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.