Cog swappin’
Poor Mark still has his rear-end woes to contend with! This month he starts to sort the chain and sprockets on his K2. Should be simple, eh?
One of the worst bits on the CB750 was the rear chain. This was apparently undersized, lacking the slightest trace of lubricant (hence the shrieking noise when moving the bike around), rusty and worn out. Apart from that it was okay, but I replaced it anyway! Photo 1 shows a comparison between old and new, and as you can see I went a bit over-the-top and bought a DID VX series gold-plated X-ring chain which should last another 100 years at my annual mileage rate. I suppose I am fairly mindful of the relationship between the old CB750 and chains as there was a bit of trouble here in the early days. Back in 1969, owners were not used to engines which could maintain a high (for the time) output and were probably not as careful about chain maintenance as they are now. Also, chains were joined with simple spring links and as I have demonstrated on a couple of examples these were occasionally installed incorrectly. The result was chains failing and this often led to wrecked crankcases when the free end whipped round and smashed into the backs of the engines. This was not helped by a couple of failings by Honda. The first was that the original sprockets were too small: if you think about it, to transmit the same torque (and hence power) a smaller radius will need a larger force – i.e. tension – in the chain. There was also an issue with some sprockets on very early bikes which had not been properly heat-treated during the manufacturing process though they should have been changed by a recall programme. Finally, there was also an issue with the battery breather pipe, which if routed incorrectly allowed acid to be deposited on the chain. You might think that the worst this could do would be to make the chain a bit rusty but in fact it can cause links to fail surprisingly rapidly. I do not understand this mechanism and have not tried the experiment but I am assured that just a couple of drops of strong sulphuric acid on a link can make it crack in a few minutes! Also, the original sprocket ratio was 16 front/45 rear. This was modified to 17/48 and later on 18/48. I think this partially explains the poorer performance of later bikes in contemporary road tests. My new chain was part of a kit which included new sprockets. After I had finished working on the rear wheel I could fit the rear one and Photo 2 shows one of the studs upon which it is mounted. The ends of these had been sawn off, though I cannot think why (maybe someone tried to fit the wheel with the spacers the wrong way round). Removing studs which have
been in place for decades can be a bit fraught, so as their threads were okay and the remaining length was sufficient for them to be perfectly safe I kept them in place. The only thing was that there was no room for the outer chromed side plate, though it was missing anyway and I believe it is decorative rather than functional. We see the new sprocket (nearly) fitted in Photo 3. You see I am being a good boy and have the recommended 48 teeth and have even bought brand new special double tab washers (no expense spared). Now we turn to the front sprocket. The old one was a 17-tooth, to be replaced by an 18-tooth, and this gives more economical cruising at the expense of a little loss of acceleration. It is probably no great loss as, let’s face it, the CB750 is not a performance machine now. I think you can see that there is something a little unusual about the arrangement (Photo 4): what is that screw in the end of the transmission output shaft? We’ll see in a minute but for now the challenge is in getting the sprocket off. With this Honda system, it is usually pretty easy: just undo the two M6 (10mm AF) screws, rotate the fixing plate slightly and both this and the sprocket slide off. But on the K2, you need to undo the hex-headed plug first as its lock washer prevents the sprocket from being removed. As an experiment, I did try to do that by hand but found it impossible as access is not that good. Using a pneumatic impact driver took about a quarter of a second so that is the answer here. I shall look at amateur pneumatic kit in a future issue. Now we are faced with a decision about the output shaft oil seal (Photo 5). This old seal (under the crud) is working perfectly well with no sign of oil leakage.
Bearing in mind though that it is now 46 years old, I think it would be wise to replace it as we have access to it right now and the genuine Honda part is not expensive. Clean the old one up first (Photo 6) to stop any grit getting in to contaminate the bearing as that is a critical component. It really does not look too bad at all now, but let us press on. To remove it I drilled a couple of holes and screwed in biggish self-tapping screws, then using a bit of ingenuity involving a couple of self-locking wrenches (Photo 7) and a crowbar was able to lever the seal out, without causing any damage to anything else. Before fitting the new seal, use plenty of grease and some tape to cover the sharp edges on the splines (Photo 8). I drove in the new seal using a big ¾in socket and extension (Photo 9) with a heavy soft hammer. To ensure it seals round the outside it is probably not a bad idea to use silicone ‘just in case’ (Photo 10). Now let us see what that screw in the end of the output shaft is (Photo 11). I did mention earlier that owners were not quite as assiduous as they might have been over chain lubrication and this was Honda’s answer: an automatic oiler. Okay, strictly speaking this was the Mark 2 version as it could be easily adjustable in contrast to the earlier attempt which relied on a choice of packing shims. Oil is collected from the transmission and directed down a hole through the middle of the shaft, passed through a porous plug to reduce the flow and then past the black rubber plug (the ‘rubber orifice’) shown on the left-hand side here. It then passed through a hole in the splined part of the shaft you can just see in Photo 5, radially outwards over the inside of the sprocket and hence on to the chain. Screwing in the rod compressed the plug and reduced the flow. The aim was that initially the owner should keep checking on that the chain was just damp with oil and adjust the screw accordingly. All very worthy, but I shall not be using it. Why not? Because it will lubricate the chain with engine oil but chain manufacturers recommend using an EP80 or EP90 grade transmission oil (the letters EP standing for Extreme Pressure, which is exactly what is experienced by the chain bearing surfaces when accelerating hard). I just follow their instructions and it seems to work okay and costs less than sprays. The natural thing to do would be just to screw the adjusting rod right in to stop the oil flow altogether. The ancient rubber orifice would be too hard to compress enough though and unfortunately a replacement is no longer available. So rather than trying to make a new one, I wrapped the old one with PTFE tape (Photo 12) which seemed to work okay. For good measure I fitted the screw-in plug with a fibre washer to prevent possible leaks (Photo 13). I used Loctite rather than the tab washer which I am sure will be enough and anyway, even if it does start to come undone it does not hold on the sprocket and I am sure I shall be able to tell from the oil leaking out. The screws which do hold the sprocket should be fitted with Loctite too, and you might be able to see that they should be high-tensile steel as they should be done up quite tightly. Notice though that the screws do not hold the sprocket on to the shaft; they merely hold the fixing plate on to the sprocket and the plate stops the sprocket from sliding off. It is normal for the sprocket to feel slightly loose, by the way – in fact, it is a design feature as it allows the sprocket to align itself correctly with the chain. Next time, we’ll fit the new chain.