Primary

Primary drive

This includes the primary drive chain, the chain sprockets on the crankshaft and of course the cover for the whole thing, made from sheet metal or cast aluminum. The sprockets on the clutch are riveted or welded on; if you need to rplace one of those, you will have to buy a clutch basket.

Quick question: why does the primary drive have its own category at W&W?

Harley-Davidson has successfully rescued a drivetrain design to the present day, that was completely normal at the beginning of the 20th century, when motorcycles were still young and average dirt roads were still called country lanes. Back then, engines, the first clutches, gearboxes etc. were all separate assemblies that often were designed by different people at different times. When these parts were then combined on a motorcycle, what could be more obvious than connecting the crankshaft and the clutch with a chain? Exactly ... then put a cover over everything and – presto! - here goes your primary drive. The historically later opposite of this design is called a unit construction (all drive parts in one block). But from the 5-speed Shovel to the Evo and Twin Cam to the Milwaukee Eight, the engines of the big Twins are actually independent assemblies. Although the housings for the primary drive and the gearbox are bolted to the engine, they are actually three separate parts. And on Sportster models, too, the primary chain still has its own housing, separate from the crankshaft.

Does every primary drive on the Harley have a chain?

There was a brief trial with a drive belt as the primary drive in the FXB and FXSB models of the years 1980-1983. The idea was good, but it in real life it didn't work well. The belt with 14 mm pitch was too susceptible. The factory could not get to grips with the heat development in the primary housing and the resulting significant change in the center distance between the crankshaft and gearbox shaft. The belt guide on the engine and clutch pulleys wasn't the best either: end of experiment, back to the tried and tested chain.

So, apart from these said models, all air-cooled Harleys from the 1915 Model F to the Milwaukee Eight have a chain drive under the primary cover ex works. But of course, the guys from aftremarket R&D have not let up, and so today there is a whole range of proven, high-quality Primary Belt Drive Kits for the Flatties with 3- or 4-speed transmission and all Big Twins from Knuckle to Evolution with 4- or 5-speed transmission.

Riding the primary dry or wet?

Opinions are often divided on this question in forums. From a pragmatic point of view, there is little to discuss.

For all models with primary covers made entirely of sheet metal, this question does not arise. The chain runs with drip lubrication from the engine ventilation; if you don't trust drip lubrication, you can occasionally add chain lubricant. (If you want to ride a belt drive, relocate the mouth of the drip lubrication system).

On Sportster models the primary drive always runs in oil, namely in the same medium as the transmission. (The fact that the clutch was dry until 1970 and therefore had a sheet metal cover does not contradict). You would have to block the connection in the housing between the gearbox and the primary: far too complicated. This is why custom belt drives never became popular on Sportsters.

Dry or wet is really only a question for Big Twins from Shovelheads to Twin Cam engines with 4- or 5-speed transmissions and aluminum primary housings. If you install a belt drive, you ride dry, if you leave it with the original chain, you ride wet.