YZ250 Motor in a YZ450F Aluminum Frame

The answer to the question no one asked....will it be easy to put a 250 2-stroke into a newer generation YZ 4 stroke frame?

I pulled apart my 2000 YZ250, it was time to freshen up the motor anyway. I put just the countershaft gear in and put the cases back together so I could align the sprockets. Then I put the rest of the shell together so I could place the motor in without all of the weight. I got a frame, swingarm, subframe, and airbox from a 2006 Yamaha YZ450F. The rear motor mount on the older 2 stroke motor has a smaller diameter bolt hole than the newer ones, so you need to press the old ones out and cut them on a lathe or buy newer ones to fit the larger swing arm bolt.

The motor fits perfectly into the swingarm with no other mods. The lower engine mounts need to be cut off so the motor can sit in the frame at the correct angle. The exhaust pipe fits into the frame and on the motor very well with no modification. I will be bringing the frame and motor to the local chopper shop for them to weld the bottom engine mounts back on where they need to be and to weld tabs on for the exhaust hangers and ignition coil.

The biggest modification I have run into so far was fitting the airbox, since the boot for a 4 stroke carb is much smaller than the 2 stroke. I ended up disassembling the 450 airbox and took out the boot. The 2 stroke boot fits into the airbox with little mods. You need to align it and drill holes for the flange to bolt in and bolt it together. It fits into the frame pretty well but does rub on the left side of the boot. I took my die grinder and ground the frame from the original cutout for the 450 boot, down another 4 inches so the boot would fit better. I also ground up top so the boot wouldn't rub as much, but found that grinding above the cutout would get into the structural integrity of the frame.

My fix for the boot was actually pretty simple, I attached the subframe and boot to the carb and got out my heat gun. I warmed the boot up so it could stretch and releive some tension from not being perfectly aligned. Where it was be the frame, I also warmed it alot and the placed two screwdriver handles in between as it was cooling. Once it cooled, I took out the screwdrivers and it kept its shape around the frame instead of rubbing on it.

I have to put my 2006 suspension on yet and mount everything else, but I figured I would share the start of the project with you. I will keep you posted on the progress. I am shooting for a better handling 2 stroke than the current frame they have now for it. I love the way my 2007 YZ250F feels and am shooting for the same effect with this one.

Sorry for the crappy picture, all I had handy was my cell phone. This is what happens when you have a long South Dakota winter. Let me know what you think or if you have any questions.

You need to upload that photo to photobucket and then click on the share button for the photo, generate a link code, then copy that direct link to the IMG tags.

You can send it to my email and I can do it for you if you do not want to sign up for an account, I PM'd you the address.

Man,

That looks awesome. Looking forward to seeing the end result, and seeing if the handling is greatly improved, and how all the finer details all come together. Good luck.

Thats pretty awesome, I was wondering if you got the chance to weigh the 450s Alum frame since I've really wanted to know for awhile?

Right on sounds and looks like a very cool project! Looks good so far. I'm sure it will handle better than the 00 yz but will it be better than an 05+? Hmm I guess you will find out.

While you will have conquered the problem of graphics sticking to the gas tank, I think all this work is going to prove that the 450 framed bike won't turn as well as the stock aluminum framed bikes. Lots of luck though.

I was kind of thinking the same thing Budlite. But I wasn't sure if he was wanting to upgrade a 00 YZ or an 05+....? He is using a 00 engine so I figured it was that year.

I will be watching this thread like a FREAKING HAWK!!!!

Nice one buddy.

I have have been eying my dads YZ250F frame for my YZ250 for too long. Its so much easier to grip the bike with your knees with the fourstroke frame. And the plastics dont look a decade old.

Every year I think yamaha will do this for us, but then they dissapoint us again. I might have to do this too.

Keep us posted!!! I cant wait to see how it turn out.

Ok i am locked in for updates on this thread!! Gotta see how this turns out! Goodluck with the build!

...The rear motor mount on the older 2 stroke motor has a smaller diameter bolt hole than the newer ones, so you need to press the old ones out and cut them on a lathe or buy newer ones to fit the larger swing arm bolt.

...This is what happens when you have a long South Dakota winter.

Great project. Keep us in the loop!

...And the plastics dont look a decade old.

Every year I think yamaha will do this for us, but then they dissapoint us again.

Wow, can't wait to see how this turns out!

I have have been eying my dads YZ250F frame for my YZ250 for too long. Its so much easier to grip the bike with your knees with the fourstroke frame.

This would make more sense to me. The 250F and 250 2T engines weigh just about the same. I believe the 2T has a lower center of gravity. If the pipe will fit in the front then this might be a good swap. Defintely worth looking at. This is the one I have been thinking about for a future project, not a 450 chasis but if this guy is not a hard core racer maybe this swap will be perfectly fine for him.

I looked at the 250F frames, since I already have on with a 307 stroker in it, and I came to the conclusion that there would have to be extensive frame mods to the front of it in order to fit the exhaust pipe. That would weaken the frame significantly in the front. I am not a hardcore racer, just the occasional hare scramble and play riding with the boys, I go too old and fat to race anymore. I have several parts on order and hope to have the motor back together this weekend and put the suspension on and then off to the chopper shop for a little welding.

What are your thoughts on updating the wire harness and cdi to the 2002 and newer. I think it will be easier to route and mount, but will there be any issues with it? I also thought of hacking the original one and making my own harness.

I wish I would of had an airbox from a 2002 or newer YZ250, then it would have been alot easier to remount the intake boot, oh well, maybe next time.

I will try to get a few more pictures of the bike in progress, and closeups in the next few days. Stay posted.

I will try to get a few more pictures of the bike in progress, and closeups in the next few days. Stay posted.

You tease!!!

I looked at the 250F frames, since I already have on with a 307 stroker in it, and I came to the conclusion that there would have to be extensive frame mods to the front of it in order to fit the exhaust pipe. That would weaken the frame significantly in the front. I am not a hardcore racer, just the occasional hare scramble and play riding with the boys, I go too old and fat to race anymore. I have several parts on order and hope to have the motor back together this weekend and put the suspension on and then off to the chopper shop for a little welding.

What are your thoughts on updating the wire harness and cdi to the 2002 and newer. I think it will be easier to route and mount, but will there be any issues with it? I also thought of hacking the original one and making my own harness.

I wish I would of had an airbox from a 2002 or newer YZ250, then it would have been alot easier to remount the intake boot, oh well, maybe next time.

I will try to get a few more pictures of the bike in progress, and closeups in the next few days. Stay posted.

What about adding a custom spacer to the engine, making the pipe come out a bit further from the engine?

No the problem is that the exhaust port on the cylinder would sit directly behind the frame. You would have to cut the frame to make it work on a 250F Frame. Got my forks, triple clamp, front wheel, handlebar and controls mounted today. I ran into a problem with the motor that will set me back a couple of weeks. The nikasil on the exhaust port has a slight chip in it so I am going to send off the cylinder tomorrow to get it redone, better to be safe than sorry I guess. I am still waiting for the new spacers for the rear engine mount to put it back in.

Don't let the weakening of the frame keep you from doing this. There have been hundreds of AF500 done by individuals and a few providers like Service and MPS. The thing is when you get it new the frame strength varies quite a bit due to different materials like castings and tubing. I've only seen pictures of one failed conversion and it looked like the result of cold welds and preparation. I'm close to the end of my AF500 and considered doing the heat treat after welding, but after thinking about it it was more hassle than it is worth. You would have to get it back to annealed condition by a solution treatment which is ~ 1000F followed by a quick dip into water. Then an ageing step for several hours. The problem is there are quite a few sealed off areas and if they're not drilled you will get a blow out resulting in trashed frame. Either way you're going to probably get distortion even with an engine blank. Any time you go from hot to immediate cold you're going to get some movement and also the finish of the frame. By the way, besides aerospace welding I'm also a heat treater in conventional and vacuum and follow AMS2750D religiously. To sum it up, you're only modifying the bottom cradle and the Y section. The engine and the frame as a unit contain the stresses so as long as it's prepped and welded good you shouldn't have any problems. We do expect pictures which goes without saying.

Craigus

Huh... It's too bad that you couldn't modify the cylinder to make a small degree turn... I suppose aligning water jackets and altering the exhaust port etc would be way to much work come to think of it.

That sucks! What are you going to do?

Worked on project Smoker today again. I put my radiators on, put the bottom end in it, put my forks, wheels, chain, handle bars, controls, and footpegs on. I might have a clearance issue with the pipe and the radiator, I will have to wait to get my cylinder back to be sure though. It is shaping up nicely though. I put all TM Designworks sliders and rollers on, cleaned up the swingarm and prepped the frame for welding.

I am getting excited for completion. I ended up ordering a wire harness and cdi from a 2003 on ebay to put on there, the mounting and routing will be easier with the newer ones. I also switched the intake boot from the YZ250 to a YZ125, the bells on the carb are the same size but the shape is a little different and fits in the frame alot better. Here are a few pictures as promised, with more to come as more parts arrive to complete it.