The rocker perches with the 'joggle' (center example) were introduced in 1924 on the OB/OW models. According to advertising (caveat-emptor), they were still being used on the OC. It might have been intended, or expedient, to use the S1 cylinder head on early models, as such is illustrated in the OB/OW24 handbook; with the cross bracing no less! It must be remembered the OB/OW was originally going to use a gearbox layout like the RA, but that apparently never got any further than early promotional illustrations.
Regardless, factory drawings were made for the 'new' heads. But, even that does not make all clear. The drawing for the OB shows (or 'shews', in keeping with the era) a 1.00" inlet port and a 1.25" exhaust port. The latter can be misleading as there is a taper opens it further to 1.375" at the gland face. The OW drawing was done a full year earlier in March of 1923. It shews a inlet port of 1.125" and an exhaust of 1.313" (no taper). This is obviously a bit large for a 350cc, and someone has penciled in a note "OB24 Inlet". Presumably a drafting mistake. The other thing to note is the OB drawing is at revision four, and the OW at revision one (with any other reversions having been lost.) So other than adjustments for capacity, they both likely used the same casting and evolved hand-in-hand. Other changes noted over the course of a year of revisions are changing from a flat head joint groove to a diamond groove. Also more mass was included around the spark plug boss, essentially filling in between the spark plug and the cylinder joint surface. The perches got slightly thicker in section, and the seating for the exhaust spring was drilled liberally to minimize surface contact area. The fins between the perch became larger, and project well beyond the spark plug face surface on the later drawing.
Douglas has some freedom in altering the shape of the fins between the perch and perch itself. These were cast separately, and then build into the sand mold of the cylinder head and fused when the main casting poured. So changes did not entail the expense of a entire new cylinder head pattern. It could also have enabled them to use the same patterns for the front and rear castings, by allowing the perch, fins, and spark plug boss to be 'flipped'.
One of the things that I have been curious about is what changes, if any, existed between the OB and OC. The OB/OW had full hemispherical heads, and an included valve angle of 90 degrees. By the time of the DT, the valve angle had tightened up to 82 degrees. I have not had the opportunity yet to examine a set of 'definitive' OC heads to see if they have the shallower combustion chamber and 82 degree valves like the DT, or if that change had to wait for the Dirt Track model. It is probable it all happened with the DT, since the casting was completely redesigned in all other respects. An easy way to check externally would be the center distances for the rocker spindles. The OB/OW is 4.125", and on the DT they had to move inward with the valve angle to 3.625" apart on center.
Doug
