This week’s lessons surrounded two bikes; a CB450K0 and 1971 CB500 Four and their issues with slipping clutches. Honda has had somewhat of a history of clutch slipping problems, dating back to the 1960s with the CB77s. While the clutch assemblies on the 250-305s are quite similar, they are also quite different in details, as they apply to each version.
Honda had a four, five and six plate clutch setup depending upon the size and year model of the bike. Early 250 Dreams had a set of four very thick friction plates, which gave way to a five plate arrangement used on 305 Dreams, as well. CB72-77s had a six plate clutch in the early years and then backed down to a five plate set, but with different friction plates than those of the Dreams. Clutch springs came in various strengths, but often the clutches were right on the edge of holding tight and slipping under loads. Many stock CB77s, loaded with OEM parts exhibited a slight “cold slip” condition that seemed to improve as the engine and oil heated up. The clutch hubs only held four springs in the pressure plates. The usual way to get more bite was to put in stiffer springs, at the expense of a light lever pull action.
Earlier stories written here about the intricacies of the Honda clutch options highlight not only the influence of the clutch plate packs and spring selections, but the release mechanism, including the clutch adjuster ( / ) and stack height. The first series of plates required a thin spring wire to help hold those plates steady during clutch release, so the whole clutch pack didn’t just shift away from the pressure plate as a lump, instead of spreading apart plate by plate. When the spring wires are left out of the inner hubs, the clutch tends to drag between the inner and outer hubs, causing torque on the gears, which then refuse to select neutral without a fight.
The first candidate to arrive was a 1965-66-ish CB450K0 Black Bomber that had a slipping clutch, performance issues and fuel leaks. The clutch cover was removed to access the oil pump for inspection and at the same time check out the clutch plate conditions. Despite the clutch slipping problems, the plates all looked pretty healthy except for the steel plates which had been glazed and showed some surface rust problems often associated with a “stuck clutch” condition that was not remedied properly. Rust and friction material will form on the steel plates, causing drag or glazing which interferes with the normal operation. The clutches on the 450 twins are enormous, hefty parts with 8 sets of plates and 6 springs. The plate sizes are similar to the 250-305s, so the extra pairs and springs help to contain the additional fourteen or so horsepower of the DOHC 450 engine.
Apart from cleaning up the steel plates on the wire wheel, there wasn’t a lot else to condemn, so it all went back together with the same parts. In the end, once the clutch release mechanism was lubed and adjusted, the clutch worked perfectly fine. The 450s use the same clutch adjuster as the 250-305s, which is out of production now and prone to wear if not kept lubed and adjusted regularly.
The early CB500 Fours had both shifting problems and clutch spinning issues early on. Honda designed a specific shift kit to help index the shift drum properly to ensure positive shifting between gears. The “fixes” for the clutch were many and varied. Some home tuners used thinner CB350 friction plates and added an extra pair, along with some stiffer clutch springs. With a four hundred pound motorcycle, plus rider and even more horsepower than the 450 twins, you would think that a similar clutch to the 450s would have been assigned the task of containing the extra loads. But NO, not really. The pressure plates and inner hubs of the CB500 Fours have only four springs and seven thick friction plates with seven steel plates. As with the CB450, the steel plates needed cleaning and the friction plates were replaced with aftermarket Vesrah units with radial grooves. The friction plates which came out had angular grooves which might actually retain a certain amount of oil causing slippage between the plate sets. Some online drawings show the early CB550 clutch friction disks with those angular grooves, but apparently replaced by the CB350 series plates instead. The K1-K2s used the early style clutch, even showing use of the 250-305 wire spring retainers on the first clutch plate. By the time the K3 versions came out the clutch pack was redesigned, using 350 plates at first, then when the engine was enlarged to a 550cc style, the friction plates carried a 392 code part, which is for the later SOHC CB750K series models.
In anticipation of the arrival of the CB500, which was a change-out for the CB500K3 which was in for other servicing, new clutch plates and a spring set were ordered from Marty at 4into1.com. The friction plates looked like normal radially-grooved parts, but the suggested springs were a bit longer and stiffer than the ones which came out. The stock springs have a 323 code and have white paint on the ends, as Honda seems to like to color code some of the spring sets. 323 springs have been the spring of choice on CB77s of late, but they do add some noticeable extra lever effort when installed.
After reassembling the CB500 clutch the lever pull was quite a bit heavier, more than I would have liked for my own bike. Having the experience of just cleaning the steel plates on the 450, I opened the CB500 back up and reinstalled the stock springs again. The lever effort was back to “normal” but a quick test ride indicated the return of slippage, once again! The cover came off again and some old 305 “YB” springs which are shorter, but thicker in wire gauge were put in place. Again the lever effort was not very stiff, but felt stronger than with the stock springs. Under hard acceleration the clutch would slip momentarily then hook-up again. I was right on the edge of the crossover between lever pull effort and clutch slipping prevention. The next choice was to install CB550 springs which have a 374 code part number. I had nothing in the spares box for that application, so ordered up a set of four from the local Honda dealer.
The CB650 clutch springs came in quickly, however they were just longer than the CB500 versions and didn’t seem to be any stronger than the originals. Ultimately, I chose a hybrid combination of the CB650 springs and the EBC replacement parts. The clutch lever pull was heavier than before but not as bad as with the full set of EBC springs. A quick test ride showed no signs of slipping even at a full throttle blast on a freeway ramp, so finally the clutch slippage problem has been solved, but not in a way that I would usually choose.
A follow-up email back from the CB450 owner, after a few days of riding was very complimentary about how well the clutch was working now, compared to the way it arrived last week. So, those are two paths to clutch repair success on these vintage Hondas. There is a CL360 waiting in the wings now with some mysterious surging, misfiring problems that need my attention, but the clutch is fine, I hope.