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426pilot

yz426f jetting question

8 posts in this topic

I'm replacing the stock piston in my 2000 426f with a wiseco 13.5:1 high compression piston. I need to know if I'll need to make jetting changes for this upgrade.

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You should not have to rejet for that.

What you should do however is make absolutely certain what the piston to cylinder wall clearance is using the new piston.

Wiseco pistons are notorious for shrinking after an hour or so running and what you end up with is piston slap fairly soon.

If the cylinder is more than .002in (2 thousandths) bigger than the piston right now do yourself a HUGE favor and send the cylinder to Millenium or US Chrome and have it re plated and honed to .0012in (1.2 thousandths) and you will be set, that will set you back about $120.00 or so not including shipping.

Otherwise don't be surprised with the new knocking sound that develops on your second ride or so, its not detonation or "pinging" it's piston slap.

I would venture to say that the Wiseco you have is closer to .003in (3 thousandths) clearance out of the box. which is too small (too much clearance).

Just sharing my experience (I have done 2 motors for myself with aftermarket pistons) and my newest big bore project is using a JE piston.

Also do not use the Wiseco wrist pin, get a brand new OEM wrist pin, they are lighter and every gram helps especially if you are on a stock Yamaha rod.

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I went leaner on the main jet (160) when I did the 13.5 piston. My Wiseco does slap a little, but you get used to it. Its a forged piston and they run looser than a cast, so its normal according to Wiseco. The performance gains are huge!

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Did you make any other changes besides the piston like new cams? Just wondering. Thanks for the info.

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Did you make any other changes besides the piston like new cams? Just wondering. Thanks for the info.

Its got a set of Hot Cams , and some mild porting done too. :thumbsup:

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I went leaner on the main jet (160) when I did the 13.5 piston. My Wiseco does slap a little, but you get used to it. Its a forged piston and they run looser than a cast, so its normal according to Wiseco. The performance gains are huge!

Yeah I used a Wiseco piston on my last build before I knew about the shrinking issue, and I don't think it is in Wiseco's best interest to admit that their product wont hold tolerance, but I have friends that build race motors for a living and they all concur that ALL aftermarket pistons shrink in the bore after little time and so the answer is to run the clearance tighter than prescribed so that when the piston actually shrinks it shrinks to fit basically.

The main thing to consider is that with tighter tolerances comes more heat so you need to control the heat by having a good cooling system intact.

I went with Fluidyne radiators for my current build and expect that to keep the heat under control.

Bottom line is that the piston will beat itself to death eventually if it is allowed to slap around in the bore for any length of time and the consequences could be costly if and when the piston decides to come apart.

Before Suzuki came out with the RMZ the Yosh guys were building DRZ big bore motors and testing them, Kevin Schwantz rode one in Supermoto for a bit.

I think Burned can attest to what I am saying, but anyway, they told me that they were breaking wrist pins and going through pistons right and left before they decided to run them tighter.

Once they started running them at .030mm clearance the problems went away for the most part and they also went back to OEM wrist pins and they quit breaking motors (as easily).

They did confirm that the pistons were in fact shrinking however.

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I see what your saying teamscream, but my noise has not gotten any worse in the 200 hours that I have on my piston.

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