Converting the CB750K6-7-8-"F" Swingarm to an Earlier Design.
In 2007, Honda discontinued the swingarm's "Collar" for the CB750K6 through K8 and all "F" SOHC versions. Without this part, rebuilding the swingarm becomes a hit-or-miss proposition, as new bushings on an old "collar" (aka "pivot shaft") will not yield the necessary small clearance of .0008" to .0012" required. Your options are narrowed to a cleanup of the old, worn "collar" and new, custom-sized bushings, provided the "collar" diameter is still greater than 0.830" after the cleanup work is done.
A more robust option is to convert this subsystem to the earlier version found in the CB750K0-K5, CB500/550, and many other Honda models of the time. This version's "collar" is still available from Honda because it fits many other bikes, as well.
By the mid-1970s, Honda was facing stiff competition from the other Japanese 4-cylinder "me too" bikes that followed the CB750K. Profit margins were slim, and riders were trading their bikes in earlier than Honda had first thought they would, so this subsystem was changed from 7 precision parts to 2 similar parts. Lost in this change was the independent end caps and the grease seals, along with reduction from an expensive (but effective) gun-drilled pivot bolt with 2 grease zerks to a single fitting located in the center of the arm itself. This latter change was responsible for most of the loss of lube to the bushings, the rest being that no provision for the grease to reach the outer half of the bushings was even provided.
The differences between the cheapened late version and the better early version are:
1. The (expensive) gun-drilled and cross-drilled pivot bolt with 2 grease zerks for effective greasing of the bushings was replaced by a solid bolt with no zerks, and a single zerk was added to the center of the pivot tube on the arm.
2. The pivot "collar" inside the arm's pivot tube, which rides on the bushings, was changed from one with holes that received the grease from the 2-zerk design and grooves to spread that grease along the whole bushing(s), to a lesser "collar" with no grooves or receiving holes at all. The single zerk in the later swingarms was supposed to receive grease and deliver it down the width of the arm to the bushings directly, but delivery to the outside half of each bushing was impossible: a slight depression in the center of the bushing area of this new "collar" would collect the wear debris and quickly create a very effective dam of crud. Unless the rider disassembled the system on a regular basis to clean and hand-regrease this area, it would only last about 15,000 miles before it became impossible to lube, then the outer half of the bushing would readily seize.
3. The bushings were changed from a forgiving (and expensive) zamak material to a phenolic material that required less grease, but absorbed much more water. These phenolic bushings were frequently undersized after the arm's tube distorted a bit wider from riding, causing them to crack soon after replacement. Phenolic also absorbs water readily and holds it, just the opposite of what is needed in this area. The bushings were also changed to be longer parts that included the "top hat" shape of the original independent end caps that aligned the grease seals, and the grease seals were eliminated. Those original seals both kept grease in and water out. Although the longer bushings seemed to promise more wear area, the depression in the "collar" bearing area removed this advantage because it did not contact the bushing at all.
How to improve it:
The simplest approach to changing this subsystem is: get the 2-zerk pivot bolt from an earlier bike, change the "collar" to the #52141-300-000 early version, get some (non-steel) bushings, felt seals, and end caps from the earlier arm, and replace yours with those. This way, your chain guard and other fittings will still fit. This may, however, take quite some elapsed days, depending on your supply of used K0-K5 parts...
The next most effective way is outlined below: you must obtain and modify the afore-mentioned "collar" from a Honda shop, and get the early-style bushings (or bronze bushings, best), seals and end caps, or make your own seals and end caps.
1. The seals are easy: get some felt or wool (.030" to .060" thick) and cut out a pair of 1" diameter discs, then cut a 3/4"-7/8" hole in their centers. That's it.
2. The end caps are almost as easy: using a long screwdriver, tap out the old "top hat" bushings from the opposite ends of the swingarm, so as to not damage the flanges. Don't try to pry them out: they will immediately break. Penetrating oil, applied profusely into the pivot tube (and onto your shoes and floor), will also help slip them out. Clean in gas or kerosene after they are out.
3. Cut the old bushings off so you end up with flanged end caps that measure 0.180" (for .060" felt) to 0.210" (for .030" felt) long. The length of (the end cap + felt thickness) should be 0.240", so let that number be your guide. (See picture of hacksaw cutting the old bushing on the lathe.) These are brittle parts, so go slowly or they will shatter. If you have a lathe or really big drill, let it rotate the bushing under the saw blade, or use a moto-tool and cutoff wheel.
