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Discussion Starter · #1 · (Edited)
Hey hey all waves at monitor,
I picked up a semi-rough 125 SX yesterday. I knew beforehand the clutch was 'problematic'. I can get going in first - barely. From there the engine just revs as gas is applied and will not go any faster than maybe 5 mph in any gear. I did notice some notching in the basket from looking in from the oil fill.

So, being the kind of guy to monkey around on stuff, I took off the plastics, tank, and subframe. Then I finally drained the oil (it has a generic bolt in there - I need to get a magnetic one) and saw it was grey/gray. I looked around the forums and figured it was clutch material. I smell no coolant and it didn't seem to separate after a while.

You with me still? Good ;) Anyhow, after draining the oil, I removed the clutch cover, pressure plate, springs, and took out the plates (all in one piece because supposedly the metal plates are both steel and aluminum and go in a certain order). I measured the friction plates and they ranged anywhere from 2.95 to 3.04 mm. That seems well within the spec from what I've read in the repair manuals (2.9 - 3 mm). Only one spring was 38.7mm - the rest were 40.x mm. The range in the repair manual is 38 - 39 mm. Maybe my micrometer was off shrugs

So, I'm trying to figure out what's going on. The greyish oil had me thinking 'def clutch'. But the specs seem to be within tolerances. Maybe the grooves? How bad does a basket have to be before it doesn't work at all? I'll try to include some pictures because ..........well...... they're just better than words sometimes lol.

The clutch lever 'seems ok'. I mean, there's nothing obvious that I can feel.

Things I have NOT looked at yet/wondering about
-master/slave cylinder
-something else in the clutch assembly (rod,pin,etc)
-How far should the push rod come out when you pull the clutch lever? I have the plates still out and the push rod is still installed. I did notice a couple of 'bluish' areas on the rod - could that be heat related?
- The one pic with the 'pressure plate' (I think that's what it's called). You'll notice a couple small 'marks'. One right by my thumb, and another about the 12 oclock position. Those are very small 'pits' in the plate. No idea what that's from.
-Oh, one more. After reading the 'aluminum/steel' clutch plate installation order, I tried my magnet on the plates - and they're ALL steel. Is that ok? I did see that the first and last plates were fiber/cork/whatever material.

Ok, I've rambled on long enough. Hoping someone out there can point an old-timey guy in the right direction :)








 

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Welcome to the forum.

If clutch plates are inside spec and clutch basket does not have too deep ridges (they can be filed away if need be, the clutch might become a fraction more "rattly" but you will not really hear it) then the issue is the inner clutch hub where the mating surface has worn down. You can test this if you have a spare steel plate: Fit the spare steel plate as the first plate in the stack (it will now take over the job the worn inner clutch hub mating surface had). Re-fit the clutch plates and adjust the clutch so it works well. (NOTE: The height of the stack will change by doing this and so you need to make sure the clutch operates as per normal). We have raced a 2 day MX weekend by doing this w/o any issues. We had to fit two fibre plates w/o a steel plate between them to get all to work well . But you can also fit two steel plates together.

Here the clutch for your bike: 1998 KTM 125 Clutch | KTM World at $92 the inner clutch hub is not badly priced!
 

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Discussion Starter · #3 ·
Welcome to the forum.

If clutch plates are inside spec and clutch basket does not have too deep ridges (they can be filed away if need be, the clutch might become a fraction more "rattly" but you will not really hear it) then the issue is the inner clutch hub where the mating surface has worn down. You can test this if you have a spare steel plate: Fit the spare steel plate as the first plate in the stack (it will now take over the job the worn inner clutch hub mating surface had). Re-fit the clutch plates and adjust the clutch so it works well. (NOTE: The height of the stack will change by doing this and so you need to make sure the clutch operates as per normal). We have raced a 2 day MX weekend by doing this w/o any issues. We had to fit two fibre plates w/o a steel plate between them to get all to work well . But you can also fit two steel plates together.

Here the clutch for your bike: 1998 KTM 125 Clutch | KTM World at $92 the inner clutch hub is not badly priced!
Thank you Axzon! I’ll try switching the plates around per your suggestion first before anything else 👍
 

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Discussion Starter · #4 · (Edited)
Ok I’m having a wee bit of difficulty (what else is new)
I’m Fixin to file down the ridges on the basket, but I can’t remove the 27mm nut. I’ve bent back the tabs for the ‘keeper’ or whatever it is, but I can’t figure out how to prevent the hub from turning. Maybe take the bike off the stand and have the wife hold the rear brake down? I see a tool is available but figured I’d ask here first 😁
Edit: didn’t work with the brake pedal method lol
 

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Ok I’m having a wee bit of difficulty (what else is new)
I’m Fixin to file down the ridges on the basket, but I can’t remove the 27mm nut. I’ve bent back the tabs for the ‘keeper’ or whatever it is, but I can’t figure out how to prevent the hub from turning. Maybe take the bike off the stand and have the wife hold the rear brake down? I see a tool is available but figured I’d ask here first 😁
Edit: didn’t work with the brake pedal method lol
OK, so you would have the clutch plates out. And the tabs on the big lock washer are down. Put motor in to top gear, stop the rear wheel from turning and give the nut a strong turn to open it.
 

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Discussion Starter · #6 ·
OK, so you would have the clutch plates out. And the tabs on the big lock washer are down. Put motor in to top gear, stop the rear wheel from turning and give the nut a strong turn to open it.
I’ll give that a (t)whirl, thx again! 👍
I also noticed (after I took the tank off) there’s no engine mount bracket 🤦‍♂️. Thankfully I found one on EvilBay……phew!
 

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Your basket looks terminal. Your milky trans fluid suggests coolant has contaminated your transmission. You can purchase a proper basket holding tool or simply fashion one from your worn out clutch parts. Take a friction plate and steel plate and drill holes in both and rivet or screw them together. It works as good as the factory made tool and it makes you look like a professional (y)
 
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Discussion Starter · #8 · (Edited)
Your basket looks terminal. Your milky trans fluid suggests coolant has contaminated your transmission. You can purchase a proper basket holding tool or simply fashion one from your worn out clutch parts. Take a friction plate and steel plate and drill holes in both and rivet or screw them together. It works as good as the factory made tool and it makes you look like a professional (y)
I'm gonna try filing down the grooves first. I dont have the coin to get a new basket. As for the fluid, I think it may be clutch material. It was slipping so bad that the bike was barely moving with it revved pretty high. At least I like to think its only clutch material lol. Good idea on the tool to remove the hub! (y)

I just got back from the auto parts store. I purchased some replacement fuel/overflow lines for the carb. When I took the carb off I noticed there were only 3 hoses attached: fuel, overflow, and maybe venting? There seem to be 3 more places to attach hoses/lines. After looking on the interwebs a while, it appears that both 'vertical' posts can go into a 'T' then go somewhere, and the 2 horizontal posts also go into a 'T' and go somewhere. Does that sound correct? I can look up exploded views of the carb till the cows come home, but can't seem to find info on how/where to connect the various lines :/


 

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The tiny hoses are vents. The slide chamber vents are teed together above carb slide chamber. I connect them with sleeve connector and drill 030 hole in plastic connector. Hole is not needed but i feel better with tiny vent hole. If it were too large it could suck dirt or water through carburetor. Best to keep hoses flexible and clear as possible. The bowl vents usually go down behind motor and hang below swingarm. Savvy racers slash cut open end claiming it helps keep mud/debris from building up. I slash cut them as well but have no proof it helps. Certainly can't hurt.
 

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Filing basket may cause its own set of problems. When they cut that deep, filing them smooth will cause clutch drag. You may find it is usable until replacement can be sourced. I have seen much worse. Be sure to purchase economy oil or ATF and change it often to reduce aluminum shavings in your gearbox. It is a bit hard on the bearings. Changing trans fluid helps drain the nasties out. Don't be afraid to use the economy oils in that gearbox. It's a sight better than the mud you pictured in your first post. You may find your clutch action improved by substituting automotive ATF for gearbox lubricant. Stuff works very well. Use Mercon/Dextronlll
 

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Discussion Starter · #11 ·
The tiny hoses are vents. The slide chamber vents are teed together above carb slide chamber. I connect them with sleeve connector and drill 030 hole in plastic connector. Hole is not needed but i feel better with tiny vent hole. If it were too large it could suck dirt or water through carburetor. Best to keep hoses flexible and clear as possible. The bowl vents usually go down behind motor and hang below swingarm. Savvy racers slash cut open end claiming it helps keep mud/debris from building up. I slash cut them as well but have no proof it helps. Certainly can't hurt.
thank you for the info :). I was looking around some more and found a post that you replied in. Included pictures too on how the hoses go :). I also noticed there's a port right below the carb at the top of the engine. I'm guessing it's a vent from the trans. And, naturally, there's no hose there either.......yeesh. So off to the auto parts store again sometime soon! 🤦‍♂️ I need to get more anyhow as the hose(s) I purchased were a 'variety' pack so there wasn't enough.



Filing basket may cause its own set of problems. When they cut that deep, filing them smooth will cause clutch drag. You may find it is usable until replacement can be sourced. I have seen much worse. Be sure to purchase economy oil or ATF and change it often to reduce aluminum shavings in your gearbox. It is a bit hard on the bearings. Changing trans fluid helps drain the nasties out. Don't be afraid to use the economy oils in that gearbox. It's a sight better than the mud you pictured in your first post. You may find your clutch action improved by substituting automotive ATF for gearbox lubricant. Stuff works very well. Use Mercon/Dextronlll
Good to know. I got some 15W-40 to use as I couldn't find 20W-40 anywhere! a 5 Qt jug of that ATF is about the same price as well ($19-$25 USD). Got some new grips, magnetic drain plug, inner clutch hub, plates/springs and a flywheel puller (just in case) come in today. I spent all day cleaning the subframe and pieces of it since it was raining and otherwise nasty out. Repacked the pipe as well. Scotch Brite/ Quad steel wool ftw! I've been unable to remove the 27mm nut no matter what I do, so a removal tool should be here in a couple days. In the meantime - replacing worn/rusty/missing nuts and bolts and cleaning and more cleaning.......

Thank you all for the wealth of information. It's been soooooo nice getting helpful info! (y)




 

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I just recently sold my 2003 SX125 a month or so ago. Little rocket. More fun than a bucket of tits (y)
 

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Well done! And yes, the 2 stroke KTM's are buckets of fun. My son races a 2016 125sx and a 2013 150sx. Both bikes have HGS pipes. Add some trickery and the 150sx has become a 250f killer!
 

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Discussion Starter · #14 ·
Just a small update:
Got all the parts in! Spent yesterday filing the basket, installing the plates/springs/cover gasket. I also took a peek around for any missing nuts and bolts and found a few. I've been replacing worn/rounded off bolts as I find them.
-Got the subframe back on (that was a puzzler getting the separate pieces together - who knew 3 pieces could be so difficult?).
-New grips on. The throttle seems to not snap back all the way - I'm not sure why. Maybe I need a 45 going into the carb rather than a straight one the curves (binds?) near the carb shrugs.
-I still need to route the carb tubing as well. Muffler is replacked and on.
-The intake boot is slathered in gasket maker. My nephews buddy has a 3D printer so maybe he can print one? He'd be a popular guy for sure! I've seen on the WWW it being done successfully. Not quite sure how the original part is measured/scanned into the computer tho
-Made a vent hose for the filler cap. I noticed a small L-shaped bent right in front of the tank on the headstock. No clue what it is/does
-There is no triangular engine bracket. I guess I didn't look close enough. TBF its kinda behind the tank and hard to see. I ordered one off EvilBay. Came in yesterday - it seems a bit 'short' tho. By about 1/4 inch. Maybe there's an aftermarket cylinder/head cap installed? I reckon I can enlarge the hole on the bracket a bit to get it to fit tho....
-One of the 2 bolts on the 'exhaust control valve' (aka power valve?) cover is missing. I removed the other one and tried it in the other hole. The hole is stripped :(. Helicoil? I think that involves drilling and such, which means removal of the cylinder itself - dont want metal shavings in there!).
-Another bolt down around the exhaust seems to be broken off inside the hole. There's a set of 4 that seem to hold the exhaust flange on. Drilling that one out seems like a nightmare. At least 3 are on there tight!
-Replaced front sprocket. It was way overdue! I'm pretty sure the 'ridge' on the sprocket goes to the inside when installed........

I'm getting closer!






 

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Hey hey all waves at monitor,
I picked up a semi-rough 125 SX yesterday. I knew beforehand the clutch was 'problematic'. I can get going in first - barely. From there the engine just revs as gas is applied and will not go any faster than maybe 5 mph in any gear. I did notice some notching in the basket from looking in from the oil fill.

So, being the kind of guy to monkey around on stuff, I took off the plastics, tank, and subframe. Then I finally drained the oil (it has a generic bolt in there - I need to get a magnetic one) and saw it was grey/gray. I looked around the forums and figured it was clutch material. I smell no coolant and it didn't seem to separate after a while.

You with me still? Good ;) Anyhow, after draining the oil, I removed the clutch cover, pressure plate, springs, and took out the plates (all in one piece because supposedly the metal plates are both steel and aluminum and go in a certain order). I measured the friction plates and they ranged anywhere from 2.95 to 3.04 mm. That seems well within the spec from what I've read in the repair manuals (2.9 - 3 mm). Only one spring was 38.7mm - the rest were 40.x mm. The range in the repair manual is 38 - 39 mm. Maybe my micrometer was off shrugs

So, I'm trying to figure out what's going on. The greyish oil had me thinking 'def clutch'. But the specs seem to be within tolerances. Maybe the grooves? How bad does a basket have to be before it doesn't work at all? I'll try to include some pictures because ..........well...... they're just better than words sometimes lol.

The clutch lever 'seems ok'. I mean, there's nothing obvious that I can feel.

Things I have NOT looked at yet/wondering about
-master/slave cylinder
-something else in the clutch assembly (rod,pin,etc)
-How far should the push rod come out when you pull the clutch lever? I have the plates still out and the push rod is still installed. I did notice a couple of 'bluish' areas on the rod - could that be heat related?
- The one pic with the 'pressure plate' (I think that's what it's called). You'll notice a couple small 'marks'. One right by my thumb, and another about the 12 oclock position. Those are very small 'pits' in the plate. No idea what that's from.
-Oh, one more. After reading the 'aluminum/steel' clutch plate installation order, I tried my magnet on the plates - and they're ALL steel. Is that ok? I did see that the first and last plates were fiber/cork/whatever material.

Ok, I've rambled on long enough. Hoping someone out there can point an old-timey guy in the right direction :)

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It's an old bike.

Clutches, gaskets and top end components don't last very long. They're basically consumable items. Replacement intervals are in hours not days or months.

The bike is past the point of just filing down the basket and replacing the plates. Might work for a little while but not much longer.

I would have concern about the top end also. It's cheaper for a piston, rings and bearing than a new cylinder and head.

I wouldn't have bought a 24 year old dirt bike without expecting to have to almost rebuild the entire thing as a project. Just be aware that cheaping out with some items might cause damage to other more expensive parts.
 

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Ah. Forgot to mention. Plastic barb fitting on steering stem pipe pictured is connection for fuel cap vent hose. Very sanitary method of fastening it. Well engineered machines, our KTMs
 
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Discussion Starter · #18 ·
Thank you Doggie! I kinda figured that but just seemed kinda weird seeing there’s a hole in the steering column where a vent hose fits as well 🤷🏻‍♂️
Got it all buttoned up, turned on the gas (choke held up using the ‘zip tie around the plunger’ system), and it barked off 2nd kick! Woohoo!
It was still on the stand (ice cooler lol) so I pulled in the clutch. It went into gear with a noticeable ‘thunk’ and the back wheel really wanted to turn (cooler isn’t tall enough for the wheel to be off the ground).
tirned it off, got it off the cooler, got on, put it in gear. I could move the bike back and forth with the clutch in, but there is some noticeable resistance.

took the cover off the master cylinder (the one on the bars) and there was no fluid. S few days ago there was. Oh great I have a leak! Next I removed the piece on the……..uhhhhh….other end (slave cylinder?). There was a bit
of fluid all around the outside, so I guess I know where the leak is. I don’t see any gasket either….maybe there So I guess I need a slave cylinder rebuild kit. Any recommendations on brand?
I’ll put up a few pics because it’s more informative 😁



 

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Rebuild kits are available for the slave cylinders. Fairly routine operation. Take pictures to remind you how it all goes together. Worst part of the job is bleeding clutch fluid after slave reassembly. Keep removed slave cylinder connected to hydraulic line if possible during overhaul and keep it above height of master cylinder. This keeps some fluid in master cylinder and hose and will help keep some of the air out during slave overhaul. You will help the bleeding process by putting small amount of correct fluid in slave before installing its piston. Bleeding these clutches once they become air bound can be a real bitch. Try pre bleeding with slave held up high above master cylinder to help remove air. I use bungee cord and zip ties to hold the bastard up from the roof rafters. The air will rise to highest point and make bleeding easier. Be sure to use correct fluid in clutch master. The cover will indicate correct fluid needed.
 
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On older KTM's the gasket thickness between the slave and the motor is important so that the slave does not put pressure on the rod that goes to the clutch while riding. The idea is to set it so that there is a small gap between the steel ball in the slave and the rod at rest (no pressure on the steel ball in the slave).

Also, if it says "Magura" on the master lid you might need to make the push rod in the master a fraction longer to get enough stroke for the slave piston to properly open the clutch plates. And the Magura text also indicates that you need to use hydraulic oil and NOT brake fluid in the master/slave!

And fit a steel bracket/cover to the slave so that if/when the rear chain comes loose/breaks it hits the bracket/cover and not the slave... That normally also cracks the left motor half... (ask me how I know...). And as your motor casings are now obsolete the extra work is worth it...

Regarding how to get the air out of the clutch: Follow Augies suggestion and hang the slave high up. Then pump the master and open the bleed nipple on the slave. Do this a few times until no more air comes out when you open the nipple.
 
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