should i go with the 470 big bore kit or stay with the stock 450

guys i am doing a top end what are the pos/neg for going with the larger bore beside cost, will it effect realiblity?, i heard it mellows the low end hit alittle, what do you guys think?, bike is an 03 450 thanks

Mellows the low end "hit"????????????????? I personally don't think so................ :cry:

im sure it will sacrafice realiability a bit but i dont know about mellow .. you add more cc's and horsepower that things gonna hit you hard

who makes the 470cc for the YZ426? :cry:

+2mm stroke X +2mm Bore = 459cc only reliable readily available package that I know of.

who makes the 470cc for the YZ426? :cry:

there is a 470 kit on ebay 300 and something bucks and a core,can't remember who makes it ,might not of said.duh i need to learn to read it is for the 450 not 426 :cry:

The only bad thing is if you want to ride at an sanctioned track they might not let you race in the 250 classes but I beleave that you could race in the modified class.

I have a yz470 that I use as a woods bike and it does mellow the midrange hit a little. It just pulls harder all of the way from the bottom. It likes to be short shifted in the woods, but it can pull from anywhere in the rev range.

Who did your kit for you? Thanks.

I have had the 470 kit in my 03 for over a year now. RPM's in Covina, CA did the boring and replating and I installed the Wiseco piston myself. On the RPM's site click on the YZ469F link on the bottom right of the page, their is a detailed description of the kit along with a dyno and some "claimed" increases.

Reliability hasn't been an issue at all, the bike has run solid for over a year since the big bore, I ride quite often and race Grand Prix / MX at least once a month.

I noticed a big power improvement over stock although I had head work done at the same time, I also run an Edelbrock carb. Just recently I installed Hot Cams which I did have a problem afterward but found out it was a carb issue, solved it and the bike absolutely rips.

For me there was no down side to installing the 470 kit, although I paid $500 at the time, looking at the RPM's site it looks like the price jumped to $750.

I am also considering doing this on my 03 vs buying a new bike...

Go for it It give's that bottom end power right off idle that the foe 50 need's Got mine from Thumper racing $470.00 a doller a cc. :cry: :cry: :cry: :cry: :lol::cry:

this one is the 470 kit from lukes racing. I did my yz250f's with his kits and they are rev monsters now.

I will be getting one from him for my 450 shortly. :cry:

The one thing you have to do, is make sure they send you an 03 or an 04 cylinder. They have the half moon cutouts taken from the cylinders to reduce the pumping losses in the crankcase. Thay are must haves.

I did it myself with a JE piston, Powroll crankshaft, a special cylinder gasket/spacer and a 450 exhaust cam.

what was the end cost?

Why the special spacer?

Piston w/rings pin & clips, bore & replate cylinder, stroke crank, gasket & spacer approximately $900 for the parts plus a "core" crank (needs to be like new or rebuild parts extra) and cylinder.

The spacer keeps the piston from overtraveling out of the bore by the increase of stroke, an alternative is to machine approximately 1mm off the lid of the piston, the OEM Yamaha piston is capable of this (Aftermarket pistons are generally lighter, thinner in the lid) along with a corresponding reduction of piston to valve which in turn limits your cam selections and lobe centerlines. That's why I had the spacer done, it enabled me to have a 459cc (instead of 444cc). Very nice package.

I know Rinaldi makes a official 479 kit for the 450. I guess reliability shuld't be an issue when using an official Yamaha kit. Check this link to Yamaha Racing

I'm kinda confused :cry:. Where would the spacer go? In between the cylinder and the case to raise the cylinder up? I was under the impression stroking meant using a smaller length con-rod. If you are having to deal with the piston overshooting the bore, then that would mean a longer con-rod? :cry:

It would mean the same length con rod, but moving the rod journel further out from the crankshaft centerline.

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