I thought It was clutch slip, but I think it actually may be a gearbox, (1st gear) issue?

Britstuff said:
Of course the question now is why is first gear slipping? I think I know why, and I am hoping someone can confirm or debunk my theory. I
Click to expand...
Bent shift fork(s) is the likely cause for gear wheel 06.4639 jumping out of engagement. Bearings (plain and ball bg) would have to be extremely worn to affect the same.
Does 06.4639 feature undercut dogs? If not, it can be modified. Undercut dogs help to keep gear wheels together.

-Knut
 
Last edited:
Britstuff said:
I ended up removing the gearbox form the bike (thank you @marshg246 for the tips on how to do so) and decided to disassemble and inspect the gearbox myself. First off, it does have the original "Portugal" layshaft bearing. So clearly this was an exercise worth doing. Secondly, I wanted to confirm that first gear was slipping / was not properly engaging. I think the evidence of first gear slipping is pretty compelling, please see picture below of the layshaft first and second gears:

Of course the question now is why is first gear slipping? I think I know why, and I am hoping someone can confirm or debunk my theory. I noted when I stripped the gearbox that the kick start shaft seemed to have a lot of end float. I foolishly did not measure it, but it was obviously way above spec. One thing I am not sure about is were layshafts shimmed from the factory? Or is shimming the layshaft something that is only done to compensate for the width of the replacement for the "Portugal" layshaft bearing? I think it is highly likely that the inner cover on my gearbox has not been opened since it left the factory and there were no shims present.

I did notice that the Index plunger appears to be be working correctly. There was also some other general wear, but nothing that stood out to me as being particularly bad. That, said once I get a better idea of what is going on with the first gear slippage, I will do a closer inspection of the remaining parts.

Thank you!
Click to expand...
Those witness marks are present from normal dog engagement.

Gears don't slip. Clutches slip.

For the gear engagement dogs to come out of engagement & allow the freewheeling you described, the selector system has a problem. Forks, cam, detent, ratchet.
 
Thank you everyone for your most helpful input. Sounds like I do not understand what the problem is yet. Not surprising as I am a gearbox newbie. I will do some more thinking and inspecting. One concern I have is that I ideally need to recreate (on the bench) a gearbox first gear jumping out of gear problem that only occurs at approx 3000 rpm (and probably higher) and only in first gear. All other gears no problem. Perhaps...... if it is first gear on the layshaft that is floating, if I reassemble the gearbox maybe I can get gravity (orientate the gearbox with the layshaft gear down while in first gear) or a good shake to prove of disprove that theory or give me some other ideas?

I do not believe the layshaft itself was floating. On disassembly the end was still firmly in the "portugal" bearing inner race and took some effort to remove. The "portugal" bearing (which I will of course replace) looks to be in great shape, the inner race looks very clean as does the end of the layshaft. As far as I can tell I have not yet found any issues with the selector mechanism / forks.
 
Last edited:
You are lucky as those Portuguese bearing usually blow between 10k and 12k miles some with lockups of back wheel, I was lucky when mine decided to blow was kicking my Norton over outside my house when it went bang then had nothing, my mates 850 blew 100 miles from home but he was able to get it into 4th gear and riding it home in 4th with a lot of clutch work and in one town the highway went through was a traffic light at the bottom of one big hill climb, but he got it home.
All the bearing balls was just sitting on the bottom of my gearbox and no gears were damaged.

Ashley
 
ashman said:
You are lucky as those Portuguese bearing usually blow between 10k and 12k miles some with lockups of back wheel, I was lucky when mine decided to blow was kicking my Norton over outside my house when it went bang then had nothing, my mates 850 blew 100 miles from home but he was able to get it into 4th gear and riding it home in 4th with a lot of clutch work and in one town the highway went through was a traffic light at the bottom of one big hill climb, but he got it home.
All the bearing balls was just sitting on the bottom of my gearbox and no gears were damaged.

Ashley
Click to expand...
I guess a good reason to not change the gearbox oil and have a lot of sludge - grabs the balls!

Just kidding!!!!
 
ashman said:
You are lucky as those Portuguese bearing usually blow between 10k and 12k miles some with lockups of back wheel, I was lucky when mine decided to blow was kicking my Norton over outside my house when it went bang then had nothing, my mates 850 blew 100 miles from home but he was able to get it into 4th gear and riding it home in 4th with a lot of clutch work and in one town the highway went through was a traffic light at the bottom of one big hill climb, but he got it home.
All the bearing balls was just sitting on the bottom of my gearbox and no gears were damaged.

Ashley
Click to expand...
Mine blew on a 70 years back but with warning . A tic - tic occasionally whilst riding , then the kicker would head for the pavement and head back to position , then do it again a few minutes later . Locked up the rear finally , dangerous .
 
marshg246 said:
I guess a good reason to not change the gearbox oil and have a lot of sludge - grabs the balls!

Just kidding!!!!
Click to expand...
I very rarely change my GB oil only when I need to go into my GB, GB oil don't lose its velocity unlike motor oil with all the continuance from burn fuel and other crap, some times I might stick my finger inside the GB and feel the oil on my finger, in 48+ years I have only replaced the GB oil maybe 6 times in it life, first time layshaft bearing when it blew, 4 kick start pawls and a broken kick start shaft where the pawl sits in the kick gear, the gear was worn on the inside where the KS pawl was worn from kicking over the motor (the slot where the KS pawl engages the gear was rounded off from wear and slipping when kicking), the KS gear was replaced with new shaft and pawl, that was well over 30 years ago maybe longer can't remember it's been so long and haven't been inside my gearbox since.
My gearbox still changes gears smoothly with light clicks, easy to find neutral, in fact I don't have any problems with my GB, it has a lot of miles on it as it was an everyday rider from new to 2013 and only been off the road for the conversion to the Featherbed frame in 1980/82 and a few upgrades over the years, I still ride my Norton regularly but the Thruxton has taken over the most of my normal ride time.

Ashley
 
Question.....

I have stripped /cleaned the gearbox and replaced the portuguese layshaft bearing with a roller bearing from Andover Norton, (also replaced both mainshaft bearings). Am now engaged in some trial fitting of the gears to test first gear engagement. In the photo below the gearbox selector mechanism is in first gear. The second layshaft gear is raised (by the selector fork) to engage layshaft first gear (removed in this picture). As you can hopefully see, for the first and second layshaft gears to fully engage the second layshaft gear needs to raise another approximately 1-1.5mm.

My question is, is this normal? I am assuming there must be some level of slop in the gearchange. Just wondering how much is normal and considered sufficient for successful first gear engagement?

Thank you!
 

Attachments

  • I thought It was clutch slip, but I think it actually may be a gearbox, (1st gear) issue?
    IMG_7524.webp
    130 KB · Views: 93
Last edited:
Britstuff said:
Question.....

I have stripped the gearbox and replaced the portuguese layshaft bearing with a roller bearing from Andover Norton, (also replaced both mainshaft bearings). Am now engaged in some trial fitting of the gears to test first gear engagement. In the photo below the gearbox is in first gear. the second layshaft gear is raised (by the selector fork) to engage layshaft first gear (removed in this picture). As you can hopefully see, for the first and second layshaft gears to fully engage the second layshaft gear needs to raise another approximately 1-1.5mm.

My question is, is this normal? I am assuming there must be some level of slop in in the gearchange. Just wondering how much is normal?

Thank you!
Click to expand...
It's probalby the picture angle, but it does not look to be in 1st to me. Are you sure the plunger is in the 1st gear detent and not neutral?
 
Hi @marshg246:

Sorry about that. Yes, I believe it is fully in first gear. the plunger is definitely in the first gear notch, not neutral. The selector fork is located firmly in the first gear (end) position of the camplate. Selector forks do show light wear but are not bent.

Here is a slightly different angle:
 

Attachments

  • I thought It was clutch slip, but I think it actually may be a gearbox, (1st gear) issue?
    IMG_7522.webp
    237.5 KB · Views: 81
Hi All:

The gearbox is now fixed, shifts smoothly and reliably!

I think the problem was a combination of excessive end float on the layshaft and ever increasing wear to the first gear dogs between first and second gears on the layshaft, causing only partial first gear selection, with a tendency to "jump" out of first gear. Not exactly sure why, but end float was actually much less once I changed out the infamous "Portugal" layshaft bearing for a new roller type bearing. I replaced the first and second layshaft gears, all bearings and a number of other bits and pieces. After cleaning, careful reassembly etc and much trial fitting, I settled on shimming the kickstart shaft with a single .020" rear isolastic shim, for a total end float of approx .011".

I found the following youtube video and its succeding episodes to be incredibly helpful. Not sure how it measures up with the Mick Hemmings vids, as I have not watched them:

You Tube Channel: Classic Motorcycle Mechanics with Chris Rooke



I wish to say thank you to all the members of this forum who have helped me with this gearbox issue and a number of other problems with my Commando! Your knowledge and patience is much appreciated. I now have a solid, usable and (dare I say) "reliable" Commando and the basic understanding that I need to keep her running and enjoy for years to come!

Thanks again!

KInd Regards,

James


1730643963127.jpeg
 
Last edited:
L.A.B. said:
The cause of my Commando occasionally jumping out of first gear was due to a worn layshaft first gear bush which had also begun to break up and move out of position.
I thought It was clutch slip, but I think it actually may be a gearbox, (1st gear) issue?
Click to expand...
You are the first one to mention this. This bushing wears out easily and I usually replace it as a matter of course whenever I go into a gearbox. I made a short steel dowel with a tiny shoulder to push it in and out using a vice.
 
seattle##gs said:
You are the first one to mention this. This bushing wears out easily and I usually replace it as a matter of course whenever I go into a gearbox. I made a short steel dowel with a tiny shoulder to push it in and out using a vice.
Click to expand...
I freeze the bush before pressing in .
 
Torontonian said:
I freeze the bush before pressing in .
Click to expand...
I use plenty of oil.
The first thing I do when installing a bearing or bush is to place it on the hole and with dial calipers measure in four spots to make sure it's going in straight. Then start pushing it in. If things are hot this must be done quickly. This why I never bother freezing the bearing.
Try putting a bearing in the freezer and measure the O.D. Then measure it at room temp. No change.
 
Britstuff said:
The plunger is definitely in the first gear notch, not neutral. The selector fork is located firmly in the first gear (end) position of the camplate. Selector forks do show light wear but are not bent.
Click to expand...
I know this reply is too late, as you have assembled your transmission. Your selector shaft is (was) unsuppoted at one end. Wiggling it may produce the desired effect.

Britstuff said:
I replaced the first and second layshaft gears, all bearings and a number of other bits and pieces.
Click to expand...
Again, a late comment. Great renovation, but you probably know gears should be replaced in pairs because of the wear pattern.

- Knut
 
You must log in or register to reply here.
Back
Top