Honda-SOHC
SOHC.co.uk Forums => CB750 => Topic started by: archmill on October 18, 2026, 03:39:29 PM
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Having rebuilt the 750F2, I discovered that when the engine is starting with the button, the starter clutch more often than not does not catch and when the engine is running, there is quite a heavy "clicking" noise from the alternator housing. Stripped it down again on that side and discovered 1) the roller end of the rotor was indented (with the shapes of the clutch rollers) and 2) the centre boss of the starter ring was indented all the way round (obviously from the rollers) see attached photo. I deduced the "clicking" noise was from the rollers catching in the dents and being launched outwards. Quite successfully actually, more successful launches than Elon Musk's SpaceX rockets.
The rollers themselves were perfect.
Only place I could find a new starter gear was CMSNL at just short of £200 posted. Mmmm, I wonder ?
How do you measure the depth of a dent on a round boss ? You don't. You measure the width of the dent and if you know a 15mm roller caused it, you can calculate the depth (I used Autocad which is only accurate to 6 decimal places of a mm if you want it to be).
Worst dent between 2.2 and 2.6mm wide, therefore dent calculated at 0.1mm to 0.15mm deep
I machined a boss the exact diameter of the original (42.31mm) and kept taking the smallest cut I can manage on my lathe and testing the rotor with the rollers fitted until I got a fail, I.E. the boss is now too small to work. That size was 41.09mm. Sooooo I can take 1.22mm off the diameter before it fails to grip and the dents are only about 0.15mm deep (0.3mm) off the diameter. Lets go for it.
I ground the boss on my lathe using a toolpost grinder (which I had to make first) and ground away until all the dents were gone.
Measurement :- 42.01mm dia. which is a reduction in diameter of 0.3mm (grind depth of 0.15mm - Yi haw, Autocad calculation correct)
I lightly skimmed the marked rotor face, rebuilt the mechanism with new rollers, plungers and springs as a matter of course and fitted it back to the engine.
Starter works first time every time and no "clicks" while running. Sorted !
Moral of the saga (even thought it's longer than War and Peace) is the starter clutch can be repaired successfully for an absolute minimum of cost.
Eat yer heart out Einstein.
Now, to fix the wet sumping.
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Fantastic recovery. Sadly, I'd need to buy a lathe to save the £200!
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No you wouldn't Martin6, thats what the club is for and Uk has a pretty good postal service. LOL
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Why not buy a spare starter clutch and do an exchange system?
Good work btw, I’d love to be able to do that kind of work.
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Isn't it satisfying knowing you can salvage parts and bring them back up to spec 👍
Well done that man, top job🫡.