511cc big bore / stroker in 2006 WR450F

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  • elcubeo

Posted September 08, 2009 - 06:51 AM


Hey there guys--

I'm in the market for a big-bore and stroker crank. Guess why?
I blew the small end of my stock con rod to chunks!

Upon initial inspection, the only things broken are the jug (windowed pretty nice!), con rod, and piston. The transmission is ok (coasted her about 1 mi down hill to the trailer, better than being 1 mi below the trailer...). The cylinder head and valves escaped remarkably unscathed, shy of a few piston-deck love marks.

I had a JE 14:1 std/std 450cc setup with hotcams shafts and an aftermarket exhaust. I liked the power, but not the $7.00/gal fuel and all the extra lo rpm detonation no matter my jetting. I race harescrambles and I dont really need that explosive of power delivery but i want to be able to pour it on when the opportunity presents itself. I see that my old GNCC hero Scott Summers has a good idea for his race Husky 510. He lowers the compression ratio to soften the power hit and make it more durable for abusive racing conditions. It's still about as fast as the 450 but it's more chuggable and impossible to break. Basically i wish i could install a 1987 XT600 engine in my awesome handling bike.....

Thus I am thinking of using all yfz parts to do a 511cc bb/stroker (478bb x 471stroker). My bike is an 06 so I dont think i will have issues with the crank snouts. I like the idea of stepping up from an 18mm wristpin to a 20mm wristpin (stronger maybe?). I might open up the combustion chamber slightly around the valve pockets while leaving stock quench area. I'm thinking about milling a slight dish in the center of whatever piston i end up using. Then it will roll over on the e-start like a lazy old utility quad.

I dont like the idea of assembling these kits without balancing something. I know from my drag car experience that a 6000 rpm v-8 can benefit greatly from a good internal <.1g balance job, so why not a 13,000 rpm single? I really dont forsee any of these parts being mass produced and being balanced to that precision. Also, has anyone found any balance shaft tech info, as far as what its actually balancing...and what i will have to change with +3mm stroke?

Maybe none of this crap matters, and I am the problem. I bet if i rode it like an XT600 and kept the rpm down I wouldnt break anything. But 250f rpm feels so good... what should I do?


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