99 WR400 Overhaul

Hi Guys,

I've done quite a few searches on this site and read many of the very informative posts. I have a '99 WR400 for which I don't know the service history. I have checked the valve clearances and all okay and inspected the clutch. The clutch has the holes drilled into the clutch hub to help distribute oil to the plates and all is in tidy condition, no notching of the basket tangs etc, no warped plates ( I have done about 20 hours enduro on the bike) so I presume someone has looked after it.

From what I can determine the following would be worthwhile doing as a minimum during an overhaul:

  • Piston
  • Balance shaft key
  • Pin
  • Conrod
  • Big End & Main Brgs
  • Cam Chain

I read that the later 426 rod is a worthwhile mod for reliability, is there an aftermarket piston that will fit my standard bore with the larger small end diameter?

I might find that once I strip I find the bore worn, at that point might be worthwhile finding a good 426 barrel (or jug as many of you guys say)

Can you recommend aftermarket parts such as Wiseco and similar for valve train components that are more reliable / better value than OEM

Should I replace all of the valves at the same time if there is no sign of them stretching or other obvious damage?

Collets / Springs / Caps / ?

I would prefer to order everyting that I might need, then be able to finish the job in 1 go, rather then continually order more bits as required and have a prolonged downtime. Alternatively I strip the motor, inspect and measure and then place an order for everything.

Your thoughts are appreciated, also any pointers of who can supply everything at competiotive prices (exhange rates make buying from the USA attractive at the moment)

BTW - I'm not after more performance, but want to set a datum from where I can log hours and regularly maintain to keep a reliable machine



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