Honda CG125 Gearbox

[ColinH]

I’m putting the gearbox back together on a 1986 CG125 Brazil.
It goes back together and kinda works but sometimes jams and gets stuck. I may have muddled up the selector forks....does anyone know how to make sure I have the forks in their right places? They have some numbers and letters cast into them - are they relevant to how they go together? I am putting the gearbox together as a test with no gasket in place - gears etc are oily.Thanks a lot for any guidance.

[stinkwheel]

Are you turning the output shaft as you try to shift it? Have you refitted the clutch?

Remember a gearbox won't always shift properly unless the bike is rolling and the clutch is disengaged. If you're trying to go through all the gears with the engine stationary on a bench, I wouldn't expect it to get them all all the time, just like if you stop in top gear and try to change to first, you need to rock the bike and fiddle with the clutch.

To answer your question, a lot of honda parts are marked with whole or partial part numbers. There have been a lot of differnt GC125s so you'll need to look yours up, an example of a CG selector fork part number is: 24211KPT900

If you go on CMS and find your model, they have parts lists which include an exploded diagram of the gearbox, part numbers and if you click on the part, usually a photograph of the part. Should be able to aork it out from there.
https://www.cmsnl.com/honda-cg125_model1192/
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[ColinH]

Clutch is not fitted yet. I have just assembled the gearbox and closed the two casings as a test. I’ll try and trace the part numbers of the selector forks as you suggest to make sure they are in the right positions and put the whole engine together and test.
Thanks a lot for the help - much appreciated!

[WD Forte]

One quick test is to try and replicate the actual running not static conditions after assembly
The closest and simplest thing I've found useful is to to keep turning the output shaft by hand when selecting gears up and down the box.

If I've done it right can go up and down the box and find neutral quite easily, if there's any stiffness or jamming I've made a mistake or there is damage, so it's go back and start again.

if youre not turning the output shaft, it's quite possible for it to feel stiff
and 'jammy'
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bikers smell of wee

[ColinH]

Thanks WD. I’ll give that a go......

[RhynoCZ]

[stinkwheel]

[ColinH]

Yep, thanks. I wondered if the gasket would have an influence. It’s all reassembled ready to go back in the frame for a proper test......

[lingeringstin...]

[quote="stinkwheel"][quote="RhynoCZ"]

Not sure about Honda, but some bikes do need a gasket of proper thickness between the two halves of the engine (gearbox) to function properly.[/quote]

A good point, vertically split gearboxes need the right amount of end-float to work properly, you may even be slightly bending the selector guide rods with no gasket in. Some are fussier than others, you have to shim the ends of the shafts on some older bikes, which is a faff (dismantle, reassemble with plastiguage, dismantle, reassemble with shims and hope you got it right). I suspect a CG is more forgiving and engineered with better tolerances.[/quote]



THIS.

I once "cured" an annoying oil leak by having the engine halves skimmed only to open up a world of gearbox shenanigans that ended up with me machining adapter collar things and all sorts of malarky. I eventually ended up with a gearbox that worked but it was a huge arseache.

I suspect the CG125 gearbox isn't far off what I've got in my Chonda clone engine. I've made gearboxes out of assorted bits from other engines that you would think should go together just fine but I've ended up having to take them apart several times and reassemble them in exactly the same manner with only very miniscule shim differences before the gearbox would function correctly.

That's why now I always test that the gearboxes will shift easily by hand on the bench before putting the rest of the engine together. I don't even put the clutch on, I just spin the gearshaft by hand and waggle the selector drum through the gears. If it works that way it's good.