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About to replace my 2002 yz426 top end, and i have a few questions.

2 posts in this topic

I have a 426 that I bought a few months ago. It had an oil blockoff kit installed( wet sump), but I returned it to a dry sump with the help of all you guys here at TT. Thanks again for all the previous advice. The bike starts first kick and runs absolutely perfect and is killer fast. The problem is that I don't know how much time it has on the top end, and I'm by nature the type of person who doesn't like to take unecessary risks. The questions I have are,

1.) What are the odds or is it possible that the piston might be larger than stock and if so, how do I identify the size?

2.) Is there anything to watch out for during the top end replacement? Are there any steps or advice that anyone has which might save me money or heartache during the process?

3.). Is it relatively basic, that someone with the right tools, service manual, and a decent mechanical skills can perform?

4.) Are the main parts I need the piston pin, clips, rings, piston and gaskets? Oem best??

Thanks in advance for any advice. I will document everything and post the results.

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Measure the piston is the easiest way to see if you have a big bore kit or if the piston has a over bore on it.

As for Top end they make complete top end kits that come with everything you need to do a top end job.

As for me I put a 3mm over bore big bore kit in my 426 so almost everything I had to order separate and find who made the parts for the 426.

As for the bore if it the stock coating on the bore most of the time you can see where it getting thin or worn though if has a Sleeve in it will be silver measure it could be over bored measure top middle and bottom of the cylinder bore at different clocking position it tell you if coned or out of round.

Rule of thumb take a look at rod end for discoloring it could tell you if has been ran hot ant check rod end end play.

PUT A NEW CAM CHAIN as long you have it apart.

I would pull the valves out head take a look at them and most topend kits come with new valve stems seals might as well install them at the same time.

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