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2000 yz 426 rebuild questions

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Let me start by saying GREAT SITE. I have been reading posts on and off for years and have gained a lot of great info. This site is a great resource. 


 Been riding offroad bikes for 40 plus years now and have owned a lot of great bikes, never bought anything new  - usually started with a "project" bike and rebuilt it. Usually put way more money in to the "project" bike than it was worth and then I would ride the hell out of it for a year or two and then sell it - at a loss of course. Some of my friends have really nice older bikes.  : ) 


 My latest project bike is a 2000 yz426. I Stripped it down to bare frame over the weekend. Dropped the frame off at the powder coater this am. hope to have it back Friday.

 Have a set of forks off of a 2003 wr that I have already rebuilt -seals and oil - and will be installing on it. Picked up a 2005 yz450 Carb off e-bay and installed the aftemarket spring on the AC pump and the extended air/idle screw, replaced the diaphram etc. Going to go thru the rear suspension and linkage, new plastics, chain and sprockets, etc, etc. So a you can see I'm serious about really going thru the bike and getting it fixed up nice.


 Finally to my questions. I rode the bike for about a hour before I retired it to the garage and started to pull it apart. Started easy, ran ok, no smoke, motor was a little noisy, idle wanted to hang, had a mild bog - not real bad. Anyways, tore the head and cyl. off the motor - Piston was a Prox. Measured the cyl for wear and taper - still within spec.

Lower rod bearing was in good shape, top rod/piston pin was trash!  Great deal of galling between piston pin and rod.


1  Is this a common problem with these motors and if so is there any fix.

2  Should I go with a aftermarket rod kit and are they dependable --- and what brand should I use ---- or should I stay with oem = more money

3  Going to replace the piston, pin, etc. Good deals out there on aftermarket kits --- or should I stay oem again.

4  Thinking about reusing the main bearings if  - after a good inspection they appear to be in good shape ---- or is this a mistake.

5  Head - Valves, Springs, Retainers, Keepers appear to be in good shape. Guides are tight. Was just going to replace the seals and touch up the seats and call it good --- or should I go ahead and replace some of these parts.

 Don't mind spending some money to get things right but don't want to get carried away ether.

 I appreciate any and all replys!







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1-3: Not a real common problem, no.  Use Genuine OEM.  Second choice is Hot Rods stuff.  Yamaha pistons for the bike are forged, and are as durable as anything you could put into it.  Should be less expensive, too, but you can't tell how they price some of the old stuff.


4: The problem is that you have no way of being able to tell if or when the bearing races may start spalling ("flaking"), or a cage break, or who knows what.  Kind of a guessing game unless you change them.  Then again, that's also true of all the rest of the bearings in the engine.  They might last years, or fail in days.  Do be critical of the left side main shaft bearing.  They seem to get roughed up more quickly than the rest.


5: If the bike is really a 2000, it will have stainless steel valves.  These can be lapped carefully to prolong their seal.  If, however, it was ever re-done with titanium valves for a later year model, they cannot be lapped without compromising the valve life considerably.  In either case, if you aren't going to replace the valves, don't refinish the seats (except by lapping SS valves).  It can be hard to tell the two metals apart in a used state.  If you have one of each it's obvious, because the steel valve is just over 40% heavier than the Ti valve would be.  For reference, the titanium center intake from a YZ450F weighs 14.2 grams.  A stainless valve should weigh about 20 grams. 

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Thanks Grayracer513, looks like I will be ordering oem on the rod and main bearings. most likly replace all the tranny bearings also. Not crazy about spending that much money on the bike -- but I don't   have hardly anything in now. From what I read should be dependable. Owned one of the 98 yz 400 years ago and liked that bike quite well. never really bothered me starting it with the compression release so I most likely won't change out the exhaust cam on this one.

Do you think for dependability  I should replace the valves,springs,keepers and retainers?  And yes they are ss. I know the 2 guys that owned the bike for 12 of the 13 yrs of its life and all that has ever been done is a piston replace abou 8 yrs ago.



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