YZ444 info request

Greetings almighty TT'ers. I have a '02 YZ426 that the motor gave up on me about a month ago. The cyllinder was considerably out of spec so i planned on replacing it with another stock one til I found a 444cc big bore kit in stock at a local shop. I've read all previous posts about reliability, performance, etc. but haven't seen if any other mods or upgrades were necessary. Currently i'm running the stock exhaust (eventually want power bomb SX header), YZ450 cam, BK mod, blue wire, Ready Racing air filter, but wanted to see what anyone else had to do to get the bike running optimally. As always I appreciate the knowledge, wisdom, and experience this group has to offer.

go for it! shouldn't need any additional mods besides jetting changes


And timing chain!

Powroll sells a 2mm stroker kit giving you a 440.


I'd go with the 444 kit if I was him. I've heard that big bores are more reliable than strokers. too much "custom" work with that powroll kit.

And timing chain!

does any company make a heavy duty timing chain for a 426. it seems really cheesy compared to the crf450 timing chain that I put on my 400ex quad. The 426 is like the same as the stock 400ex chain.

I don't believe that there has ever been any particular reason to want a beefier timing chain in the 426. I haven't heard of any problems with the one they come with.

That's good to know, thanks Gray!

You may need to "diddle" with jetting wen you go big, but other than that, just go big. The rest of your mods will work fine with it. When I went big on my WR, I already had hot cams installed, so I don't know how the big bore kit would work without them. What I can tell you is that the extra mass of the big piston and the higher compression of the 13.5:1 piston kinda calmed down the powerband a bit compared to the stock bore.

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