SOHC.co.uk Forums > CB350/400
Gearbox Stuck - The last straw?
K2-K6:
I'm not immediately , in fine detail, familiar with 400 gearbox components.
In answer to this though "What actually holds the gear shift shaft in place? Is it just the gear shifter? Does anything hold it in place on the clutch side?" The shift drum will be limited for end float to control that lateral movement. Turning it to change gear, it has to remain still while pushing & pulling the selector forks across the gearbox to select the ratios. Insufficient restraint and it will move instead of the forks, making selection poor, possibly bringing a severe clash that will break parts.
Usually it'll be something of circlip, thrust washer or bolted through a bearing that's fully retained in the case, this to give that assured sideways location.
A workshop/ parts schematic should show the relevant components.
Dolomite_:
--- Quote from: K2-K6 on November 23, 2025, 11:55:58 AM ---I'm not immediately , in fine detail, familiar with 400 gearbox components.
In answer to this though "What actually holds the gear shift shaft in place? Is it just the gear shifter? Does anything hold it in place on the clutch side?" The shift drum will be limited for end float to control that lateral movement. Turning it to change gear, it has to remain still while pushing & pulling the selector forks across the gearbox to select the ratios. Insufficient restraint and it will move instead of the forks, making selection poor, possibly bringing a severe clash that will break parts.
Usually it'll be something of circlip, thrust washer or bolted through a bearing that's fully retained in the case, this to give that assured sideways location.
A workshop/ parts schematic should show the relevant components.
--- End quote ---
Its the shaft that goes through the casing and the foot shifter connects to the other end. not the shift drum.
Is it just the foot shifter that holds the shaft in place? I cant see any circlips or anything else on the exploded diagrams.
K2-K6:
Apologies for delay in responding.
Ordinarily, the change mech (lever fork, indent disc/spring etc) don't locare the drum in lateral movement. They are usually just free and "floating" to keep their primary function competent and easy to use in changing gear.
Looking at the schematic, it doesn't show much in the way of any specific means to control that movement.
It appears that its controlled by the bearing in the case being positively located, the shift drum passing through that, then the outer pin mech etc being restricted enough to hold it positively in that bearing.
If you remove fork and disc etc, can you push and pull the drum in and out of casings to any significant degree ?
SeanFD:
This video https://www.youtube.com/watch?v=vR-MiqdHi1w&t=234s which I referenced before is a very clear demonstration of re-assembly of the gear selector mechanism.
This one https://www.youtube.com/watch?v=NdYqec2u_4U&t=95s at about 13 min, shows on a CB750 how the mechanism fits together and how it operates. The 750 selector mechanism is almost the same as the 400.
The selector drum is held in place by the bearing on the left side, and there is nothing holding the selector.
They might help.
SeanFD:
Good morning all!
I'm back at it now and about to go and uncover the bike.
Dolomite, did those adjustments you made solve the problem?
If not, have you had any success?
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