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Voila!...'02 WR flywheel on '05 YZ

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Finally got the answer I was looking for.

The '01-'02 WR250F (as well as the WR426F) flywheel and stator WILL fit '03 and later YZ250F's with the following modifications;

1) The 10mm hole on the outside of the flywheel (where the nut is) needs to be opened up to 12mm (about .475").

2) The woodruff key needs about .010" shaved from the top.

3) Flywheel clearance is VERY tight with the cover and may require a little lapping of the taper and/or double gaskets for the mag cover.

4) Cut the pink wire from the charge coil and ground it to the stator frame (if you plan on using with YZ CDI)

Here's how it's done;

Flywheel set up in a lathe. Make sure the hole centers up within .001"-.002"

CIMG1136.jpg

Using a drill or boring bar, open hole up to 12mm

CIMG1137.jpg

'02 WR stator bolts right into '05 YZ case (all '03 and later are the same)

CIMG1139.jpg

Install '02 WR flywheel (rotor) onto '05 YZ crank.

CIMG1140.jpg

Make sure pickup coil lines-up with pickup bar on flywheel.

CIMG1143.jpg

Install '05 YZ cover and check clearance with flywheel.

CIMG1141.jpg

Older flywheels will require 3 screw puller to remove.

CIMG1138.jpg

Haven't run the bike yet, but I don't anticipate any problems.

Edited by 2grimjim
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I knew there was a way to do this without too much trouble. Thanks for the write up. 50watt charging system correct?

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50watt charging system correct?

Closer to 60. A ton more than you can get from the crappy Moose/Electrex/MSR lighting coils available for the YZ.

And there is always the option to rewind the WR coil for up to 200 watts.

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Holy moly 2grimjim - :bonk:

How to put a 200 watt coil on a modern YZ250F. :smirk:

This needs to be a sticky! (once the bike and charging system works of course :banana:)

That may be a little while since I have to rebuild the entire bike.

My only issue (and a relatively small one at that) is the flywheel sits 2mm farther out on the YZ crank. It shouldn't be an issue with the stator or pickup coil, both of them can be shimmed out 2mm if it's presents any problem. The only other issue with the 2mm offset is it barely contacts the inside of the YZ cover where the relief is cast in for the gearshift. I could make a .062" alumininum spacer, glue 3 gaskets together, or just do a little grind-and-weld on the cover.

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Steahly FWW comes with a spacer coz of this too.

Yeah, I saw that. I suppose I could buy a spacer, but I'm such a cheap-ass...

So far I haven't spent a dime. I guess I should give Steahly a call and see if they will just sell me the spacer.

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Yeah, I saw that. I suppose I could buy a spacer, but I'm such a cheap-ass...

So far I haven't spent a dime. I guess I should give Steahly a call and see if they will just sell me the spacer.

NOOOOO!!! That's the easy way out!! With all the access to the talent, tools and machinery you have you should be able to make a spacer. I agree buyin a spacer is tempting but making one puts you at pimp status!!

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Grimjim, Is it possible to change the taper to achieve moving the flywheel in the desired 2mm? Or are you not concerned with the flywheel weight being located that far out on the crank? At 13,500 rpms, I think I would be concerned.

This is amazing work so far!!! Keep us informed!!!:bonk::worthy::worthy:Maniac

Edited by MANIAC998
added some wording

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Grimjim, I had heard some talk abou useing the early WR stator and rotor years ago but no one went into detail except that you would use one half of the wr crank with the yz crank. Your method seems much simpler. I have a question or two will you be able to retain the yz ignition timing? Can you staticly time the yz, and then adjust the pick up sensor to the yz timing? Anyhow great job and thank you for shareing it with all of us, you are a asset to this forum's community.

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Grimjim, I had heard some talk abou useing the early WR stator and rotor years ago but no one went into detail except that you would use one half of the wr crank with the yz crank. Your method seems much simpler. I have a question or two will you be able to retain the yz ignition timing? Can you staticly time the yz, and then adjust the pick up sensor to the yz timing? Anyhow great job and thank you for shareing it with all of us, you are a asset to this forum's community.

Well.....don't have answers there yet, but, sometime today I'm gonna mock up the ignition and see if I get spark with the '02 WR magneto hooed up to the '05 YZ CDI.

Since the position of the pickup coil is exactly the same on both machines, whatever static timing was on the WR is what you will have on the YZ (now that I think about it, I've made a big assumption abouth the position of the woodruff key. Hmmm....). The only difference would come from the pickup bar on the flywheel being in a different location. So...if the static timing on the WR was 2 degrees less on the WR than the YZ, you will 2 degrees less timing equally distributed on the YZ advance maps. Not something that can't be overcome with programable CDI, but my bet is that it will run fine with the stock YZ CDI.

Oh, and thanks for the compliments guys! :bonk:

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Grimjim, Is it possible to change the taper to achieve moving the flywheel in the desired 2mm? Or are you not concerned with the flywheel weight being located that far out on the crank? At 13,500 rpms, I think I would be concerned.

This is amazing work so far!!! Keep us informed!!!:bonk::worthy::worthy:Maniac

Yeah, you could special order a tapered reamer and open the taper up a tad, but then you would have to broach-cut the woodruff key slot to make it a little deeper. During the reaming process you'd have to make sure you don't lose your center or the flywheel would definately be off-balance.

I'm not really concerned with 2mm displacement since it is neutral balanced. As long as the flywheel is balanced dynamically, that extra 2mm isn't going cause any strange loading on the end of the crank. The tip of the taper (where it's threaded for the nut) is pretty beefy on the YZ, 12mm threads vs. 10mm for the WR.

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We have spark!

But not without making a small change to the wiring.

CIMG1144.jpg

Not without some detective work though. The '05 YZ magneto (and all other YZ's) use two charge coils. The primary charge coil (Brown/Green wires) have a resistance of about 820 ohms. The same wires on the WR have a resistance of 770 ohms. The spec in the YZ manual calls for 720 to 1000 ohms, so the WR primary charge coil is well within these specs. The difference is with the pink wire. On the WR, the pink wire is a center tap on the primary charge coil, and on the YZ, the pink wire is connected to a seperate coil with a resistance of about 50 ohms (this is referred to as the secondary charge coil in thr YZ book). The black, red, and white wires all serve the same function. Red and white wires are for the pickup coil and have the same spec for the '02 WR and '05 YZ. The black wire is grounded on the stator plate.

When I put a Moose lighting coil on my '06 YZ, I noticed that the pink wire is grounded at the same point as the black wire and does away with the secondary charge coil. I have used the OEM CDI and the Dyna programmable CDI with the Moose coil with no problems. Whatever the intended purpose of the secondary charce coil is, it's not something necessary for any YZ CDI to function. I verified this with a 10 year old NOS MSR lighting coil for '98-'03 YZ's and it is wired the same way; the pink wire is grounded at the same point as the black wire on the back of the stator plate.

So, with all this in mind, I disconnected the pink wire at the CDI connector and grounded it with a jumper to the engine.

CIMG1146.jpg

Now that I know how to make the '98-'02 WR/YZ stator work with the "03+ YZ's CDI's,

I just have to cut the pink wire at the charge coil and ground it to the stator frame.

Edited by 2grimjim

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TDC mark lines up prefectly with '02 WR flywheel and '05 YZ mag cover. This means that the woodruff key and timing marks index exactly the same on the WR and the YZ. The other thing this means is that the timing marks on the cams will be correct.

If there are any differenceces in ignition timing, it's in the design of the WR flywheel and the placement of the pickup bar. Not anything that can be changed easily unless you are using a programable CDI.

So, whatever static timing that the '02 WR had is what I will have on the YZ.

CIMG1147.jpg

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Grimjim, Excellent work! I'm sure there are lots of people watching this intently. Threads like this are what makes ThumperTalk the greatest source of dirtbike information on the planet. My hat is off to you!!!:bonk:

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Later today I'm gong to hook up the Dyna CDI and make sure it functions too. If the Dyna works, I'll reprogram it for the 250 and call this mod a success.

I have some .063" aluminum sheet that I think I'll use to make a spacer for the cover and I'll post some pics of the modification needed for the pink wire on the stator.

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Is the '09 crank the same as the '05 crank?

Yes. All of this should apply to any YZ250F up to '09 and maybe later.

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