Hi all - my posting of a couple of weeks ago drew no response, so I am starting to see the Big 4 engine as a bit of an unknown, generally speaking. When I look at the rest of the bike, there seems (certainly in WD days) a fair bit of commonality with the 16H, but the engine in particular seems to be quite different.
I saw from the cylinder layout that the valves are splayed (i.e not parallel) and now I have some crankcases I see the tappets run directly on the cams - see picture - so there are no cam followers as such. That also explains the big chunky anti-rotation, square fit tappet guides I suppose.
Big%204%20timing%20chest%20_0.pdf
So I have learnt the cams are not the same as the other common singles, but the valves, springs etc are per 16H. Looks like the standard oil pump/drive will work too.
Does anyone know if the cams and timing cover are common to any other model (c. 1948), as searching for Big 4 parts on-line is proving futile.
p.s. why is it called the "Big 4" if it's really a Model 1?
Big 4 -16H variations.
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Big 4 scratch-build engine - beware EBay!
OK, so many thanks to Richard Cornish for his very useful info.
I had noticed the oil feed/scavenge on this looked like the later twin cyl manifold type, not the individual feeds per 16H.
The drive side casing has "42532" and "C7" which does indicate 1948, so I think I have mixed casings here.....an original '48 case paired with a later timing side one. The exterior raw casting profiles match quite nicely, which is why I was not initially suspicious.
Caveat emptor (again).........
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Engine number.
Hi Russ,
I think your letter "C" is a poorly stamped "G" which would make it 1952 and this would tie in the number 42532 as the 1948 number will be somewhere around16000 to 19000. Each crankcase half should have matching pairing numbers so you probably have an original pair.
All the SV and OHV singles changed to the mono-block oil pipe fitting when the lay-down gearbox was fitted. You could use ES2 cams, but this will take the edge off the top end performance.
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Hi Russ,
Your engine is later than 1948 as it has the later fitting for the oil pipes, first fitted in late 1949 when the lay-down gearbox was fitted. There are very few parts that are common to the WD models, so make sure you source the correct parts for your year.
All the components in the timing housing are the same as the ES2 except for the camshaft profiles and cam followers which are slightly different only where the tappet adjusters screw in. The valves are parallel, it's the cam followers which are angled to suit the ES2 which shares the same crankcases as the side-valves. The valves are 1/4" longer on the Big 4 than the 16H due to the longer engine stroke. If you need to find a timing chest cover make sure it is correct as these changed slightly over the years, the small oil hole at 10 o'clock is a good indicator. If you are starting from scratch, it might be easier to try and find a complete engine so you have a definite starting point.
The reason the Model 1 became known as the Big 4 was the power rating at the time was devised by the RAC to rate vehicles for tax and insurance reasons. The Model 1 was rated at 4 nominal horse-power while the 16H was 3.5. none of these ratings had any exact relation to the engine's power out-put or even it's true cylinder capacity. I believe these ratings continued until WW2.