There have been many subtle but significant changes to the internals of the CB750 engine over the years. Many of these changes were to increase the "comfort" and "feel" of the bike rather than increase performance. Manufacturing processes and tolerances drifted over the years, which with all these things combined, effectively dropped the output from the noteable 65 rwhp of the sandcast engine, to the 40-45 hp dyno pulls seen from some of the mid 70's bikes. By reversing some of these changes, one can regain this horsepower using stock parts and a little machine work.
One such change to the valvetrain of the CB750 was the modification of the rocker shafts. The rocker shafts on the early engines (K0-K2) were not bolted in and were free to spin within the rocker tower. This freedom provided significantly less friction and wear at the rocker pivot. Less friction = more power. However, with this freedom came a tad bit of top end noise. Based on some information in My CB750 Book by Mark "Hondaman" Paris, Honda was interested in reducing the overall noise and increase ride comfort after noticing that many people were using this bike for touring. That or quell some warranty issues.
In order to reduce some of the top end noise, Honda bolted the rocker shafts in place. This prevented them from spinning and eliminated the chatter created by the shaft walking side to side as it rotated in the rocker tower. It also prevents the shafts from completely walking out of the tower if they broke. Simply bolting the original shaft down created wear problems on the underside of the shaft where the rocker pushed against it. The wear was significant and Honda responded by adding a small groove on the shafts under the rockers to catch and hold the slung oil in the top end. This reduced the wear from the original bolted shaft, but since this shaft was also bolted down, the wear was focused on the underside of the shaft. Preventing the shaft from rotating may have reduced noise and increased the "comfort" of the riding experience, but it increased wear and friction losses.
Making the rocker pivot on only one side of the rocker shaft focuses the wear and can cause the shaft to become oval at this point. The chatter from the original unbolted shafts was minor, and if you can live with a little bit more engine noise, you'll be rewarded with less top end wear and friction losses.
Below is a picture of three different rocker shafts. The lower one is the early model, with no hold-down bolt holes. The middle one is a Honda factory replacement shaft produce from the early 2000's. The top one is the final design with the oil grooves under the rockers. It's interesting to note that Honda made a replacement shaft with a beefier middle, but without the hold down bolts "to minimize top end noise".
Here are a few more pictures of the rocker tower assemblies as they developed. The first picture below is an early model rocker tower. Note the abscence of the 5 mm bolts. The second picture below is the final design, with the 5 mm bolts and grooved shafts.
According to Hondaman, the ideal fix for this in order to maximize power to the wheel and reduce friction and wear, is to unbolt the shaft and free it from the shackles of those small 5 mm bolts. The early shafts can be put in place of the bolted shafts, using the later rocker towers with the bores for the 5 mm hold-down bolts. This is one fix, but if you already have the later model shafts with the bolt holes and the oil groove, ithey can be used to increase oiling. Take the late model shaft, deburr the 5 mm bolt holes, and reinsert into the rocker tower. Don't put the 5 mm bolts back in. Let it rotate and the bores for the 5 mm bolts through the rocker tower will collect oil and lubricate the shaft where it runs through the longer bore of the rocker tower. The grooves under the rockers will aide in preserving oil between the metal surfaces.
Unbolting the rocker shaft and allowing it to float does present a risk. There is potential for the shaft to break in the middle where the groove is cut to clear the 6 mm bolt. If this happens, the shaft can walk out and jam the rockers and the cam chain. This would destroy your top end and potentially send lots of metal down into the cases. If you are using a high lift cam, heavier spring rates on your valve springs, or if your engine typically sees rpms near or above redline, it may be safer to keep them bolted down. If you really want to keep the shafts floating with a high lift cam or other mods, find a suitable way to block the shafts from walking out in the event they break.
Additional modifications are discussed in Hondaman's book My CB750 Book - on putting them back on the road. I firmly suggest that everyone who owns a CB750 have this book as a supplement to the Honda shop manual. One of the other modifications that Hondaman suggests is adding more oiling holes to the rocker tower by drilling small (approx. 1/8") holes into the 8 round bosses on top of the rocker shaft bores. In the picture above of the late model rocker tower two of these bosses are between the 5 mm bolts. These holes will add more oil to the now rotating shaft and may reduce the friction even further. It is also suggested that the four holes in the small outer rocker shaft bores be omitted if a high lift cam is being used.
For the work I recently did on the top end of my engine, I used late model rocker towers with very low wear early model rocker shafts. I did not add the extra oiling holes as I am running a very high lift cam and also feel that the 5 mm bores are sufficient oiling cups. Though a few extra SMALL holes are certainly not going to hurt. I will put up a few pictures of my final setup soon.
For more nearly free modifications and everything you've ever wanted to know about the CB750, check out Mark "Hondaman" Paris' book at: http://forums.sohc4.net/index.php?topic=65293.0
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