About Sheffield Henderson motorcycles

Sheffield-Henderson motorcycles

Sheffield-Henderson motorcycles were produced between 1920 and 1923 and although there are very few surviving bikes, this rarity does not equate to high monetary value nowadays, with fairly modest values nowadays.

Due to the short production time and a scarcity of surviving bikes there is little information on the marque, but some can be found here:

https://cybermotorcycle.com/euro/wikig/Sheffield-Henderson.html

https://www.gracesguide.co.uk/Sheffield-Henderson

A search on Google images finds some interesting old pictures:

There appears to be only 3 bikes with modern photos online:

A beautiful 1922 350cc OHV showing the original design of fuel tank

Another lovely 1922 bike fitted with an oil-cooled Bradshaw engine

And of course the Stan Johnson bike, photographed outside the London Motorcycle Museum 

The 1922 Stan Johnson Sheffield-Henderson

The 1922 Stan Johnson Sheffield-Henderson

The history of the bike
Around 1930 Stan Johnson aquired a Sheffield-Henderson 2 3/4hp OHV sports motorcycle from a gentleman named Frederick Thomas Moore. At that time a secondhand older motorcycle would have been fairly cheap to purchase, and with it's simple design, sporty geometry and low weight it would have been a good candidate for an enthusiast like Stan to convert into a vintage race bike. 

 

Stan with the bike in the early 1930's

The bike has been modified in several ways, and was used initially for grasstrack racing, later Stan stripped the bike down and further changed the riding position in order to use the bike for sprinting, and this is how it remains to this day. 



The bike is now quite different from the original one that left the factory but comparing images from now with some older images it is still recognisably similar, particularly the frame and fork design.

Frame - The biggest modification to the frame is around the area of the rear triangle, it has been cut and re-joined to bring the seat position lower. The frame has two top tubes and there is some evidence that the lower of these was heated and moved sideways and upwards to allow a taller engine to be fitted.

Forks - It looks as though the tubing remains as standard. The suspension was originally provided by leaf springs and at some point this was converted to coil springs.

Petrol tank - Sheffield-Henderson used a quite distinct design of tank, elongated, tapered and square in section. The tanks had a seperate compartment for oil. The area between the top tubes (where the sprint fuel tank was fitted) was a space for a small tool kit. Stan used a tear-drop shaped tank for grasstrack and moved the oil tank behind the engine. Later when the bike was used for sprinting it was fitted with a very small fuel tank and the oil tank was further reduced in size (it appears to be fashioned from a small screwtop tin!)

Wheels - As can be seen from the catalogue the original bike had no front brake with just block brakes on the rear wheel. This would have made braking a bit sketchy to say the least! The wheels were replaced with ones that had front and rear drum brakes. Wheel size was also increased from 26" to 27".

Gearbox and clutch - The gearbox is a variation on the Sturmey Archer countershaft type gearbox. The CS gearboxes were used on many different makes, from daily runabout machines to prestige bikes such as Brough Superior. The gearbox is a straight cut dog box, where the gear wheels of the mainshaft are permanently meshed with the gear wheels of the lay shaft. Gear changes are enabled by a fork that slides the pairs of gear wheels simultaneously along the splines on both shafts. The actual change in gear is by engaging the driving dogs of adjacent pairs of cogs.
The model of gearbox is the rare CS-TT type which is a close ratio sports box that was produced in the early 20’s and used mainly on racing Nortons. There is no kickstart on this model to save weight, push start only!
The clutch would have originally been fitted to the main shaft by means of a taper and at some point this clutch was replaced with the later splined drive version which would have meant replacing the mainshaft.






 

Gearbox and clutch

Gearbox & Clutch
The gearbox is a Sturmey Archer CS-TT model. This is a rare box and after contacting Simon Grigson from http://www.vintagenorton.com/ it turned out that this model was also used in early 1920's racing Nortons. The box is notable for close ratios and the lack of a kickstart; in a racing machine this would just be extra weight to carry around. The gearbox is detailed in the catalogue as this and the images of the bike show that too, so it is believed to be the original gearbox.
The spline-drive clutch is a later addition, having been introduced by Sturmey Archer around 1926 and this would entailed replacing the mainshaft from the gearbox as the original clutch was a taper fit.

The piston

The piston
The piston is not the original and is quite highly domed and would have been fitted to increase the compression ratio of the engine, Blackburne did supply their own high compression racing pistons though at the moment there is no indication as to what engine this piston would have originally come from.

After the head was removed the bore and stroke was measured: 71mm x 88mm, which straightaway says one important thing: it's actually a 350!

Removing the cylinder barrel revealed another interesting find; only the lower piston ring was fitted. In between the two ring grooves there is quite a bit less 'varnish' than above where the upper ring would have been, which suggests the piston was run for much longer with both rings and then run for a shorter period with the top ring removed. Why a ring was removed is a bit of a mystery, it may have been that Stan removed a ring to gain some more engine speed when he was using the bike for sprinting, though leaving the top ring would have given a bit more compression. Perhaps he tried the top ring and there was some piston instability? I think this will always be unknown!

The con rod was found to be very highly polished and the paper gasket between the crankcase and cylinder barrel appears to be from a magazine, a peer into the crankcase revealed plenty of old Castrol 'R' slopping around and a clean looking crankshaft, to be investigated further when we get the time...

The highly domed piston crown

A view of the underside of the piston , note the undercut machined into the lower inside of the skirt, as well as the highly polished con rod

The piston from the side, note the clean surface under the missing top ring groove you would expect it to be as dirty as the crown...

 

The cylinder head

The cylinder head
One of the first jobs on the bike involved removing the cylinder head for an inspection of the head, piston crown and cylinder bore, which is a fairly straightforward job.
On removal there was some expected carbon build up but generally everything looked ok.
One of Stan's specialities was vintage cylinder heads so it was expected that the head would have had some tuning work carried out on it.

Cylinder head at first inspection - looking OK, but what is that circle around the spark plug hole?

Carefully removing the carbon from the inside of the head revealed it to be very highly polished, as was the intake port. The circular line around the sparkplug hole was from where it has been plugged and then re-drilled for a 10mm plug at a slightly different angle, possibly to fit a racing plug.

The cleaned head with valves removed showing a high level of polish and the re-positioned plug.

Sheffield Henderson 1922 Catalogue

Sheffield-Henderson 1922 Catalogue

Below is the catalogue for two models produced in 1922 by Sheffield Henderson. The model that Stan Johnson acquired in 1930 is the 2 3/4HP model, which was a 350cc Blackburne engine sports model. 

Priced at £110 this would have been quite a sum of money in 1922 when the average yearly wage was around £140. By the time that Stan bought the bike it would likely have depreciated greatly as many older motorcycles at that time did. 

As can be seen the bike was already quite low before Stan then lowered it further by cutting and re-welding the rear triangle. The area where the sprint petrol tank is now located below the top tube is where a small tool kit was fitted. The original fuel tank as seen below was much more elongated and had a separate compartment for oil. The bike was quite extensively modified by Stan, who went on to become a highly renowned tuner of vintage motorcycles, particularly of vintage Nortons.  

Many thanks to Rick Parkington for sending images of the catalogue.

https://rickparkington.co.uk/

The Blackburne Engine

The Blackburne engine

Publicity for Blackburne engines at a 1929 exhibition.

Advert for Blackburne engines in 1922, the year the bike was made. The engine was the 2 3/4 hp (OHV) Air Cooled model

One of the first investigations of the bike was to do with the engine. Shortly after we collected it from the museum I did the customary internet searches and being such a rare marque, very little came up about Sheffield-Henderson. What did get plenty of results was searching on Blackburne engines. Blackburne engines were built for lots of motorcycle manufacturers (as well as cars and aeroplanes) and I quickly found Rick Parkington’s excellent blog https://rickparkington.co.uk/. Rick is a motorcycle journalist who writes for Classic Bike magazine where he is known as ‘The spannering supremo’ with an encyclopedic knowledge of vintage bikes. He is also a Blackburne engine enthusiast and has been cataloguing them for some time. He already knew of the motorbike and had previously recorded the engine number and from this was able to tell that the engine was manufactured in 1927 and was therefore was a later addition.
On the 1930's logbook the engine size is given as 285cc, which was not a standard Blackburne engine size and originally we thought that it was maybe a 250cc engine modified with a longer stroke, but more on that later...

Researching the history of the engine (and the 285cc/350cc confusion!)

On the bike's logbook from 1930, the engine had the number EC152 and size was given as 285cc, this continued when the logbook was transferred to the DVLA computer database.

Rick Parkington had an idea about why this may have happened and also the engine number:

"I think the 285cc comes about because there was a confusion about ‘horse-power’. In the early 20s a 350cc bike was known as a 2 3/4hp, a 500cc was 3 1/2 and so on. This was a taxation rating and didn’t refer to brake horse power, although it might have done when it was first devised but by then even a side valve 350 put out at least about 10 bhp. Around 1925 it was decided to simplify things and 100cc became 1 hp, so all the 350s became 3.49hp. This meant that people in the licencing office confronted with a log book saying 2 3/ 4hp often wrote it down as 2.75hp, or 275cc (maybe 285cc if they were crap at maths!) It’s quite a common mistake on old log books...
...The original engine number prefix EC doesn’t really make sense but it would if it was CE, C being 350 ohv and E the ‘iteration’ of it, not necessarily an annual change maybe a specification; still not entirely sure about this. For example, they apparently made the 350sv  L and LA engines at the same time; I have both and the LA seems to have a sportier cam so maybe the A meant a sports model there. The 350 ohv started in 1921 as C. For 1922/23 I have CC and then later CF. CG was 1924 so maybe CE was somewhere between"

Thanks to Rick we also know that he current engine CK2994 would likely have been made in 1927.
So the bike would have originally have had a 1922 350cc OHV engine but this has been changed more than once!
At sometime in the early 1930's it appears to have possibly been fitted with a 175cc Blackburne when he set the bike up for grasstrack racing; this is described below a photo from Stans obituary and also he won a grasstrack trophy in 1931 riding a 175cc bike.

In a well-known (and high quality) photo of Stan standing behind the bike in the early 1930's there is a 350cc engine fitted, though this is with different valve rockers and crankcase from the one currently fitted(the current engine is late 20's and has more rugged valve rockers.

Once again Rick came up with some more great information:

"Little bit more Blackburne history. The original Blackburne OHV engine appeared in 1921 and was essentially the side valve bottom end with a new cylinder and head. Like the side-valve, the cylinder was bolted to the crankcase with three studs and the tappets came out of the crankcase parallel even though the push rods themselves were inclined toward the top. This remained the same through 1922 and ’23 but in ’23 a round timing cover was introduced with a separate magneto chaincase – this was an option to the standard cover that incorporated the inner part of the case. In 1924 the round cover remained an option, y the tappets were angled to match the pushrods and the cylinder had four fixing studs. In 1925 the breather on the back of the timing chest was lowered slightly,  possibly to remove it from the line of fire of oil off the camwheel. This necessitated moving the nearest timing cover screw and the round cover was now fitted to all models with the new screw pattern and a ridge around the outside edge. In 1926 the rockers changed to the heavier type.

Looking at that believed 1931 photo, you can just about make out that the cylinder has the 2 studs on the left, making it a 4 stud cylinder. The original would have been three stud. The round timing cover on the Stan standing pic has no lip, as you say, the rockers are early type and the magneto drive is missing from this side – maybe the only good mag Stan had was a clockwise drive one which would either need converted or run the other way round. The combination of that timing cover/ 4 studs and early rockers makes this a 1924 engine"

So it looks like the bike has had several engines: the original 350, in the 30's a 175 and another 350, then finally the current 350 engine. Also there is some anecdotal evidence that Stan may have had a supply of Blackburne engines that originated from his brother, Bert.

The current engine appears to be a tuned model, fitted with a high-compression piston and racing cam. There has also been extensive polishing of some of the internals, such as the con rod, cylinder head and ports and the cam followers.