In the earliest days of motorcycling, virtually all motorcycles had their transmissions in separate housings from their engines, and the two were connected by chain (and in a few cases by leather belt). When the Japanese led the way into another technological era, they surprisingly kept the chain connection despite their engines and transmissions being in one housing.

Or at least Honda did, and Yamaha as well. Kawasaki and Suzuki preferred the more efficient gear connection. But transferring crankshaft power to the clutch and transmission by gear was considered noisier and required more engineering. So Honda stuck with chain primary in all of the first-generation inline fours.

It's interesting that Honda included a roller tensioner for the primary chain in their first four, the CB750. Because the next four, the CB500--and its later permutation the CB550--was missing a tensioner, as were also the CB350F and CB400F. Far from a bad thing, having no tensioner simply meant that as the miles accumulated the chain grew more slack. This slack doesn't have to present any problems, but it can if the engine is not kept in perfect tune, and it can create a very small glitch even in a well-kept engine.

The first issue is rattle, and a kind that due to the thick hy-vo type chain's mass is deep and loud--unsettling to many. But this is easily tuned out in most cases. The second issue is an unavoidable (and at times irritating) driveline snatch as the powerflow changes from accel to decel and back again. Especially when the engine is hot. It can feel like a slipping clutch to the uninformed. And there is no recourse, no remedy.

But actually there is. Quite a few owners of CB500s and CB550s have fitted the primary chain tensioner from the CB650, which is very literally the higher-evolved, more developed cousin of the 550 and thus the natural source for improved pieces. The adaptation of the 650's tensioner is not a backyard operation. But given some ingenuity and tools and equipment it's not difficult either.

One more thing. Though in most cases a primary chain rattle in the non-tensioner models can be eliminated by careful engine tuning (Honda mechanics got very good at this), an associated and often misdiagnosed issue is a clutch basket gear whose rivets are loosening. This will of course add its own kind of rattle at an idle, just like the loose primary chain. However, back in the 70s I rode a CB500 for over 90,000 miles with a loose clutch gear and a worn primary chain so I could have experienced some noise. But I didn't. So, fitting the 650 tensioner, just like fitting the later 550 pinned camshaft cover, is by no means essential, despite what some may say.