02 yz426 top end rebuild

30 replies to this topic
  • 02_yz426f

Posted May 02, 2016 - 10:40 AM


I no when the bike went back together timing was deffently on point over rev could be possable but I wasn't rly on it like that what damage do y'all thank is done do you think the valve is bent an it scored my intake cam a lil not on the lob tho but can feel I lil rough patch in the when spinning on the cap

  • grayracer513

Posted May 02, 2016 - 11:37 AM


If the piston stopped the engine against the valve, both the valve (and probably the other two intakes) and the piston skirt are bent, and the crank is likely in need of realignment.  Inspect the cylinder carefully, and realize that the rod and rod bearing may also be damaged.  Additionally, the valve guides of all three intakes need close inspection to see if they were damaged.

  • toten

Posted May 02, 2016 - 12:30 PM


If you're lucky, the head isn't too beat up. 


Will it really bend a piston skirt that easily? Wow. And throw the crank out of alignment? I wouldn't have expected that either, but I guess when you're building them to be lightweight and make good power that's how it works out.

  • grayracer513

Posted May 02, 2016 - 01:21 PM


It isn't going to fold the piston into a wad, but it can tilt the piston hard enough to distort the skirt, yes.  Normal clearance is less than .003", and they're unserviceable once they get to .004".  If you tip the bottom of one skirt in no more than .003", you'll hear what real piston slap sounds like, and accelerate the wear on the bore dramatically.


Same for the crank and rod.  The wrist pin only has to be a couple of thousandths out of square to cause trouble.  Likewise the alignment of the left and right crank halves, and the possibility of the rod bearing rollers "stamping" grooves into the surface of the crank pin is quite real.  Understanding that you can throw the crank out of line by using a brass hammer once or twice to install it should give you something to compare it to the force applied when the piston encounters a solid object in the combustion chamber and brings the whole show to an abrupt halt.

  • 02_yz426f

Posted May 02, 2016 - 08:38 PM


I didn't hear it slap like that so I pulled it apart once again 3 bent intake vales they did put a little mark on the top of the pistion but can't find any evidence of the rest of the stuff but where do I take my head to get checked out

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  • toten

Posted May 03, 2016 - 05:52 AM


I don't think there are many options for hidden damage to the head, as long as there aren't a bunch of dents and the cam bore and valve guides seem ok. If a piston bend of .002" is enough to cause problems, there's no way you're eyeballing that, though. Same with the crank.

  • 02_yz426f

Posted May 03, 2016 - 02:45 PM


So how do I check that out then

  • 02_yz426f

Posted May 03, 2016 - 03:09 PM


An how do you get bent vales out of the head

  • 02_yz426f

Posted May 03, 2016 - 03:11 PM


An also their are also some scratches in the valve gides how do I no if their too deep

  • toten

Posted May 03, 2016 - 08:07 PM


You'd need a micrometer to check the piston. 

  • grayracer513

Posted May 04, 2016 - 06:30 AM


Take the head to a machine shop for the repair work.  Have him check out the piston.

An also their are also some scratches in the valve gides how do I no if their too deep


How do you know that if the valves aren't out of the head?

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