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2003 Kx 125 piston seizing

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Went to freshen up my bike , got new top end vertex piston and boysen reeds and pro circuit  pipe , jetted carb to pro circuit specs , did the break in period , went for a rip was in 3rd gear bike just stopped started right back up again let sit overnight took out the next morning n totally locked up , took apart sent jug to machine shop again got another new piston jetted carb to oem specs , did break in again , went out for a 30min ride everything was fine , came home let sit for a couple hrs ,warmed bike up went again same thing just shut off went to use the kicker would not even move waited 5 mins and it started right up and I rode it right home , checked compression this morning cold engine 180psi any suggestions 

IMG_20170304_124841274.jpg

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You said you sent the cylinder to the machine shop again. What did they do to it the first time and what did they do to it the second time? I mean no offense, but if you used all of your words and a period sometimes, it would be easier to understand what is going on.

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Did machine shop have piston when they prepared cylinder?


I think what FreeBird65 is hinting at here is the same thing I was going to ask. Was your cylinder bore matched your new piston after it was machined/welded/plated/honed? I list all those processes because you haven't said what was specifically done to the barrel, but it's tolerances being matched to piston spec is essential.

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Each time it seizes, do you flush out the bottom end for any debris that might be in there?

Did you measure the ring end gap?

How old are the crank seals? 

no matter the suggested jetting you use, if you have an significant enough air leak it will run lean/seize.

 

Meticulously cleaning your entire bike before tearing into the engine is something you should also consider to avoid problems.

 

 

Edited by mlatour

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Using redline racing oil and 92octane gas and I mix it 40:1 and the plug i use is br9iex , and this is the same setup I have ran the previous 3 seasons before the pro circuit pipe and boysen reeds and i put a fresh plug in and porcelain half black and tan

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Yes second time I flushed the bottom end out and the ring I measured at .10mm spec called for .06mm to .09mm, and yes I am planning on checking the left crank seal today , but have no symptons of crank like high idle or reving up 

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does your exhaust valve show any signs of contacting the piston?

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That was my first thought the first time it happened so I gave the machine shop the jug with the piston and exhaust valves and they said no it wasn't from the exhaust valves , I haven't taken it apart again yet 

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That was my first thought the first time it happened so I gave the machine shop the jug with the piston and exhaust valves and they said no it wasn't from the exhaust valves , I haven't taken it apart again yet 

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That was my first thought the first time it happened so I gave the machine shop the jug with the piston and exhaust valves and they said no it wasn't from the exhaust valves , I haven't taken it apart again yet 

Nope it's vertex 53.96 size b

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You may have worn out crank bearing/s witch will make seal leak. The new compression could be to much for it type thing. Sounds like everything else is ok. I'd do a bottom end atleast bearings /seals.  Before you try again :thumbsup:

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