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RasmusDK

Gearing YZ400F

4 posts in this topic

Iam in doubt what gearbox is in my bike, Iam thinking its maybe a WR400 gearbox.. Can I check that without spliting the engine?

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Iam in doubt what gearbox is in my bike, Iam thinking its maybe a WR400 gearbox.. Can I check that without spliting the engine?

don't know about that.

You should ride a similar bike to yours. If gearing feels the same (RPM change between gears is about the same, and sprockets are the same size) then you probably have stock gearbox

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Put the bike on a stand, mark the front sprocket, and put the bike in first gear. Have someone hold the compression release (or tape it down) while you turn the engine over through the timing port with a T handle or some other tool that makes it possible to count how far the engine rotates. Turn the engine until the rear sprocket makes one full revolution, and record the number of turns the engine made. Do the same thing in 5th.

Then you do some math. Here are the ratios of the two transmissions:

WR400

1st 2.416

2nd 1.733

3rd 1.312

4th 1.050

5th 0.840

YZ400

1st 1.929

2nd 1.563

3rd 1.278

4th 1.091

5th 0.952

As you can see, the biggest differences are in the 1st and 5th ratios. That's why I suggest using them for this check. The primary ratio of 2.952 has to be taken into account, too, so what you do is to multiply the 1st gear ratios by 2.952 to get the net internal ratio in low gear:

YZ: 2.952 x 1.929 = 5.69

WR: 2.952 x 2.416 = 7.13

So, then, if when you turn the front sprocket one turn you find that the engine has rotated just less than 5 3/4 turns, that matches up with the YZF ratio. If it turns just over 7 turns for one at the sprocket, it's a WR. Repeat the process for 5th, just to make sure.

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