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.