhodes)

The above 2 pictures are the Beasley DOHC cambox fitted to Bill Stewarts engine

The Eldee engine above (picture taken by myself at Pukekohe 2000)

Above: The Woods Cylinder Head and Desmodromic Conversion Kit for a pushrod Velocette engine (picture courtesy of Dennis Quinlan’s blog)

I could go on…. but my point is that I cannot find any Velo engine that has the 2 characteristics that I mentioned above.

Is it a home-made concoction? I do not believe it is home made. Whilst it is not beyond the realm of possibility, I believe that the detail of the design – which is extremely well thought out and the complexity and effort required to produce these parts coupled with certain specific design features indicate that these parts originated with Veloce. I’ll explain why I think this is.

I spent the entire 45+ years of my working life in the automotive and engine industries and, working for consulting engineering companies (AVL, Ricardo and CD-Adapco), for 35 years of that time I was involved with the Engine Research Departments of OEMS around the world. There were many projects that came and went and were rarely discussed or published outside of those research departments. Why was this? Well because any lessons to be learned were subsequently applied to product development or because the ideas simply didn’t work and were discarded.

A couple of examples of projects from my early days at Perkins Engines – it’s not a problem talking about these as they had already finished 50+ years ago:

When I joined Perkins Engines in 1969, I spent much of my first few years studying mechanical engineering at college and the "holiday" periods at Perkins in Peterborough being assigned to different departments to understand how an engineering company works – part of the training. I managed to get into the research department in 1970 (as a 19-year-old student) and was assigned a competitor’s engine (a Fiat 806A 6-cylinder diesel) plus an engine dyno plus a tester and, under supervision, tested this engine – power curves, fuel loops, heat balance, friction (the dyno could motor the engine), cold start etc..  for some months. In hindsight, I was given a lot of freedom for a young kid. The research department dynamometers were, at this time, located in a site called “Queen Street” – right in the middle of the city of Peterborough – long ago demolished to make way for shops.

Anyway, being in the research department and having the freedom to wander around the dynos gave me a bird’s eye view into other projects, most of which never saw the light of day and were never mentioned outside of the company.

For example, there was “The Duplex”; this was a uniflow scavenge piston-ported split single (in this case, a diesel engine – not a gasoline engine like the Puch) with an external supercharger. By the time that I joined Perkins this project had already finished – apparently the combustion system gave the highest smoke reading ever achieved and the engine was so loud that the dyno testers nearly walked out in protest. I never saw it running but I do remember the one-and-only prototype engine lying forlornly in a passage at the back of the test shop collecting dust.

Then there was the BICERI (British Internal Combustion Engines Institute – now defunct) VCR (Variable Compression Ratio) piston – a clever invention for which the piston was made in 2 main parts – the crown and the part attached to the gudgeon pin and these could be moved relative to each other to vary the compression ratio by changing the volume of oil contained in a chamber that separated them. An oil feed through the crankshaft and up through the connecting rod controlled the amount of oil in the chamber. Apparently, it worked quite well but never got beyond the single cylinder stage (singles were often used for testing ideas and concepts).

The last project that comes to mind is the Differential Diesel Engine (referred to as the DDE). This consisted of a 6-cylinder in-line diesel engine (the 6.354 was a very successful engine from Perkins) to which an epicyclic gear arrangement (often referred to as a sun-and-planet gear system) was used to drive a supercharger such that as the engine speed started to reduce under load, for example when climbing a hill, the supercharger speed would increase, thereby increasing the boost pressure. Quite a few of these units were built and installed in trucks - I remember speaking to one of the drivers who told me that he could overtake pretty well anything on steep hills on the A1 road that was often used for testing. For anyone interested, there is a full technical description of the project in an SAE paper here.  This project suffered the same fate as the others – it was not pursued and did not go into production. I don’t know why – cost, complexity, some technical reason…. But I do remember a long line of these engines that were discarded in a passage at the back of the test shop after the project had finished and awaiting collection from the scrap man.

So, what was the point of this little ditty? The point, quite simply, was to illustrate with concrete examples that the industry has a strong track record of coming up with ideas and pursuing them until the time when someone decides that a project has served its purpose. For the most part, these projects are never mentioned outside the research department – they are simply discarded, lessons learned and the engineers move on to the next project.

I believe that this engine falls into this category and was probably designed sometime in the early 1930s before the Mk 2 engine came along and was subsequently dropped because it was decided that enough had been learned and/or there could be a better solution to DOHC based on the Mk 2 engine.

It’s worth exploring some of the design features of this engine and how they relate to Velocette engines generally.

Starting with the cylinder barrel, I believe that a standard (not a KTT) barrel was the basis; the fins were machined off and an aluminium outer was cast around it. Why? My guess would be that they were testing the Alfin (or some related) process that was subsequently adopted more generally.

For the record, the dimensions of the DOHC barrel are shown on the left in the pictures below. A standard “K” barrel is on the right. The dimensions that I’ve indicated on the Vernier are the same on both, within the tolerance of a casting (note that on the picture of the DOHC barrel the material above the base flange is aluminium that has been cast around the ferrous cylinder)

However, the observant will have spotted that the diameter of the holes for the through-bolts for the cylinder head is disproportionally large (just over 0.5” in fact).

Why? Well, the reason became clear when I tried to figure out how the barrel was mounted on the crankcase. In the pictures below, I have shown the pitch of the fixings – studs, bolts etc on a standard set of “K” crankcases

and on the DOHC engine.

(It’s easier to measure the pitch on the head – which is the same as the barrel, as 5/16” steel rods are a good snug fit. The actual measurements were made in a more accurate way than shown – the picture is for clarity of illustration)

The important point here is that the holes/studs/bolts are ¼” closer together on the DOHC engine and the head, barrel and cambox (which all have the same pitch) can’t be simply bolted down onto a set of “K” crankcases. It is not a simple matter to change the crankcases – there would be many knock-on effects on other aspects of the design and moving the pitch of the 3/8” BSW threads in the crankcase to be closer together would not be a good solution as the threads would come perilously close to the inner surface of the cases and be much weakened. Essentially, each stud would need to move 1/8” towards its neighbours and which equates to 0.176” inwards towards the cylinder centreline – too much!

The concept of an eccentric stud had occurred to me and as I was pondering this it became apparent as to why the holes at the base of the barrel were "pear-shaped".

When I first saw these, I didn’t understand why they were this shape (it looks like a machining error but it’s too consistent) and then it became obvious. Velocette must have experienced the same thought process and came up with the solution of eccentric studs and the pear-shaped section is to house the "pear" . The extent of the pear shaped section is the thickness of the base flange, ie around ½”.

I had already sent pictures of the engine to Ivan and we discussed this at length on the phone. He sent me the picture below which shows exactly the concept that I had envisaged (the bit in the top right)

Picture courtesy of Ivan Rhodes

and, apparently, this solution of using eccentric studs to change the position of the cylinder/crankcase bolting pattern was used widely at Veloce – he told me that he had a box-full of ex-works eccentric studs that were used on race engines.

For the cylinder head, at first glance, any connection to Velocette is not immediately obvious. However, if the head is viewed alongside a bronze head (this is the head from KTT 581)

then the similarity between the structure and the number of fins becomes apparent. The fins are not as deep on the DOHC head (the cambox would not otherwise fit on top) and the overall shape is squarer but there are distinct design similarities. I would also expect to see a stub, as is present here, rather than a threaded exhaust port on an aluminium head as a thread would be liable to damage. Both heads use a similar design of hairpin valve springs.

The inlet valve on the DOHC head is the same size as that on KTT 581, shown for comparison below.

By the 1930s there was an acceptance in engine design that exhaust valves can be smaller than inlet valves and this is seen here.

Regarding the cambox, I don’t yet have anything additional to add to what I have already written above plus the numerous pictures. Ivan kindly sent me some pictures of the internals of the 1936 DOHC cambox

Picture courtesy of Ivan Rhodes

but this is a completely different design and for the Mk 2 engine.

The question arises as to how to adjust the valve-to-pusher clearance. Simple; by inserting shims between the cambox and the cylinder head studs on which the cambox “sits”. These are shims that I removed from KTT 581 that were sandwiched between the magneto and the mag platform used to set the timing chain tension.

A simple solution that does not require removal of the magneto as these can be easily slipped in with the mag in place. It doesn’t take much imagination to see that the same solution, and one with which Veloce were conversant, would be applicable to the DOHC cambox.

I have not yet stripped the cambox to look inside and some similarities may emerge but that will have to wait a while.

Before I leave this topic, I’ll just quote some words from a letter that Ivan sent me having seen all the above pictures: “ ….it may well have originated at Veloce, somewhere in the back of the works where these things seemingly happened. I believe Eugene had a go at such things independently….”

In reality, we will probably never know (unless someone comes up with some genuine factual evidence) of where this engine originated but, at least for now, I would put my money on it being a “skunk works” project at Veloce.

So, what is the plan? I didn’t acquire this engine to put it on a shelf to look at, interesting as it is. It has also turned up at just the right time. For the 3 early cammy Velos that I am currently building, I have completed the dry build of KTT 305, I have nearly finished KTT 55s dry build (another couple of weeks should see that done) and I was then going to start the build of the engine for the “cammy special” – that plan has now changed. Instead of the engine that I had originally earmarked for this bike, and that will now be used elsewhere, the DOHC engine will take its place.

I have no way of knowing whether the DOHC engine was ever fitted to a bike or whether it only ran on a dyno but it’s going to be a fascinating project to resurrect it and see (and hear!) it run again. But there’s a lot of work ahead…. I guess that is why we have winters…