Forked off!
Our man Mark finds out that his K2 is a bit of a bitsa. Does this mean he has a mountain to climb?
Imust admit I did buy the CB750 K2 on a whim without even seeing it, though I did have a few not-very-good photos to assess it. One thing I had noticed was that the front stay of the mudguard was missing, and possibly there was something not quite right about the brake caliper.
When I had got the bike home it became clear. Not only was the stay missing, there were no mounting holes for it and the caliper was definitely not right for a K2 (Photo 1). Then it all added up: the front-end is not from a CB750 but a later model, either an F or F1 from 1975 or 1976! So the message was clear: I would need to find the correct forks and caliper for a K2. Except that I had already decided I was not going to be over-zealous about originality and the later parts were actually an improvement over the original ones! One point that came up in contemporary road tests for the pre-k3 models was that the forks displayed a high degree of so-called ‘stiction’ i.e. static friction. This meant that the forks were reluctant to respond to sudden small movements such as crossing over the joints in roads made of concrete slabs, then commonly found in the USA. The result was a rather annoying regular jarring, and this was later cured by a redesign of the forks which we shall see. The caliper was also redesigned and we’ll take a look at that later on. So on with the work. The main issue was that the forks were completely worn out (Photo 2) and leaking. The worn plating would be an obvious MOT failure as it would not be able to seal properly, so the forks needed to be dismantled. The first step is to remove the front wheel and straight away I could see that the chunky tyre which had been fitted (it looks more suitable for a tractor than a motorbike) was much too wide and the lumps at the edges were catching on the mudguard (Photo 3). Not a great choice of tyre, even though it was (it said) a Qualifier. This later design of Honda fork is very simple and following the usual procedure to access the forks (which we have covered many times) the first step is usually to drain the oil, but I do this later on as it is less trouble. It is easy enough to extract the fork legs and take them to bits and then the oil can be poured out of the bottom cases (this is the Honda terminology for the lower castings). One side contained a small quantity of thick black oil in (Photo 4) and the other had a gloopy substance similar to what I dredge from the bottom of my pond (Photo 5). This would probably explain why the fork tubes looked like this (Photo 6). You can see that the hard chrome layer has disappeared, as has the nickel underneath, and the steel tube itself has started to
wear. Not surprisingly, the leg using mud instead of oil was worse. I sent them off to AM Philpot’s for the usual treatment, i.e. checking, straightening if necessary, grinding off the old chrome, re-plating and finish grinding to size. Their turnaround time was six weeks, which was an improvement over the 12 weeks when I last enquired. The seals are held in place by simple wire clips, which were easy enough to extract as the oil leaks had kept them rust free (Photo 7). I know other correspondents use a special tool to extract the seals but I find a simple small crowbar or pry bar (Photo 8) works perfectly well though a small piece of wood prevents the end of the lower case being damaged. I know it sounds sacrilegious but even if it did get damaged this way that would probably make no difference as it would have no effect on the fork’s operation, nor would it be visible under the boot. But why not use the bit of wood anyway? One reason why these forks are very much simpler than the original pre-k3 type is that they lack bushing. That does mean that the bottom cases will eventually wear out but fortunately mine were acceptable. I suppose conceivably it would be possible to hone them out oversized and have the tubes re-plated and ground to a suitable oversize as a solution. While the tubes were being processed I renovated the cases and the easiest way is to use a Scotch-brite wheel in a bench grinder which produces a finish reasonably similar to the original: they were not highly polished when new, though they were lacquered. I shall not use lacquer as it is not durable and looks a mess after a while, as we have all seen on old Japanese bikes I suspect. So a little polish from time-totime is the way forward for me. One of the cases would have been not too bad (Photo 9) had it not been scratched in places so it needed to be refinished, just like the other one which has been completed in this photo. The fork legs were returned from Philpot’s after the predicted time and the results were very pleasing. A quick check with the tubes in the cases showed that the wear in the cases was not excessive as there was no detectable play, and that the tubes had been ground to the correct size. Rather than using the genuine Honda oil seals I went to my usual supplier, Simply Bearings, who offer a very fast service at reasonable prices (about a tenner for a pair including VAT and delivery). You can choose between different types of seals, the differences being in the seal lip design and I went for the TC4 option as that is closest to the original Honda design. Funnily enough, the seals I took out were different. Photo 10 shows a view of the top of each.
Reassembly of these forks is pretty straightforward, but let me point out one or two points: you can assemble each leg either by putting the seal in the case and then the tubes through the seals or by assembling with the tubes in place, then fitting the seals into the annular gap between. I choose the former because it is easier and it is not necessary to use a special tool: a suitable sized socket works perfectly (Photo 11). One thing I find rather curious is that the seal appears to be not deep enough as if it is pressed down on to the step in the case, which is presumably where it is meant to be, the top is nowhere near the groove for the circlip (Photo 12). It looks as though a spacer is meant to be inserted between the seal and the circlip, but no such part is specified. The other point is that the design is perhaps slightly imperfect as it is possible to assemble the bits in the wrong order with them fitting together perfectly well. Naturally, if they are in the wrong places they will not work properly. Looking at Photo 13, you can see that the little spring fits perfectly on that aluminium component. The latter is known as the oil lock piece and it sits right at the bottom of the fork case, its function being to help to stop oil leaks and to locate the damper rod. So it is easy to conclude that the little spring is supposed to act as a bump stop between the bottom of the fork tube and the oil lock piece, if the fork ever became fully compressed. The clue though is in the name: this is the rebound spring and its function is quite the reverse in that it cushions the contact between the damper piston and the fork tube if the fork were completely fully extended. So it needs to be assembled as shown in Photo 14. Finally, if you do use my method to assemble the forks, you need to be aware that the oil lock piece will not fit through the oil seal (Photo 15), so the first operation is to drop it down to the bottom of the case. The damper rod seems to fit into it quite reliably later on. At the bottom-end of the cases a socket screw carries a copper washer to stop the oil from escaping, which you will of course replace with a new one. You will probably get a more reliable seal by ensuring that the old washer or one previous to that has been removed and disposed of rather than added to (Photo 15). Lastly, when you fit the axle holder right at the bottom (Photo 16) you will find that it fits perfectly with no gaps one way round, but the other way round it leaves a gap on one side. As far as I am aware, the gap is correct. I have always understood that the idea is that the first nut without the gap is tightened, then the other one with the gap, and this ensures that the axle is gripped tightly. Next time we shall look at the caliper and the steering-head bearings.