Big bore options? 12.5 or 13.5 pros and cons.

I am rebuilding a 2002 426 that is in need of a new cylinder, piston, and valve work. I was thinking of going with a big bore kit since it is just a little bit more then reworking what I currently have. Just looking for the pros and cons of 13.5 compression. Do you have to run race gas with this set up? Other mods you could suggest while I have it torn down would be appreciated as well.

Bustersat-

I have a Luke's Racing 444 kit with 13.5:1 Wiseco piston on my 01 426. Holy crap! I had to go mup a tooth on the countershaft sprocket to make firts gear even remotely usable. It will yank your shoulders out of their sockets. Depending on the quality of the gas I get, I do sometimes have to add some octane booster or mix pump premium with race gas to keep the death rattle away...but that's usually on gas from "Buddy's Quicky-Mart" or other less reliable sources. Shell, Mobil, and even ARCO work fine on 91 octane pump premium. That's important to me because I'm plated and do some DS rides that use gas stations for gas stops. I usually carry a small plastic bottle of octane booster on those rides just in case...

As far as other mods, I have hotcams intake and exhaust cams, a JD jet kit, PowerNow and Quickshot. As far as jetting goes, the recommendation for jetting in the temp and altitude chart included with the JD kit are pretty good. I know 642MX from this forum went with just the big bore (no hot cams, etc) and was still working out his jetting...

Thanks for the reply! How do you like the longevity of the Luke's kit? That was the exact one I was looking at! Have you put some hard hours on it? Thanks again! I normally run a 50 50 with race gas and premium already.

I know 642MX from this forum went with just the big bore (no hot cams, etc) and was still working out his jetting...

Yep. I've got a YZ444. Birdy talked me into it. :confused:

I do have the hot cams and 444 kit, but I went with 12.5:1. I had a 13.5:1 piston in the stock cylinder before I did the BB kit. I thought going to a larger displacement with lower compression would make a nice smooth power curve. As it turns out, I was right. The hit is a little mellow compare to the previous set-up, but the more I ride it, the more I like it.

I did struggle with the jetting at first, but Birdy got me close using his specs. I went ahead and made it a little better, read this http://www.thumpertalk.com/forum/showthread.php?t=610318&highlight=leak+jet+on+a+YZ426 and now it seems perfect.

Now, if it will stop raining, I can start riding. I'm just about ready to load up my new enclosed trailer and head to Grayracers house. :excuseme:

Now, if it will stop raining, I can start riding. I'm just about ready to load up my new enclosed trailer and head to Grayracers house. :confused:

What kind of beer do you like?
Thanks for the reply! How do you like the longevity of the Luke's kit? That was the exact one I was looking at! Have you put some hard hours on it? Thanks again! I normally run a 50 50 with race gas and premium already.

Bustersat-

I've got somewhere in the neighborhood of 3000 miles on the Luke's big bore kit with no issues. I ran a compression check after the first 30 minutes of riding, and run one every 4 or 5 weekends of riding the des, figuring that when it starts to drop off a bit it will be time for a piston and rings. So far, no change.

The customer service at Luke's is top notch, as well. Good people and a good product.

What kind of beer do you like?

Michelob Amber Bock. :confused: or just about any dark beer will do.

The customer service at Luke's is top notch, as well. Good people and a good product.

+1 they where very helpful and quick to ship out my kit. I'd use them again.

Thanks a ton guys. I have my head and cylinder getting checked at at the machine shop as we speak. Depending on the results I think I will be ordering from luke's soon. I called luke's and they said they were not taking on any new projects right now but their online store is still open. Kind of sucks I was thinking of sending them everything to make sure it was done right. Thanks again for the advice guys!

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