Gearbox Stuck - The last straw?

[SeanFD]

Just trying to think outside the box, could this be caused by the retainig clip coming off the gear selector fork?

You mean inside the box, Ted? 

Which clip are you referring to?

Dolomite: I won't be updating too soon because I'm not going across until mid-Dec. If you've by any chance solved the problem by then, please please post the solution.

Dave: So, gasket oil seal and Hondabond. ✔️ and ✔️ and ✔️ That's No 270 on the parts fiche? 91205-333-005?

[McCabe-Thiele (Ted)]

When I rebuilt my 400 there was a clip on the gear selector forks inside the gearbox.
It a few years back iirc there are two shafts with  selector forks on each shaft that move the gear dogs. Without the clip the selector  fork just moves along the shaft thus failing to engage a gear.
If you drop the sump the clip might make it's way down there.

I might have miss remembered, I lost one of the clips on dismantling. I swept the floor, tried
a big magnet to no avail. A few weeks later I found it on the garage  floor a good distance away.

[Bryanj]

Anything is possible, as the engine was apart were all the thrust wazhers put back in corect places

[Ken4004]

I have just seen this video for assembling of the Honda cb 750 gear change mechanism, this is of course not the Cb400 but explains the principle of operation and might be useful to somebody.

When I assembled my Cb 400 four gear change mechanism I took a lot of photos when I took it apart because I realised it was quite complicated and would be easy to make a mistake.


https://youtu.be/NdYqec2u_4U

[Ken4004]

I have just found another video which again is not the CB400 four but does explain nicely the principle operation of the gear change mechanism and I think it shows the cir clip that has been mentioned earlier, again might be useful to someone.

https://youtu.be/Xtghypx8mII

[SeanFD]

I had forgotten about this series of videos:

400 Four Engine Rebuild:
https://www.youtube.com/watch?v=vR-MiqdHi1w&t=62s

I think the retaining pins Ted mentioned are the two he shows right at the beginning.

[K2-K6]

As a general guide to fault finding, there's as many sliding gear items as selector forks, obviously 

Each of these should move freely with the forks removed. The important bit in contrast though, is that ALL of tbe remaining gears should be fully located on their shaft such that they absolutely cannot move along the shaft. Anything substantial (probably more than 0.5 mm ) for these "fixed" gear items, and you'll likely have thrust washer, circlip etc that's compromised, missing, misplaced etc as a fault.

With the gear set in neutral, there's very little clearance between moving items and stationary gears (for movement along each shaft) that stops the whole thing getting into a mess with potentially two gear ratios part engaged simultaneously. 

Of course, the selector forks must have positive and functioning engagement, usually via spigot pin to the selector drum. Also the selector to be not obviously bent such that it can cause out of tolerance position in moving gear item.

[K2-K6]

Additionally, the end float of both gear shaft and the selector drum will cause variance of control between the two as gear selection is then a moving target, which it shouldn't be.

[McCabe-Thiele (Ted)]

Just realised the clip I was refering to earlier was not on my 400 gearbox.

Its the end clip that holds the pin on a 500 K1 selector fork, its my age I guess.

[Dolomite_]

I spent a few hours investigating the issue today. Drained the oil, took the clutch casing and clutch drum off and also the sump.

I noticed that the bolt circled in red had become loose enough to allow the disc to have some play. I think this then meant that the shift arm in blue wasn't snug up to the pins. It was catching them and then slipping off. Because the bolt was loose I think it also meant that the part behind the disc wasn't straight like it should have been.

I took the disc off and put the bolt back so I could see what was happening and when I held the arm in place it shifted fine. I added the disc back and tested it again and it felt like it was slipping off the pins again. The arm had some stiffness against the disc when it tried to return flush against the pins. This turned out to be because the gear shift shaft had moved out slightly and was putting some pressure on the arm, I pushed the shaft back and it seems to be fine.

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?

One other thing to double check. Does the arm held in place by the yellow bolt help shift it that last little bit? When I removed the arm when testing and I would shift I would need to manually rotate the part behind the disc slightly to get it to engage the gears. This gave me a little panic because I was hearing sounds from the gear box I thought that something had become loose inside.

Lastly does anyone know the toque settings for the red and yellow bolts and also the bolts that hold the clutch springs in place? I will also be putting a bit off loctite on them.

Im not sure why the image isnt showing Ive done [img ]http://[\img ] (without the spaces)



https://imgbox.com/ZkK3587b

[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_]

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.

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?