Hi all, I bought this YZ250 '02 late last to ride post 1990 2 stroke class with Viper VMX club but and also
 to do the odd trail ride with
amtra as well. The only draw back is the gearbox ratios being an mx bike first gear being a too high and
top gear to low, not ideal for trail riding. 
I have heard from an amtra member (I cant remember who)  that a WR400 gearbox will slide straight in,
anyone know about this mod?
I could sell it a buy another more suitable bike. An expensive option and I really like this bike so this seems a good idea.
Wade

I love the yz250s Iv often thought about gettin one. 
Why can't you just change sprocket ratios and see how it is

Yep tried that Tim it makes 1st gear way too high.  

I think Flipper is the bloke you want to talk young Wade.
Al

The complete WR400/426 gear cluster will fit with some very light case grinding using a dremel style tool - it is just a whisker that has to come off to allow for the larger size of the 1st gear. I have heard of people just putting the gears in without bothering to do the grinding, but I personally wouldn't want the aluminium dust floating about in my gearbox.

If you don't like the idea of fitting the full cluster, 3rd, 4th & 5th can be changed to WR gears to spread the upper range ratios, allowing you to run different sprocket ratios and drop your existing YZ 1st and 2nd down to a more usable range.

Look at the US ebay site for gearboxes - they are much cheaper over there, even with the postage costs.

Oh, and you still use the existing YZ selector forks and shift drum with the WR gears.

I am fitter machist and no probs splitting the cases and machining some clearance if nessesary.
I would like to change all ratios, for the WR gears. I gather the WR450 is not capatible? 

Sh!t Pete,

You know stuff! 

Thanks Pete, I found this on evilbay

http://www.ebay.com.au/itm/271553487129?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649

Yeah, but only boring technical stuff Serge.

There have been a couple of unconfirmed reports of people fitting the earlier WR450 gear clusters into the YZ cases, but there have also been others that said it didn't work, so your guess is as good as mine in that regard. I chose the 400/426 box because I knew it would work.

It is a popular mod in the US and South Africa. To give you an idea how little has to come off the case, the gear actually fits in place without the mod, but rubs on the casting imperfections.

I fitted the gear shaft into place, and spun it to mark where it rubbed, then ground down the high spots. The casing material is quite soft, so be gentle when machining it. Also, use the old bearings whilst doing the mods, and then fit fresh ones when ready to put it back together - saves having to worry about getting filings in them that way.