Some history and development
The Harley-Davidson JD from Jurgen is from 1929 and is a 1200cc inlet-over-exhaust engine. “There are many details to report,” says Jurgen. “The cylinders are special Ricardo cylinders, we believe. Sir Ricardo went from England to America in the early 1900s at the request of H-D to help develop an engine for their new racing bike. He stayed there for three months, result being that H-D had a team that was always experimenting with the tips they got from the world famous English tuner.
“They then worked with inletover-exhaust engines and with these engines, the spark plug in the pocket valve. They learned that it is best to have the spark plug as close to the piston as possible to have the most power, but then the temperature in the engine will also rise. The two cylinders that are in the same line are therefore disadvantageous to get the heat away on the rear cylinder. That is why a lot of rear cylinders were damaged during the tests.
“In this way they learned experimentally what the best compromise was between power and heat. These cylinders are almost impossible to find and certainly not as a matching set of two. You cannot buy two separate cylinders because the place of the spark plug is always different and will therefore also give a different combustion pattern. On my cylinders, the spark plug is also in a different place and the displacement is also slightly larger.”
It also includes a ‘One year only’ I-beam racing fork. The fork legs are round on standard forks. That’s how these forks were made at the time, but since these Harleys often rode on dirt tracks or were used in hill climbing, racing, speedway and the like, they noted that this was a weak point.
In the year 1928 it was decided to bring a new model of Harley to the market and strengthening the front fork was one of the priorities. In mid-1929, they discovered that an I-beam (I-profile) was much stronger than the traditional O-profiles. They cast part of these forks and after the tests they decided in 1929 to quickly distribute these forks among the large dealers (due to limited numbers). It was also only the major dealers for H-D who had the race catalogues and where you could order racing parts.
They naturally made sure their own racers were the first to receive such an improved fork. For example, most forks ended up on the JDH sports models that were also used on the racetracks. This was still a kind of prototype fork because the model V in 1930 has the further developed I-beam fork.
The 1929 I-beam fork was not in the catalogue at the beginning of the year because it had not yet been developed, but would only have been an addition to the race catalogue by mid-1929.