HONDA CB750 K2
Mark Haycock knuckles down to get his rear hoop sorted out. He hopes!
Idid need to replace the rear tyre on the K2, as one might expect, and this gave me the opportunity to take a look at the wheel itself. There are a number of critical parts within this area and with this type of project their condition is unknown, so it is important to make sure all is okay. Apart from wheel parts such as the spokes and rim, the hub houses the rear brake, wheel bearings, cush drive and rear sprocket. So, let us start with the brake… The first thing to be aware of with old brakes is that they will not be using asbestos-free pads or shoes so you need to take great care. If you don’t hold with all that ‘nanny knows best’ stuff, I should tell you that I have known a couple of people who have died of mesothelioma, one a mechanic and the other a shipyard worker, both of whom worked in environments where asbestos dust was occasionally around. A coincidence? Maybe not. So it is a real danger, but by taking a few precautions you should be okay. The main thing is not to get very fine dust into the surrounding air and you can do this by keeping the dust damp and in particular not blowing out the brake with an airline. The latter was common practice by mechanics until relatively recently. After carefully removing the brake plate we put it down and wet it so the dust is eliminated (Photo 1). Then wipe down the drum with a wet cloth to mop up the dust and rinse out the cloth thoroughly and dispose of it. This way, we have eliminated the problem. Looking at the inner side of the brake plate, we can see one or two interesting things. The shoes are genuine Honda items and they are pretty much unworn, as was the drum. The thing is, the mileage this bike has covered is unknown so I want to examine the wearing parts to at least get some idea about what it might be. I suspect that most owners would fit pattern brake shoes to an old bike because they are cheaper, but of course only genuine Honda items would be fitted by a Honda dealer. I do not think the shoes can be original, but it is likely that these were fitted by a dealer a long time ago. But look at the green and blue markings, which appear to have been made using a Magic Marker. They must have been done by someone doing what I am doing now, maybe not recently but not very many miles ago. I shall carry on anyway. While the parts are still damp, you can remove the shoes by separating them at the cam end and lifting them off, bit by bit, at this and the other (pivot) end (Photo 2). When working on drum brakes, particularly bigger ones, you need to be aware that the springs are quite strong and if you get things slightly wrong you can end up with the shoes snapping on to the ends of your fingers like a rat trap. This is not a good idea.
Having got the brake to bits you can clean it all up with hot detergent solution, dry it off and take a look at all the parts. In Photo 3 I am examining one of the cams and I can see that the sharp corner is a little worn, as might be expected (in fact I do not know why the corner is made sharp rather than a more normal cam shape) but the pads on the shoes (Photo 4) show no wear at all so I think we can conclude that the shoes are not original. Before reassembling them, lubricate the working parts with a small dollop of your favourite high-temperature grease. Mine is silicone grease now. Do not forget the felt dust seal on the cam-shaft (Photo 5). Now let us have a closer look at the shaft. The end is splined, as is the operating arm. In the past, stingy owners would move the arm round repeatedly to allow the brake to appear to operate even when the shoes are worn out. It was even possible sometimes to move the arm round so much that the cam inside moved more than 90° which meant that if the brake was applied hard it would not release, meaning that the wheel could easily lock solid until the bike swerves to a halt and this can often be a little disconcerting. Honda were evidently aware of all this when they designed the brake, so they did this with great care. Looking at the end of the cam-shaft (Photo 6) we see that some of the splines are missing. Now, looking at the operating arm in place (Photo 7) we see that both the cam-shaft and the arm have dots and of course these are meant to line up like this to indicate that the lever is in the correct position. But when you think about it, you wouldn’t need dots if there were the same number of missing splines on both shaft and arm, as the arm would only fit in one position.
But it is not as simple as that as there are more missing splines on the arm than the shaft. The effect of all this is that the ‘aligned dots’ position is standard but you are allowed to move the arm by only one spline to take up wear – a very reasonable arrangement – after that, it is time to cough up for new shoes. On the other side of the hub we have the cush-drive. This is designed to provide a buffer between the chain drive and the wheel hub, which helps to increase the service life of the chain, wheel bearings and suspension components, as well as providing a smoother and more relaxing ride. It works by mounting the rear sprocket on a plate (called the final flange by Honda, and this is separated from the hub by a set of rubber blocks. There are four drive projections on the inside of the flange, which engage in spaces between the rubber blocks mounted in the side of the hub (Photo 8). There are a couple of things to note here. You can probably see that if the projections turn anti-clockwise (which represents the sprocket driving the wheel) they will press against the large specially-shaped blocks, but when the throttle is closed (and the forces are acting in the opposite direction) they are pressed on to plain thin blocks. This is because there is no need for so much damping on the overrun. The second point is that obviously black grease has been flung out from the bearing in the centre. This is generally caused by over-packing bearings with grease or using the wrong grade (i.e. one which is not high temperature resistant. But curiously this bearing has metal shielding so it is not possible to apply grease without removing the shield, and this is not an area which should get hot as it is remote from the brake. Anyway, I decided to replace both the wheel bearings as they are cheap enough. Fortunately, the blocks have not yet been contaminated by the grease, and they appear to be in pretty good condition. On the cush-drive side of the hub, a screwed-in retainer holds the bearing in place and you will look at your neatly laid-out set of Honda CB750 special tools on a board in your workshop and take down tool number 07910-2830000 (retainer wrench) and… Oh, you don’t have the special tools? Then use a hammer and punch (with care) the same as any other amateur would do – the retainer uses a normal right-hand thread. But then if you have not done this before you will then see that both this bearing and that on the brake side have been pressed in from the outside and there is no obvious way to get them out. Photo 9 might help. Between the bearings is a hollow cylindrical component called the collar, which is held centrally within the hub by three tabs at each end. You need to displace the collar using a lever such as a small crowbar or pry-bar (Photo 10) thus bending the tabs, then you can get a long drift in to reach to the opposite bearing and using a hammer, knock it out. Don’t worry about damaging the bearing as you are replacing it! Then turn the wheel over, take out the collar and do the same to the other bearing.
My ‘drift’ in this case was the remains of a CB72 kick-start shaft extracted 45 years ago: not much use for the CB72 then, but it has been successfully (mis) used many times over the years! Because the cush-drive flange turns relative to the wheel (albeit very slightly) in operation, it needs its own bearing and I thought it best to replace that too. It is accessed by removing another screwed-in retainer, this one with four drive holes which suggests that the special tool must be a ring with four pins in it, though in fact it has only two. Naturally I do not have the special tool but I mention this because it shows that Honda have found that only two holes are needed at any one time to unscrew the retainer. I came up with this simple idea (Photo 11) as I found that the holes were just the right size to tap an M6 thread in each. There is an oil seal underneath which will be damaged but again, that does not matter as it will be replaced. The retainer had been staked to prevent it from undoing itself but it was possible to unscrew it anyway with a long bar. I was able to press the old bearing out using a ½in socket and short extension like this (Photo 12). For replacement bearings you will need two 6304s for the hub and a single 6305 for the flange. They are not particularly expensive as they are common sizes. The old bearings had metal shields (designated with a Z suffix) but as I do not intend greasing bearings and they definitely should not leak grease over the sensitive areas we have seen, I used bearings with rubber seals each side (designated 2RS). There are one or two points when re-assembling. Again, I used a socket and vice to install the flange bearing (Photo 13) and I modified my technique for screwing in the retainer by putting a nut on each screw which helped to lock it in place (Photo 14). The thread on the retainer is very fine compared with its diameter and consequently it is very easy to cross-thread it. You discover this after you have screwed it in quite some way of course. I did use silicone grease on the first attempt to prevent corrosion but I realised this was a mistake as it acted to increase friction, so later cleaned the threads very carefully and used a light machine oil (sold for sewing machines) instead which worked well. Do not forget the odd-shaped spacer which is fitted between the flange and hub (Photo 15). You can reuse the collar by bending the three tags at each end to the right shape again. To press the bearings into the hub, I used the wheel spindle and nut along with the old bearings, tubes, washers and spacers to get them in place (Photo 16). Unfortunately, I found that the brake plate did not fit correctly as it was scraping against the hub. I thought I might have missed out a spacer but that was not the problem. Photo 17 shows what it was: it is not immediately obvious but if we compare it with the original bearing in place (Photo 10) you can see that it is meant to be flush with the end of the hub. In other words, the bearing does not press against a solid step so it is best to fit the other bearing (which does) first. For the retainer for this latter bearing, I just got away with using a punch again (Photo 18). Yes I agree, what a bloody mess but it worked and you can’t see it from the outside. Next time I shall replace the retainer (promise), but I don’t think there will be a next time. In this view you can see what appears to be an O-ring which fits outside the hub –though in fact it is a special D-ring, i.e. the same as an O-ring but with a flat interior surface. All the seals are still available as inexpensive Honda parts, by the way. Next time we shall look at fitting the new chain and sprockets.