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Discussion Starter #1
Hey guys.
New to the forum, and to dirt bikes in general.
I got a 99 125 sx, and I’m looking at doing a top end rebuild on it.
Was wondering what you guys would suggest for brands to use, and what else I should be doing too


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Welcome to forum.

Top end:
PISTON
We use Vertex. That is same piston KTM supply the bikes with. But you can get others.They come with ring/rings, pin, bearing and clips. (We race MX with one ring pistons and replace ring at 20 hour and piston at 40, but a 2 ring piston will last longer before you need to change the rings.)
Check if the old one has any info on it re size (in 0.01 millimeters) so you can get same (or a fraction bigger) size. They come in A, B and C sizes. Very small size difference and you will need a micrometer to measure size. But if you have no idea re size, then you have three options:
  • Measure the bore
  • Measure piston.
  • If you know how many hours the bore has had, then you can use this as an approx re size: 0 - 80 hours A size, 80 - 160 hours B size, 160 -240 hours C size.
BORE
Check the condition. Look for scratches/scrapes and areas where there has been much wear. If they are too bad then you will loose compression there. You can tidy up the bore by using a green brillo pad and CRC, but only the surface stuff. If there are deeper marks or scratches, then you are up for a new barrel or a re-coating...

HEAD GASKET
There are 2 big O-rings between the head and cylinder. Replace them every time you take the head off!

POWER VALVE (PV)
Dismantle, clean and check for wear. Reassemble (fit new bits if needed).
Check that PV does not hit piston!!!

ADJUSTMENTS
  • Set the squish (gap between piston and head). You adjust this by adding or reducing the cylinder base gasket thickness (mix and match the cylinder base gaskets so you get right gap. 1.1mm is a good middle range. Measure by putting soft core solder on piston. (You find You Tube videos explaining this).
  • Adjust PV to specs according to manual.
  • New O-rings on header pipe.
  • Torque cylinder to spec.
  • Fit head and torque to spec.
Come back if you have questions.
 
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Discussion Starter #3
Well that was a lot more info than I was actually expecting.
My riding season is just about over now so I’ll pull the engine and get to dealing with that in about a week.
Thanks SO MUCH
I really appreciate it


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