As with all Sunbeam owners I found that whilst my S/8 was a very pleasant bike to ride it was well down on power , even my son’s Honda CG125 had the legs of it!
At the same time I also owned a Kawasaki KL250 trail bike which I realised had exactly the same bore and stroke as a Sunbeam and moreover the camshaft rotated in the same relative direction . A KL250 puts out 20 BHP at 7000rpm so it seemed to me that by doubling up two sets of valve gear in a Sunbeam type head it should be possible to produce an engine putting out about 35 - 38 BHP .
I was fortunate at the time to be working in the R&D department of a large firm and we had our own machine shop , I got permission to go in on Saturday mornings and spent the next six months machining the head from a £97 block of Dural . The only bit I didn’t do was to put in the valve seats . All the Kawasaki valvegear was used as standard except for the two pairs of cams which were bored through and keyed to a common shaft 180 degrees apart .
This head was first fitted to the “Standard” Sunbeam then later on had the “Sports” Sunbeam built around it . It has been completely successful , giving much better acceleration and a cruising speed of 70MPH with 85 MPH easily within reach . I don’t know what it will do flat out as it goes so well that I worry about the con-rods and crankshaft --- you just can’t win !!
It is now entering it’s third incarnation as the motive power for the S/12 .
