I've heard from several this past month or two that Honda has discontinued their 48T rear sprocket for the 750s.
When they did this for the swingarm parts for the K5/6/7/8 and "F" swingarm pivot collars and flanged bushings, I came up with the workaround rebuild methods I use now on these swingarms.
Now, I'm contacting the sprocket manufacturers to see if any of them can make the "silent run" version of these sprockets that made the Honda parts so successful and superior to the aftermarket parts. I've started by talking with JT Sprockets' engineering people, and will talk with others. Keep me posted if any of you have some insider info from other sprockets you have used, or builders you know. If I have to, I will make 48T sprockets for sale at SOHC4.net: I have friends with CNC machines...
Here's what is unique about the Honda sprockets that are found on the CB750/500/550/350F/400F:
1. The base circle of the sprocket is 0.5mm to 0.8mm larger than ASME specs. This causes a gentler settling of the chain into the base of the sprocket and improves the power transmission across far more teeth of the wrap. This single change is the most important feature of the Honda sprockets (it's also true on their countersprockets).
2. The teeth are tapered further toward the root than on ASME sprockets (even further than on ANSI sprockets), all the way to the centerline of the chain's bushing pin. This causes very gentle alignment settling, side-to-side, as the chain engages the sprocket: this is very important when there is more than 20 HP being delivered to the rear wheel, because the rubber blocks tilt a little and the sprocket misaligns. (Those of you who have watched this action while on a rear-wheel dyno know what I'm talking about...) BTW: this is one reason why the stiffer O-ring chains deliver less power to these sprockets: they don't bend sideways easily.
3. The tips of the teeth are 2mm shorter than ASME spec. This lets the chain slip quietly off of the teeth at the end of the wrap, which greatly reduces the "hooking" wear that normally appears so quickly. The loss of this little collision leaves more power in the sprocket, less lost to friction.
In the day, the side-tooth tapers were also polished smooth: Honda's vendors have not done this to their parts since the 1990s. If I have them made, they will be polished. This puts more power on the ground, keeps the lube on the chain, and reduces noise considerably. It also makes for less vibration, especially at 75 MPH and above.
In dyno tests in the 1970s, the then-popular (because they were cheap) aluminum sprockets cost nearly 1 HP on 60 MPH, 4th-and 5th-gear dyno runs. Those sprockets were ASME standard types. The smart street racer and stock-class production racer of the day stuck with the Honda parts, despite the extra 1.5 pounds it added to the rear wheel (compared to the lighter aluminum ones).
Stay tuned...
