I'm pretty angry right now. I put all this money into my KX and it seized up on me over the weekend.

I warmed it up for about 5 minutes, then took an easy lap around the track to familiarize myself with
the jumps. About 3/4 into that lap, the power cut off, and I heard a nasty noise in the motor. The kick
starter wouldn't budge; the motor was completely locked up.

I have no idea what could have caused it. I rode the bike 2 weekends ago in the mountains and did
about 60 miles of singletrack with out any issues.

I was on the same tank of gas from my last trip when the bike siezed, so I know it had mixed fuel. The only thing I did to
it was change the gear oil and air filter.

The only conclusion that I can come to is the top end was shot and I should have replaced it since the total hours was unknown, but
the bike felt like it had great compression. It was also ridden by an old guy who only took it out
a handful of times. I maybe put 15 hours on it since purchased.

You need to tear it down first to see what happened.My money is on a broken piston skirt(ran too long).otherwise we could guess all day.

I will second your guess of a broken piston due to fatigue failure from too many hours.

gotta do a top end when u buy a bike

NorCal, on 07 May 2012 - 02:26 PM, said:

Wait.  Where did you spend "all this money"?

Well, you can only hope it was the crank before the piston. Piston can usually take out cylinder walls, power valve, etc. if it grenades. A crank just locks at the big end.

It's more work getting the motor in and out of the bike than it is to split and reassemble.

So, you didn’t do anything to the motor since you purchased the bike? Maybe change the oil or wash it, but mechanically you didn’t do anything? Just trying to get some info on what you may/not have done to the bike.

Could be any number of things. Sounds like you get to disassemble the motor and take a look.

KDXGarage, on 07 May 2012 - 06:49 PM, said:

I just put like $700 into the suspension, and a little money here and there for skid plate, tank, etc...

I don't have too much invested, but it feels like a lot since it is my backup bike.

SAPPERS, on 08 May 2012 - 02:06 AM, said:

Yeah.. I hope it didn't take the cylinder walls out.

My buddy who's dad owns a suspension shop said I most likely blew the crank out. He said the OEM cranks are known to
fail. Can anyone confirm this? If I need to replace, would I go afermarket, like Hot Rods, or stick with OEM?

youngztr, on 08 May 2012 - 06:53 AM, said:

I know... I'm an idiot. Honestly, I'm a much better rider than a mechanic.

All I did was change the oil, air filter, and grease up the linkages/chassis.

I'll tear the motor down maybe tomorrow and report back.

capt justin, on 07 May 2012 - 06:25 PM, said:

I found that out the hard way!!!

My money is on the OEM piston shattering with "a dozen rides" on it.  I hope it isn't though.

I can't say I have heard of OEM 2005 cranks being a problem.  Hopefully it is not too bad.

You are no worst case scenario on spending money somewhere besides the engine.  There have been a few folks on here over the years with new, shiny plastic and expensive stickers to go with a broken engine.

Look at the bright side. Worst case, the piston messed up the cylinder and now you get to put a big-bore kit in it. Lucky bastard.

NorCal, on 08 May 2012 - 04:12 PM, said:

OEM is very fine and so is Hot Rods.

shrubitup, on 08 May 2012 - 08:20 PM, said:

Nothing wrong with OEM I wouldn't say they are known to fail either.

I dunno, Maybe Wiseco sounds pretty good.
But I feel your pain. Those 2fitty cranks aren't cheap. I think I'm going to invest in the Jig to rebuild cranks.

NorCal, on 08 May 2012 - 04:12 PM, said:

What?  Maybe he's thinking of the old 250F's.

I hate when people just spread rumors like that.

KX25O, on 09 May 2012 - 02:23 AM, said:

Does anyone know why cranks for 2006 are so hard to find? I can only find cranks for a 2005.
Is it the same for 2006?

No.  It is not the same.  Look in the service manual, and it shows the pictures of the two different ends.

KX25O, on 09 May 2012 - 02:23 AM, said:

Good coz I didn't say OEMs fail either.