2003 KTM 125sx

Getting there
Getting there
125sx Ready
Josh KTM Crash
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Getting there
125sx Ready
Josh KTM Crash
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125sx parts list and exploded diagrams
Jan 2019
Race sags: Josh/Jake = 95mm, Stu = 114mm, static sag = 44mm
Jet needle set to middle notch
24 Aug 2018
Compression damping on rear shock = stock + 2 clicks
Compression damping on forks = stock – 8 clicks
Jet needle lowered to 2nd notch from top to address mid throttle sputter – we’ll see??
Race sag (Stu) = 105mm, static sag = 30mm

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To Do List

  • Sort out the front brake, it’s not grabbing at all – maybe clean the pads again, don’t want to buy knew until I know what’s causing the issue

Completed Tasks

Q1 2020

  • Ride it again, didn’t ride it once last season! – Rode it to get confidence back after wiping out badly on the RM-Z and bending the gear shift lever, it was fine but I prefer the smoother power delivery of the 4 strokes.
  • Diagnose fluid dripping from bottom right side of engine – seems to spread fairly clear on the paper towel, wouldn’t expect oil to do that.  Seems to be weeping from the oil drain plug, torqued it to 15 Nm and noticed that it was a little loose.
  • Still weeping oil, drained the oil and it was low, maybe this has been weeping for a while. Also found that the soft washer for the drain plug was missing, got a new one from TSR
  • Turned out to be oil weeping out of the hydraulic clutch cylinder gasket, installed new gasket with RTV sealant
  • Repaired front brake cable with a plastic tube, cut and then placed around the outside and taped
  • Repair left side air scoop mount on tank – found a large self tapping screw that fits nice
  • New front mudguard ~ $40 (or full set of plastics? ~ $180) – ordered new mudguard from Fortnine $36
  • New front brake cable – $180 for a KTM cable! Repaired with a plastic tube taped into position
  • New rear sprocket bolts – Josh installed them with thread lock

June 2019

  • Dropped the forks through the yolks so they were flush to try and address tank slappers


  • Give it a good wash
  • Fix wobble in front mudguard – Front brake cable clamp is missing that provided a spacer, put large washers in for now; new brake cable clamp installed
  • Fix huge cracks in rear mudguard – Used the old tie wrap stitching trick
  • Fix dent in exhaust
  • Remove seat cover – original cover still underneath with only minor tears
  • Bend radiator mount tabs back, prevent them from fouling the forks
  • Threads damaged on oil level check screw – replaced with radiator mount screw since replacement screw had larger (10mm) head
  • Replace worn vent screw in fork cap – not needed, with good screwdriver not much effort is required to undo
  • Check rear brake cylinder, push rod comes out? – looks like it could be normal on parts diagram but the rod is well corroded
  • Replace front wheel bearings
  • Replace rear shock lower mount bearing – replaced with MSR bearing, not teflon
  • Fix seized rebound adjust screw on rear shock – got a 2007 XC-W shock PN 12178C04 mono compression control
  • Fix spring on kick-start shaft – ratchet gear was mis-aligned on shaft – rectified
  • Replace fork seals – refilled with AMSOIL SAE 5 synthetic
  • Thread in left side fork protector mount hole damaged – one or two threads seem ok, new screw held in with thread lock
  • Install new piston – plenty of blow-by on piston skirt, got Wossner piston and ring (awesome!) and new small end bearing
  • Cylinder stud broken, the nut and top of stud were glued in place!!! WTF?!?! – got the old one out with stud remover, installed new
  • Screw and bucket holding the rubber chain guard on the swing arm missing – threads in hole on swing arm trashed, aluminum is thin on swing arm not much there to hold heli-coil, tried tapping to M6 and thread locking, not overly happy with it though
  • Ball bearing for kick start – thanks Dan and TSR
  • New right side drain plug M10x1 – threads in clutch cover are trashed!  There is a bit of the old threads left but the drain bolt slips straight in, tried installing an M10x1.5 Fix-A-Thred insert in the remaining threads but the hole was too small.  Plugged with high heat JB Weld
  • Threads in oil level check screw are trashed – repaired with Fix-A-Thred repair kit
  • Fix broken arm on sub frame – Josh welded it up in auto class.  Update: the weld failed so a small steel bar was fashioned and inserted.  Thought about filling around the bar with JB Weld or drilling holes and holding in place with screws but the bar was a tight fit and there should not be any load trying to pull it out the back of the bike so I think it’s good now
  • Spring for rear brake lever is missing – bought new one
  • Clutch basket and inner hub are both damaged – replaced with used parts found on ebay
  • Clutch holding tool required – used impact driver
  • Replace spark plug – ordered from Fortnine
  • Install new clutch actuator gasket, screw and banjo washer – screw was too long?? had to cut one down
  • Get mineral oil for clutch, top up and bleed clutch actuator – also bled the front brake again
  • Little play in rear wheel – replace bearings, ordered from Fortnine with grips
  • Installed new grips
  • Set race sag and suspension settings to stock – race sag = 105mm, static sag = 30mm, compression damping on rear shock = stock + 1 click
  • Clean rear rim
  • Install new Shinko rear that came with the bike
  • Adjust chain tension
  • Loctite the handlebar perch mount screws
  • New front tire – picked up a very lightly used Dunlop Geomax
  • Split engine and remove crank (big end bearing seized)
  • Install new Hot Rods crank and new bearings
  • Check power valve z dimension
  • Teach the lad how to short shift
  • Properly repaired the rear subframe using a strip of steel drillled and tapped to take 4 M6 machine screws and replaced all the broken zip ties
Getting there

7 thoughts on “2003 KTM 125sx

  1. Well apart from the missing knobbies on the front tire and the forks weeping a tiny bit of oil and still feeling a bit sticky, I think it’s finally ready to go!! Fired her up and she sounds wicked.

  2. First time out on the track, I was amazed at how well this bike handled (I had some concerns over the rear shock). It ran great and the only issue was that the handlebar perch screws came loose – loctited them in per the manual. When I got back I found that the front tire was flat?? When I installed the new tire, I found that the rim tape had disintegrated and there was some corrosion on the spoke nipples inside the rim. Cleaned all that up and installed a lightly used Dunlop Geomax I picked up from Kijiji and a brand new inner tube – Top Tip, always put a bit of air in the tube before installing to avoid pinches!!!
    Second time out, I rode a few laps and the power seemed a little inconsistent sometimes. Then the lad rode it for a while and not being used to 2 strokes he was over-revving it a bit but he loved the light feel compared to the YZ250F. Sadly on the last practice session, the bike seized solid, turns out that the big end bearing in the con rod failed. Got the engine out last night but local shop is too busy to work on it so I’ll get the crank out and hopefully he can at least install the con rod in the crank – it’s going to be tight for getting out to the track this weekend – must make sure I don’t rush the rebuild and F something up.

  3. The crank was trashed so we returned the con rod and ordered a new Hot Rods crank assembly. Also noticed that a bushing on the gear selector mechanism was missing and the screw that holds the spring loaded arm for the shift drum was loose. Turns out the threads were damaged in the hole for the screw, but there were some decent threads further down so I got hold of a longer screw with a suitable shoulder and loctited it in.
    Got the engine back in the frame and did a quick blast with the impact gun to tighten the clutch nut, now I can’t find the Woodruff key for the flywheel! Fortunately Denny at TSR says he has some in stock so I’ll pop round with the old crank later, need some gear oil anyway.

  4. Been out to the track twice since installing the new crank and both times the SX has been sputtering when accelerating from low revs. Also the hydraulic clutch failed half way round the first lap! That seems to have been due to only having one copper washer on the banjo bolt at the slave cylinder (the part list only calls for one??). I thought it was running fine before it seized but the lad says that it was sputtering when he rode it. I took Andy’s advice and set the air screw to the default setting of 1.5 turns out (was at 1.75) and dropped the tapered jet needle down by moving the circlip from the 3rd groove to the 2nd. Might go out to the track again on Sunday so I’ll check it then, hopefully that’s got it.

  5. Finally got out to the track the other day and … the effing thing was exactly the same! I put the jet needle back to the 3rd groove and checked the spark plug lead and cap again but no sign of improvement. I sat and thought about it and the only thing I really changed when installing the crank was the power valve Z dimension, I measured it at 46mm and it was supposed to be 43mm. All you need to do to adjust it is remove a cap, undo the lock screw and turn it so I thought I’d give it a go since I can always put it back. I watched some videos to determine which way I had screwed it when I set the Z dimension and then backed it off one full turn and shot over to the park – it ran great!! No sign of any sputtering at all, woohoo. So I backed it off another half turn to try since it’s so easy to change, looking forward to getting back to the track again now.

  6. We were out at the track again on Sunday, with the jet needle clip in the 2nd groove from the top there didn’t seem to be any hesitation but it seemed to bog a little at low revs. Then Jake got the KX100 stuck in a deep rut in a corner; he was struggling a bit with clutch control to feed the power in so I did it and when it got moving and into the power band it was ripping!! I’ve got to get the SX running like that, so I wound the power valve back in half a turn.

  7. Managed to get to the track yesterday and test the SX, it ran great! The power delivery is pretty fierce but I think it just needs getting used to now. I’ll still remove the carb and check the main jet size when I get chance.

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