Is it possible to make a 02-07 YZ250 a six speed?

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#1
I would really love to have that extra gear when riding with 450's in the desert. I actually out pull the 450's I ride with. When I get pinned in 5th, they pass me. My bike has plenty of engine left and would have no problem pulling an additional gear.

I remember hearing back in the day that you could do something with a YZ125 to make a 6 speed YZ250. I can't remember if you put a 250 jug on the 125 engine or the 125 gearset in the 250 case (seems more likely to me).

Any thoughts or experiences would be appreciated.
 
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#2
I don't know about a 6 speed, but you can swap in some parts from the WR426 transmission to make a wide ratio 5 speed. You can change all 5 gears, or just do 3rd, 4th, 5th (which I think most people do).

Sorry Adam, that is not allowed. Mully
 
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#3
hey, i replyed a while ago but it was deleted...... again. so basically what adam said is what i said as well. change around your gear ratios and you could pump out some more power. But doing so remember that your bike is only meant to have 5 gears and if you supe it up to make it run like it has 6 gears you might have to re-jet.
 
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#4
The trouble with a 6-speed is that the gears would have to be cut narrower to fit them in the same case as the 5-speed. This would weaken the gears. The wide ratio transmission and a change of sprockets would be the way to go.
 
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#5
Can we do it this way? I hate to leave a guy hanging when all this information is already available.

Absolutely. Contrary to another members' post, the WR426 mainshaft, countershaft, and tranny gears will all retrofit to the YZ250 (02-on).
You actually have two options for doing this. If you want a taller 5th and lower 1st, you can replace the entire tranny with the WR parts (m/s, c/s, and all 9 gears.) This is kind of expensive but cool if you ride super tight trails. The other option is to just replace 3rd, 4th, & 5th gears with the WR gears (6 gears total.) This is what I did to my YZ last summer and it works great. It buys the bike about 10mph more top speed with stock sprocket gearing. For the desert riding I do, 1st & 2nd are fine where they are.
The guys with large displacement thumpers I ride with don't check-out on me in the fast washes anymore.
I just picked up the parts today for my tranny. 3rd 4th 5th gears, '99 WR 450, all three shift forks and a crank shaft key were 321.00 Not too bad.. Now I just need odds and ends...
Here ya go friends

Gear, 3rd Wheel 5BF-17231-00
Gear,3rd Pinion 5BF-17131-00
Gear. 4th Wheel 5BF-17241-00
Gear, 4th Pinion 5BF-17141-00
Gear, 5th Wheel 5BF-17251-00
Gear, 5th Pinion 5BF-17151-00
 
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#6
Thanks Adam for taking the time to do the cut and paste.

If I can not find a way to do a 6 speed, I will look into the wr gears next. I wonder what the difference is in the primary reduction between the bikes. That would make a difference as well.
 
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#7
I don't think there is a difference. Dirt bikes seem to be pretty universally 3:1 on the primary. I think the 500's are the only exception.
 

BSWIFT

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#8
Thanks.
 
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#9
Primary reduction ratio per Yamaha's website:

YZ250
06-07 3.000
04-05 3.353
02-03 3.000

WR426f
02 2.952

I guess I answered my own question if the website info is to be trusted.

I do not completely trust the website because it shows secondary ratio (sprockets) as 48/13 for the 2005 yz250. I thought all the 2000+ model yz250's came with a 50 back and 14 front as the stock setup.

So I am still confused.

OK. I must sound really neurotic. As I was typing, I just dug up my factory manuals for 2001, 2003, 2004, and 2005. They all say primary is 3.000 (63/21) and secondary is 50/14 (3.571). It looks to me as if the website is wrong.

Does anyone have a factory manual for a 2002 wr426f (or other 100% positive answer) to verify the primary ratio?
 
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#10
The website probably is wrong. 3:1 is pretty much the universal standard. There is a a little fudge room in there, would you consider 2.952 anything but 3:1? That's like a 2% difference.