® The vintage Honda starter clutch

Electric starters are of course in wide use in powersports vehicles. These high torque electric motors turn at an rpm that is only a quarter of the rpm of the engine once it is running on its own. Therefore, a means of disconnecting the starter motor from the engine is needed to prevent damage to both when the engine is running. Traditionally, powersports application has seen two methods of facilitating this. The first is the "throwout" mechanism familiar in automobiles. Technically the Bendix system, the gear that the starter motor drives the engine with is electromechanically retracted on engine starting, thus disengaging the two. The other system is simpler and more compact, and in Japanese powersports products at least a lot more common. It is the "sprag" system, also known as the "starter clutch". In the starter clutch system the starter motor gear is in constant mesh with the gear on the engine. It is not retracted when the engine starts. This works because the gear is mounted on a one-way bearing called a sprag. When the starter motor rpm and engine rpm are equal, the torque on the sprag is applied in one direction, the direction in which the sprag does not slip, it is locked solid, providing efficient torque transfer. Once the engine rpm exceeds the starter motor rpm however, torque forces reverse their direction, and the sprag then freewheels, essentially releasing the starter from the engine. All the while the engine runs, this sprag (starter clutch) is freewheeling.

This freewheeling is not harmful, but it does present wear potential. Cylindrical rollers inside the starter clutch bridge the gap between starter motor and engine. These rollers are mounted offset to give the starter clutch its attribute of freewheeling in one direction and locking up in the other. Early versions of the starter clutch relied on only three such rollers, with the result very heavy loading on each and fairly fast wear. This wear takes the form of waviness in the surface the rollers roll on, and eventually this waviness promotes skipping and sliding of the rollers. Essentially, bouncing around, which in turn generates even more wear. After some time the starter clutch starts to freewheel in the direction it's not supposed to, so now it has lost its usefulness and the engine is hard to start and mechanically noisy when doing so. In addition, with many of the smaller Hondas -- mostly the singles and twins-- having the starter clutch bolted to the aluminum alternator rotor, the added mechanical banging and whirring causes the bolts to loosen, and the starter clutch's steel housing soon cracks.

Later versions of the starter clutch exchanged the three large rollers for twenty or more small dogbones, making for a much more durable system and one not known for the classic deterioration of the earlier setup.

On some Hondas the starter clutch can be accessed easily. On others however it is in the center of the engine and requires engine removal and disassembly to replace. Worse, at this late a date all the rebuild parts for vintage Honda starter clutches are long used up. Only used parts remain. The options are to find used parts that are the least worn, or as some have done, fit starter clutches from later model vehicles.

On a bike whose starter clutch is difficult to access, and it is making noises and skipping, it is useful to do one thing before disassembling the engine. This is switching to a light viscosity motor oil such as 10W-30. This has proved effective. In addition, briefly running a solvent in the crankcase in an effort to clean out rust and moisture in the starter clutch is also helpful.

Last updated January 2022
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