Honda CB750 Four Gearbox Problems

A customer recently bought his Honda CB750 four into our bike shop for restoration and complaining of gear shifting problems. After inspection I found there was a gear selection problem. My customer compounded this by saying he had trouble selecting gears and sometimes deliberately double shifting to miss the offending gear.

I stripped the engine and set the bottom crankcase up in a holding jig. I then set the gearbox shafts and gears in their respective positions. Then I ran through all the gear selections up and down the gearbox looking at such things as selector fork barrel position and fork position.

Honda CB750 four gearbox selector

I noticed the selector forks (2, 3, & 4) were not pushing the gears into full driving mesh, with some gears heavily meshing and some only with a shallow mesh. This would cause them to jump out of gear under load. After closer inspection I noticed that the selector forks were tilting or moving poorly under selection with the drum (6). This immediately pointed to the selector fork running shaft (5) being worn or undersized.

I measured the selector fork bores and found them all to be well within tolerance. I then measured the shaft and found it to be .005″ undersize from the selector fork bores. I informed the customer about the poblem and he supplied me with another new shaft. I measured the new shaft to ensure it was correct only to find it too was .005″ smaller than the fork selector bores. When ever I have made fork selectors and shafts from scratch I have a running clearance of .0005″.

I then machined a new selector fork shaft with a .0005″ clearance. This provided a very close push fit into the running holes in the bottom crankcase that supports the shaft. Anyway, when it was all fitted I looked at the selector fork operation and found that all the gears meshed perfectly throughout the gearbox.

My conclusion from all this, is that I wonder if gearbox selection on Honda CB750 four’s was an inherent problem or whether the gear selector shafts had been badly manufactured following poor engineering practice. The shafts I make and made for this particular bike are manufactured from material of the highest specification. They are then case hardened and precision ground.

All our restoration work is carried out at our workshops. We do not farm work out to other engineering companies or other so called bike restorers who simply buy a new part and bolt it on. We pride ourselves on the ability to look at a problem, understand it and provide a solution through skilled and competent engineering.

by Terry Ives