WR450F engine in YZ frame?

Here we go,

I just realized I posted this in the wr/yz250 forum, sorry.

First of all, this is my first post. I just got a 2003 yz450f for a steal, the only thing wrong with it was the engine, seized piston. I was able to pick up a complete '03 wr450f engine. I have a few questions for you experts:

1. will the engine fit? from what I have read, it should. I know there are some differences between the two frames but just rake, trail geometry, not engine mounts. right?

2. can I use the YZ carb? if not why? exhaust?

3. I've read that the yz exhaust cam is a bit more aggresive but if I use it I need to do the 'grey wire' mod. what is that. Should I use the yz electrical system? is it worth using the yz cam?

4. I know there are different wheel sizes, should I use the yz sprockets of wr sprocket sizes?

I'll probably have more questions but these are for now. Any help is greatly appreciated.



from what I understand the motor will bolt right in . but I know the electrical system is differant . I'm not sure when the wr's went to a dc powered cdi but this will be a major issue if it has one.. as far as thr gearing goes I'd use the wr's . tires on a wr and a yz are really the same size on the outside it's just the rim that is bigger . the wr has more sidewall to help stop pinch flats. you are most likely going to need to rewire the yz or get a wr wiring harness.

You could possibly try to piece together the yz motor with good parts from the wr, like the cylinder, head and piston.

You could possibly try to piece together the yz motor with good parts from the wr, like the cylinder, head and piston.

My question about doing that is . whats locked up? I'm not sure if the crankshaft will swap from the one to the other I know it's differant in the later years but I'm not sure about an 03 . if the piston siezed there is a good chance that it could have messed up the crank too. I think what really needs checked here is the stator side of the crank . if they are the same swap the stator and all is good if not then you'll either have to swap the crank or swap out the wiring.If you swap out the wiring you could use the electric starter.

I'm sure that he could make it work, he is after all Mr. Abe Frohman, the sausage king of Chicago.

Thanks for all the replies guys. The advantage of the WR is the 5 speed. I do a lot of desert/trail riding. The connecting rod is bent so the cases would have to be split to replace the con rod in the YZ. Could I replace the entire electrical system on the WR with the YZ system? What could be the worst that could happen? It doesn't run or worse? What is the 'grey wire' fix that I've heard about?

Thanks again

to be honest if it were mine I would use the wr wiring system for the e-z button and a headlight . I would also toss in the yz cams. The wr's wide ratio five speed gearbox is awesome . Can you get the wr wiring ? If not , I'm sure I could figure out how to wire it , but it may require a stator mod of some sort. it would be best and easiest if you can get whole wr wiring harness and cdi. the carb ,pipe ,and all the other yz stuff should work .

You won't find a grey wire on the YZ wiring harness and either way there is no relationship between cam swap and the grey wire... if it were a WR I'd say research the grey wire and cut it but you have a YZ foundation that you are putting WR power plant in...

I think you have some tough times ahead figuring out the wiring on this thing. I would try and get my hands on a WR CDI and wiring harness if I were you.

Good Luck:ride:

hey guys,

Thanks for all your help and input. I think Don hit it right on the head. Electrical is going to be the hardest to figure out. The CDI from the YZ is the same as the WR, same part #. I'm not too concerned about not having electric start and I don't have a headlight, not to mention no battery. I had read that to swap out the cams required the grey wire mod or else the hot start system would not work as designed. I have been looking at the generator systems at both. I think I'm going to try just swapping out the entire electrical system from the YZ to the WR. I just got the engine today and was looking it over. This is my gameplan:

YZ frame, everything but the engine.

WR engine with yz cams, electrical system, carb and pipe.

I also figure since I've got the engine on a bench, I'll replace the piston/rings while I can.

My backup plan is to find a wr wiring harness and go from there.

Thanks again for al your help


There is no effect of grey vs. no grey wire and starting with the cam swap. You are not using the e-start so there should be no issue with the YZ exh cam. Check cam P/N's between the two motors, the intakes may be the same cam so it would only be necessary to swap the exh.

I know the radiators are different on the bikes but you should be fine with YZ rads if you ride desert alot... pack and carry stuff and she'll overheat fast.

Try and download the wiring schematics for both bikes (if you search in this and the YZ forum I bet you'll find them) that way you can see the major differences.

Have fun, can't wait to hear how it works :worthy:

Good catch on the intake cam. Turns out the intake cam is the same thanks. Going over all the wiring and parts, I feel comfortable swapping out electrical systems. It looks like I can change crankcase covers, which will allow me to toss the starter motor and connect the YZ generator assembly in place of the WR stator and rotor assembly. Then I'll essentially have an 03 YZ with a 5 speed. I know it all sounds good on paper now, hopefully it will all just fall together. I'm planning on taking pics and documenting this FrankenBike build.

Thanks again,


Not sure if the 03 is the same as 05 ( I thought they were), but I have an 05 WR motor with YZ ignition, cams, head and it also has an 06 YZ barrel. Its now a 5 speed YZ !

I lunched my YZ motor and got a cheap WR to replace i, I assumed at the time that they were the same and only discovered the differences when I stripped it down..hey, I am new to all this.

On the 05 model, the ignitions were totally different, dc and ac, and the YZ stator is inboard and the WR is outboard on the ign case and the flywheels are also different. No idea on cam timings as I am not an expert on the whole YZ/WR intrechangeability thing, but we altered ours anyway by taking off and re-pressing the cam wheels for more top end as I use it for SM racing.

To get the YZ ign on mine, we machined the WR crank taper end and made a plate to mount the YZ stator onto the cases.

I was afraid of that. Looking more at the parts, th WR has a different crank on the generator side to accomodate the starter clutch.

I was afraid of that. Looking more at the parts, th WR has a different crank on the generator side to accomodate the starter clutch.

I think someone makes a starter block off kit for the WR... you should search the forum and see what pops up.

Can anyone tell me if there is a significant difference betwen an 06 CDI and an 03 CDI? The connectors should be the same right?



Assuming we are still talking WR450F

The 2006 CDI is:


5154912-001 $231.77

The 2003 CDI is:


1825539-001 $250.59

Whole different part number.

And the wire harness itself is:

2003 WR450F:


1825529-001 Enter 0-99 $107.24

2003 YZ450F


1745409-001 Enter 0-99 $58.92

As best as I can tell from BB.

From the schematics I'm looking at it appears to be much easier to just get and swap in a full WR wiring harness...but thats just from pictures.

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