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YZ426F/2002 transmission gear upgrade? (only MAIN AXLE in a YZ250)

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The only difference between yz426 and yz250 is first gear, 2-5 are the same (part numbers also matching)

YZ426F/2002

1gear 24/13

2gear 23/15

3gear 23/18

4gear 24/22

5gear 20/21

YZ250/2006

1gear 27/14

2gear 23/15

3gear 23/18

4gear 24/22

5gear 20/21

Because I have a YZ426F/2002 tranny in good condition want to try this on my '06 YZ250:

YZ250/2006

1gear 27/13

2gear 23/15

3gear 23/18

4gear 24/22

5gear 20/21

That means the MAIN AXLE with 13T pinion from the yz426 and the 1ST WHEEL (27T) from the YZ250 working together.

To the other gears 2-5 there will be no changes.

Is this possible ? What do you think?

If someone can tell me the OD (outer diameter) from the 27T 1ST WHEEL and the 14T 1ST PINION from the YZ250, that would be a good starting point.

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Should work, normally you can swap it directly (YZ/wr426 tranny) although you seem to need to dremel some material of the Yz250 case.

I don't want a direct swap of the yz426 tranny - i want a combo between yz426/yz250 for 1st gear.

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It will not work properly.

You will have to use both of the 426 1st gears (driving and driven) as a matching set. Each tooth on the 13 tooth 426 gear will be larger and they will not mesh with the smaller YZ250 27 tooth gear.

The reason is that the pitch circle diameter is determined by the ratio of the gearset and the distance between the shafts. It is a fundamental law of how gears are designed and cut to a particular ratio.

The YZ transmissions have ~54mm distance between the shafts.

Your 13:27 ratio has pitch circles of 35.1mm and 72.9mm.

Your 14:27 ratio has pitch circles of 36.878mm and 71.122mm.

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Another note... In these transmissions, the driven 5th gear has dogs which engage with the driven 1st gear to lock the bike into 1st gear.

Because the driven 5th gear is the same part number between the YZ250 and the YZ426 you can be sure that the dogs on your 5th gear will be compatible with the driven 1st gear.

This means driven (mainshaft) 1st gear from the 426 should drop right into your YZ250 transmission.

The only other concern would be the mainshaft on the clutch side. If the mainshaft is longer for a larger clutch assembly (like it is on the YZ450F) then you may have some issues on that end. I created the following diagram of the 2003-2012 YZ250 transmission for another project I'm working on, but you can compare your 426 measurements from 1st gear to the end of the clutch-hub splines to determine if your shaft will fit.

YZ250_2003.jpg

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thanks for the diagram, i will measure the mainshaft from the yz426.

But no interest for the stock yz426 1st gear (24/13=1.846), i need lower 1st than my yz250 (27/14=1.928) not a taller like the yz426.

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YZ426 13:24 ratio has pitch circles of 37.946mm and 70.054mm.

YZ250 14:27 ratio has pitch circles of 36.878mm and 71.122mm.

Combo 13:27 => 37.946 + 71.122 = 109.068mm

109.068/2=54.534mm

Is the +0.534mm difference so bad ?

Anyway i will not try this , i wait for a WR426 tranny.

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you can compare your 426 measurements from 1st gear to the end of the clutch-hub splines to determine if your shaft will fit.

the measurements (from 1st gear to the end of the clutch-hub splines) on the YZ426 are exactly the same

Edited by dmikel

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