Yz 426 piston/ head


7 replies to this topic
  • Tevonini

Posted October 30, 2012 - 07:10 PM

#1

Ive come to the conclution that I'm going to rebuild the top end of my 00 426 and I was wondering where is a good online store to buy top end kits? Plus what's the best piston out there for my bike? I'm looking for an all around power gain but if I had to choose it would be mid low range...
Puls I was going to have my heads ported and polished to increase power as well... Is there some where I can send them off to have this done or are there shops that do it? Or is it kind of a back yard mod?

I was also thinking about this's 444 mod I've been hearing about but can't fin the kit...

Thanx in advance


  • grayracer513

Posted October 31, 2012 - 06:43 AM

#2

Google Luke's Racing for the 444 kit. Piston's included.

I don't recommend having the head ported unless you can locate someone who has a provable record of actual documented power gains from the modification, and can do it for something less than half the value of the bike. The 5 valve Yamaha head is exceptionally difficult to work with because the configuration of the branch of the intake into three ports is critical to power production, and it's easier to lose power by reworking it than to gain it. The shape of the valve pockets is also critical. A really good job can gain you as much as 3 HP or more, but it's very expensive, and the guy doing it has to know what he's up to.

  • Tevonini

Posted October 31, 2012 - 07:29 AM

#3

Google Luke's Racing for the 444 kit. Piston's included.

I don't recommend having the head ported unless you can locate someone who has a provable record of actual documented power gains from the modification, and can do it for something less than half the value of the bike. The 5 valve Yamaha head is exceptionally difficult to work with because the configuration of the branch of the intake into three ports is critical to power production, and it's easier to lose power by reworking it than to gain it. The shape of the valve pockets is also critical. A really good job can gain you as much as 3 HP or more, but it's very expensive, and the guy doing it has to know what he's up to.


Hey grey how much power will the kit actually give me compared to putting in a high comp piston? What one will be more reliable?...
Can I find high compression pistons at that same website?

  • grayracer513

Posted October 31, 2012 - 08:05 AM

#4

The 444 is 4% larger than the stock displacement, so should gain roughly that in power across the board. That would be along the lines of 1.5 - 2 HP. A high compression piston will do roughly the same thing, and both would provide most of that increase at low/mid RPM. Together, I'd guess 2-3 HP, a significant gain. The kits include the piston in either standard or high compression. Luke's has pistons for the stock bore, also.

Nothing will be more reliable than OEM parts, but Luke's had a good reputation that they earned rather than claimed.

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  • Tevonini

Posted October 31, 2012 - 02:44 PM

#5

Now if I just do a high comp piston, will I have to run racing fuel? Or can I keep running chevron 91 like I do now?

  • grayracer513

Posted October 31, 2012 - 03:06 PM

#6

In most cases, pump premium is enough, but at or near sea level in the summer, a higher octane fuel may be required.

  • Tevonini

Posted October 31, 2012 - 03:17 PM

#7

Perfect thank you grey

  • peaceofwork

Posted November 01, 2012 - 06:10 PM

#8

grey will a yz400 cylinder take being bored to the 444 size then replated or is it best to start with a 426 cylinder?
any reason not to go with the 400 crank and 426 rod?





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