Parts differences between chain driven and belt driven shovelheads
The difference between a chain driven shovelhead and a belt driven shovelhead is not just about sprockets versus pulleys and chains versus belts. The belts (originally 1-1/2") are wider than a chain (even the widest o-ring chains will be less than 1"), and hence HD made numerous changes to several parts to accomodate the belt drive. The main differences are related to the primary, starter axis and housing, and swingarm:
- Primary related: The main difference is in the inner primary. Through a different casting, more room has been made to accomodate a wide pulley in stead of a (narrower) sprocket and chain. In addition, the pulley has a larger diameter hence the starter housing mounting boss on the primary has also been changed to accomodate the larger diameter pulley (as a side note, the chain driven inner primary barely has enough room to clear the chain and sprocket. On most shovels you will find the inner primary and starter housing eaten away by the chain because of lack of chain tension or if the bike has been lowered).
Because of the different inner primary and starter shaft/housing, the starter ring gear on the clutch shell had to move further outwards on the shell. In the process, HD also modified the riveting of the blocks inside the clutch shell. In addition, the outer primaries of the rear belt driven models are slightly different. Since the starter axis sticks out further out of the inner primary, the boss in the outer primary for the starter axis bearing sits deeper in the outer primary. Furthermore, in some other areas the outer primaries provide some more clearance for the starter ring gear. All these differences in clutch shell and outer primary are minor though, and both a chain drive clutch shell as a chain drive outer primary can be converted to accomodate a rear belt drive starter set-up (see here).
Note that HD also made bikes with both front and rear belt drives; the 80 to 82 FXB, the 83 FXDG, and the 83 FXSB (from 84, the FXSB had a primary chain). These bikes obviously had different clutch hubs (equipped with a pulley in stead of a sprocket), front pulley in stead of a sprocket, and primary belt in stead of a chain, but they also had a different inner primary. The front tranny mounting holes to attach the primary housing to the crankcase were elongated. This allowed the primary to be moved back and forth slightly to put proper tension on the belt.
For a break-down of parts and part numbers of the final belt drive parts related to the primary click here.
- Starter axis and housing: As explained above, to accomodate for the larger width and diameter of a belt drive pulley compared to a chain, the starter axis and housing had to be adapted. The starter axis is thinner and longer, the housing consists of two parts in stead of one, the starter yoke inside the primary is different, and even the starter gear that meshes with the (Hitachi) starter motor is different. In fact only the bendix gear and a retaining ring are the same as the chain drive models, although the shifter collar and a thrust washer were used in earlier chain drive shovelhead starter setups.
For a break-down of parts and part numbers of the final belt drive parts related to the starter axis and housing click here.
- Swingarm: Because of the wider belt drive and pulley (compared to chain and sprocket) a wider swingarm is used on the belt drive models. This also means that the rear axis is longer along with a different spacer on the drive side. In addition, a different (wider) belt guard is used compared to the chain guard. Also, some models had some special debris deflectors mounted to protect the belt and sprockets from foreign materials as dirt and rocks.
For a break-down of parts and part numbers of the final belt drive parts related to the swingarm click here.
- Transmission and drive: Contrary to popular belief, there are no major differences between the transmission parts of a chain or belt drive shovelhead. The only difference is in the main drive gear spacer. HD started using a special main drive gear spacer for belt drives in an attempt to reduce leakage around the main drive gear and pulley. Although not necessary, it can be a appropriate upgrade even for chain drives.
For a break-down of parts and part numbers of the final belt drive parts related to the transmission and drive click here.