Honda-SOHC
SOHC.co.uk Forums => CB750 => Topic started by: Johnny4428 on May 08, 2022, 08:09:59 PM
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http://www.stotfoldengineers.co.uk/stotfold-engineering-blog/2009/09/10/honda-cb750-four-gearbox-problems/
Found this while browsing and gleaning info on Honda 750 gearbox problems.
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I've used Terry a couple of times and have no complaints with his work or prices. He writes a few blogs on work he has undertaken that is out of the ordinary. As he mentions, he does all the work himself. He matched the shoes on my twin shoe Triton brake to the hub that had been pulled very slightly oval after a respoke.
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I too used Terry (back in 2016 thereabouts) to restore a 400F engine. Nice down-to-earth bloke with traditional engineering premises - he wrote an interesting 3 part blog on the job.
On the subject of CB750 gearboxes, I’m in the throes of rebuilding mine, for no other reason than I want to restore the bike to factory fresh. There is no issue with the ‘box, though once a month it might drop into a false neutral from 4th - it’s such a smooth box that I generally blame myself for being too lazy snicking it into each gear. However, my local engine builder suggests this can typical of 2nd & 4th gears, and so will undercut (by a couple of degrees) the dogs on those gears - ie to encourage them to stay selected once engaged. Seems such an obvious solution that I’m wondering why not standard practice - comments pls?
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Cost mainly.
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I've read quite a bit over the years on his site and see that there's good skills and competence there, however would caution that it's a single view and needs to be considered with some care against how many in pure numbers of gearboxes running over the years without problem.
To offer a contrasting position;- the fork stability (wobble on the shaft) ultimately comes from the geometry as they widen the pivot along the shaft specifically to spread geometry and bearing loading. The tolerances Honda have put on this components must be there for a reason which may be lubrication, ability to handle large temperature range etc, that through their r&d brought this manufacturing tolerance to final production. Let's face it they really did choose tolerances mostly beyond accepted practice at that time, and quite freely too with their engineering skills.
The fork (any selector fork for than matter) doesn't usually hold the gear in mesh as it will just roast the fork ears against the gear slot. It is just to move the gear to location and not much more. I can see the view that it places the gears/dog further in mesh IF it wasn't doing that in the first place. But I don't see a large error existing before that.
One of the potential faults in appraisal of existing design is to think you've error only in one direction, and that your modification can only improve something, often there's a backside to APPARENT error that's not considered.
Obviously I can't see exactly what he's seen, and don't even know if that's with a box fully installed and correctly shimmed etc to make the modification valid. It may be all gain, but feel you'd need to see that proven to believe it fully.
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Yes agree with what your saying Nigel, that’s the great power of the forum to get different views. In my case I am mainly trying to sort a problem I had with my 5th gear, and anything I can improve on now hopefully will prevent this happening again to me. I do believe my biggest problem so far as far as I can see was excess lateral movement on the 5th gear cog, (2mm +)that coupled with general wear. Seems these replacement bearings too can compound this problem with the inner race being narrower than original, according to Hondaman.
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I too used Terry (back in 2016 thereabouts) to restore a 400F engine. Nice down-to-earth bloke with traditional engineering premises - he wrote an interesting 3 part blog on the job.
On the subject of CB750 gearboxes, I’m in the throes of rebuilding mine, for no other reason than I want to restore the bike to factory fresh. There is no issue with the ‘box, though once a month it might drop into a false neutral from 4th - it’s such a smooth box that I generally blame myself for being too lazy snicking it into each gear. However, my local engine builder suggests this can typical of 2nd & 4th gears, and so will undercut (by a couple of degrees) the dogs on those gears - ie to encourage them to stay selected once engaged. Seems such an obvious solution that I’m wondering why not standard practice - comments pls?
My inclination would be to not modify one IF it's inspected and verified as not worn. Careful critical assessment of the existing parts etc should show if it's realistic to go with what you've got.
Change on these I've not felt needs altering as such, well not in a way that says an inherent fault from design. Certainly they are very direct on lever action, with even a change of footwear likely to make it different (for example adjusted for riding boot with gear lever pad, then riding it casually with normal footwear) if not paying attention to move the lever concisely.
When contemporary and used really hard as a road bike (not using racing as example) they all worked fine but with that 4th to fith being the one, because of gear rather than dog engagement, needing just a little consideration to make certain you'd properly pulled the lever up fully.
Also depends on the skill and accuracy of the person carrying out the mod. I've ground them to undercutting by hand on moto-X gearboxes that have the dogs particularly massacred, initially to get them going without spares for a race meeting, but pretty effective nonetheless in working fine afterwards.
In this case, quite a personal choice and based on that skill level noted above.
Hopefully gives something to evaluate in proceeding.
Nigel.
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Appreciated Nigel - thanks.
Given the CB750 engineering pedigree of Lindsay I’ll bow to his experience & knowledge - he’s been building / racing / restoring CB750 engines since 1972. I take your point on footwear, technique etc - and if it weren’t for the major strip down I definitely wouldn’t be bothering. The nagging doubt however would be that it was the beginning of something, so I might as well take the opportunity and advice.