Tips/Help on KTM SX125 top end Rebuild

[Blaster528]

Hello blasterforum members, im looking for opinions on how i should go about rebuilding a sx125 top end. I know that the cylinder is plated and sometimes needs to be replated if excessive damage occurs. My question(possibly for Ken O Connor himself) should I replate the cylinder or not. Ive heard this 2 ways. Some people say slap a piston and new gaskets on there, You should hone the cylinder lightly, and You have to get it replated every top end................So as you might be able to tell im confuzzled. Hoping someone can point me in the right path.

 

[best]

What year KTM? I have 2000 and 2003 KTM 125 engines.
These engines usually just need a new piston and rings and good for another 5-10 years.
Plating damage usually only comes from not running an air filter or engine damage.
Do NOT hone it. It is hard and smooth on purpose, leave it that way.

I run mine on the street. Easily does 140 kph(75mph) with the right gearing.
Almost no mods, it runs with the big boys.
Learn to tune your jets, powervalve, squish and head design, it will scream.

Steve

Steve

 

[Blaster528]

It's a 2003. Far as I know the only modification that has been done is full pro circuit. It's not in my hands at the moment so I can't take it apart. I know it has low hours by the current owner. I'm the only one that rode it this year and prior to that hasnt been started in 2 years. Compression is OK I can push it down with my hand but I'm estimating 100psi. So I know it will need to be rebuilt. So your saying I can get away with just new piston and gasket kit? If so, if you don't mind me asking, about how much do you spend.

 

[Blaster528]

 

[best]

I run a 2003 125SX motor in my 2000 EXC for street riding. The close speed gearbox and a tight powerband DOMA pipe gives great power delivery on the highway. Screaming peaky power makes me feel like Valentino Rossi as I go up and down the gears at highway speeds.

I am used to paying about $100 for a piston kit and about $50 for gaskets(Cdn$), although I often get complete kits for less than $100cdn. Be careful about the year, KTM changed piston size slightly around 2001 and many people have swapped engines and top ends. Check what you have.

Unless you are racing, I'd leave it alone if it starts, runs and idles. I dunked mine and yet ran it for ages at 90psi. Still did great top speed. Oil makes a big difference in dynamic compression. Run 40:1 Motul 800 and it will tighten up. Mine was 7 year s old before I dunked it and needed a piston change. My 2001 300 and 380 have the original pistons in them.

A good set of Bark-busters and another bolt in the seat and you are ready to go!
Not really. Bring it home, download a manual and do a complete check of everything.
Oil change, filter clean and oil, carb clean and write down the jets, set up the suspension to the manual.
Check every bolt for tightness. Check the steering stops so you don't smash your CDI (or relocate it).
Check the spokes, ping-ping! Scored rotors are a biatch, how are the pads? Replace the brake fluid.
Clutch takes mineral oil, ATF will do. Check it all out, then ride with confidence.

Me in green, my 2000 125EXC
The 660 Raptor was mine as well, I sold it once I felt the potential of a Blaster!

 

[speedyslyder]

if that is a recent photo?
i would check your chain slack it looks too loose
could be photo but set it as per specs,don't want that to fall off lol
if it is stretched too far then oh well time for a new set of sprockets and chain.

 

[Blaster528]

Thanks best. It's actually kinda funny that you mention the seat bolt because I just noticed it yesterday when I had to fix the back fender (someone decided to stack multiple 2x4s on the tail until it bent out of place). The spot is actually snapped off but it'll be an easy fix, I hope. I'm definitely going to lightly power wash it since it's greasy and dirty. Then drain gas, oil, coolant. Once I get it home I'll post back. Thanks again

 

[Blaster528]

if that is a recent photo?
i would check your chain slack it looks too loose
could be photo but set it as per specs,don't want that to fall off lol
if it is stretched too far then oh well time for a new set of sprockets and chain.

Yes that is the most recent photo. The picture doesn't lie, that chain is loose. Rear sprocket looks to be in newer condition whereas front sprocket you can notice slightly bent teeth. I might replace it but I'm pretty sure I can just pull the axle back or if there is one too many links just remove one.

 

[Blaster528]

I run a 2003 125SX motor in my 2000 EXC for street riding. The close speed gearbox and a tight powerband DOMA pipe gives great power delivery on the highway. Screaming peaky power makes me feel like Valentino Rossi as I go up and down the gears at highway speeds.

I am used to paying about $100 for a piston kit and about $50 for gaskets(Cdn$), although I often get complete kits for less than $100cdn. Be careful about the year, KTM changed piston size slightly around 2001 and many people have swapped engines and top ends. Check what you have.

Unless you are racing, I'd leave it alone if it starts, runs and idles. I dunked mine and yet ran it for ages at 90psi. Still did great top speed. Oil makes a big difference in dynamic compression. Run 40:1 Motul 800 and it will tighten up. Mine was 7 year s old before I dunked it and needed a piston change. My 2001 300 and 380 have the original pistons in them.

A good set of Bark-busters and another bolt in the seat and you are ready to go!
Not really. Bring it home, download a manual and do a complete check of everything.
Oil change, filter clean and oil, carb clean and write down the jets, set up the suspension to the manual.
Check every bolt for tightness. Check the steering stops so you don't smash your CDI (or relocate it).
Check the spokes, ping-ping! Scored rotors are a biatch, how are the pads? Replace the brake fluid.
Clutch takes mineral oil, ATF will do. Check it all out, then ride with confidence.

Me in green, my 2000 125EXC
The 660 Raptor was mine as well, I sold it once I felt the potential of a Blaster!

Rotors look good and pads have plenty of meat left. The bike does run and idle fine, I'm just worried how much abuse has been done to that piston, or when it will finally end. Also if I do end up putting a new gasket kit or new top end kit on it how should I go about breaking it in. Ride it like I normally wood on trails? Or is it more like a blaster?

 

[best]

Rotors look good and pads have plenty of meat left. The bike does run and idle fine, I'm just worried how much abuse has been done to that piston, or when it will finally end. Also if I do end up putting a new gasket kit or new top end kit on it how should I go about breaking it in. Ride it like I normally wood on trails? Or is it more like a blaster?

KTM pistons don't suddenly grenade like Blasters, and if you are trail riding they don't need a new piston every year either. If it starts, runs and idles fine, forget the piston for now. If the front sprocket teeth are hooked your chain is probably well worn too, but you never know. Buy a new front sprocket for now and adjust the chain tight. If you want to spend money on something, buy a Case-Saver, visible in front of the chain on my bike:

And here on my KTM Blaster:

That 125 has about 40hp and if the chain snaps or jumps the sprocket, it will destroy the engine case and clutch slave cylinder. A case saver is about $50, a case is about $500-$700 installed.
First chance you get, replace both sprockets and chain is your best defence.

The chain tensioner screws tend to stick in the aluminum of the swingarm if not greased frequently. If the adjuster screw does not turn, DON'T FORCE IT. Remove the wheel and use penetrating oil and a propane torch to heat and cool and work the screw free gently. If you cannot get it, remove the swingarm and take it to a machine shop BEFORE you break it off. Waterproof grease or even Locktite will prevent this.

I have a 300 and a 380 KTM but I love the 125 the most. Makes me feel like a real rider, rather than barely holding on to a wild Bronco. I hope you have as much fun with yours.

Note how small my back sprocket is. This is my 75mph wheel/sprocket setup for highway riding. I run a big front sprocket to save chain wear and allow me to change sprockets for nasty trails at the end of a long higway run. 15min to change a front sprocket. This trip in the picture I believe I did 450 miles in one day, about 80% at highway speeds. This 2003 125SX will keep up with with the 250-650 machines on this ride with no problem and weighs 100 lbs less than most of them. The oversize and extra tank hold 20L/5gal of fuel, over 300 miles worth.

 

[Blaster528]

Alright so bike is now home as of 6:30 last night Very exciting!!

 

[Blaster528]

I also 3D printed a case saver but I'm not sure it'll be as effective as I imagined. It would be great to have one on hand for measuring purposes unless someone would like to give me some rough estimates of theirs??? Cough cough best

 

[Blaster528]

Soooooo I'm a little nervous. I started the bike up for about 10sec today. Sounded great but I noticed a slight ticking sound coming from right side of motor I believe. I immediately turned it off. It had oil and very little amount of gas. But when I checked the coolant I couldn't see any in it. I researched and read that someone was riding there KTM and the hose came off and drained all there coolant. I guess they kept riding it like that and noticed a ticking sound. They tore the motor down and needed to do a complete rebuild. I really hope this didn't happen to my bike. I also don't know how long the radiator has been empty or low.
Need advice on what I should do.

 

[best]

The level goes down a bit naturally from heat expansion and overflow.
About 5 tablespoons fill it to the top again.
If you don't worry about it, it will always be 5 tablespoons low. No worries.
Me? I worry about it too. I top it up every chance I get. Always another 5 tablespoons.
I know better, but it is an emotional thing. Give baby what she wants. 5 tablespoons...

I am not going to take measurements off my case guard.
You need a good metal one, not a plastic one.

Steve

 

[Blaster528]

The level goes down a bit naturally from heat expansion and overflow.
About 5 tablespoons fill it to the top again.
If you don't worry about it, it will always be 5 tablespoons low. No worries.
Me? I worry about it too. I top it up every chance I get. Always another 5 tablespoons.
I know better, but it is an emotional thing. Give baby what she wants. 5 tablespoons...

Ok thanks for that My heart rate is going back to normal. I got Prestone 50/50 coolant since it doesn't have silicates in it. I am going to drain rads and put in new fluid. How does your 125 sound? Does all KTM's have a ticking sound to them? I might be able to get a video of it this weekend.

 

[best]

I use Prestone concentrate. Rarely change it.
With 3 bikes I forget what ones might have lost coolant for whatever reason over the summer months, so come freezin' season I check them all with a 10mm wrench and a jug of Prestone concentrate. I undo the drain bolts and see if it is clear or dark green. If it is clear I drain a bunch and refill with Prestone. Not very scientific but never had a problem. I do own a hydrometer and use it if I am winter riding. I use the concentrate so I don't have to thoroughly drain the rads.

I don't think mine ticks? Chase it down, exhaust leak?
I've owned it for 15 years so I'm normalized to its noise.

Steve

 

[Blaster528]

So this is a bit weird. I went to air up the tires and couldn't find the valve stem for the back tire. Only 1 rim lock and 2 open holes, which I know aren't for spokes. Am I missing something???

 

[best]

Yes, the nut on your valvestem. The tube has spun.
It will be a nightmare to get it out again because you cannot air down.
I would buy a new tube, poke the present one to deflate it, and install the new one properly with another beadlock.

 

[Blaster528]

Yes, the nut on your valvestem. The tube has spun.
It will be a nightmare to get it out again because you cannot air down.
I would buy a new tube, poke the present one to deflate it, and install the new one properly with another beadlock.

So your saying I'm going to need a new tube and 2 rim locks? There is air in the back tire so the valve couldnt have shredded off. You think I might be able to break the bead and try to snake the valve back in there? What's the difference between 1 or 2 rim locks? AGAIN thank you very much best for your help. I would've had a hard time without this forum

 

[best]

2 beadlocks are needed mainly for balance at high speeds (your 125 will do 70mph) and to keep the bead in place when you are running 15psi and putting 38hp on it.

You might break the bead, inspect and rescue the tube, find another valvestem on another old tube and put it all together, but I've found that after tube has been kinked and chafed for a while, the odds are about 80% it won't last long. A new tube is $10-$20 and a flat tire in the middle of nowhere is a pain. I always keep a spare tube around for a Sunday flat anyway.

There is a KTM forum but is mostly concerned about spending $$$ on bling for 2012-2016 bikes. Some good experts on there however. Thumper Talk has a KTM forum as well, a few experts on our era of bikes. I like this forum very much. Polite, knowledgeable. High info, low bull ratio.

Steve