KX250/YZ250 Donor Chassis

Discussion for swapping a KDX motor in a MX frame...
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KDX-thusiast
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KX250/YZ250 Donor Chassis

Post by KDX-thusiast »

Hey everyone, first time posting. I’ve lurked this forum for a while, as I’ve been a long time KDX enthusiast (dads owned air cooled KDX), but I just recently picked up a 220R.

I’ve fixed up the 220R, swapped a 35 mm carb, and have since been riding the hell out of it. I really love the bikes bottom end AND top end. For the trails I ride, I don’t think I could ever need more from the motor. However, definitely want more in the suspension and chassis department.

I’ve been doing lots of research on this forum, and I would really like to do a hybrid build. I’d be wanting the ‘feel’ of the bike to improve, but I would also like to have a project that’s much more unique to me. I have been looking for the recommended K125 frames, but I can barely find any of these bikes for sale, yet alone rollers. However, there are a lot of good Y250 and KX250 chassis for cheap, and I would enjoy using one for a swap. I’ve been specifically eying a 1991 KX250 roller (minus rear wheel) for 150$.

My questions:

Are the 250 chassis actually any bigger in feel than the KX125? I am 6’3” and really don’t like bikes that feel “short” to me.

What years of YZ250 and KX250 would be best for the swap? Specifically will a 1991 KX250 work?

I’ve also been looking at Suzuki RMs. Are those the same as the KX equivalents?

Sorry for the long post, just super excited for the build. Figured the collective knowledge on this forum is the best bet I have. Thanks in advance!
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Re: KX250/YZ250 Donor Chassis

Post by Chuck78 »

I believe the KX250 frames are very similar, with the engine cradle areas being the only big difference (250 is a bit larger in the front cradle areas), and the swingarms are longer on the 250's almost always. The engine cradle front tubes would pose more clearance issues with the KDX exhaust outlet location I believe.

Keep it green if you can!

The YZ250 is a GREAT platform, but 2005+ are aluminum frames, so those may be a bit harder to work with TIG welding and all. The 98-04 frames are great though. Steel. Proven. The 2006+ suspension is the really good stuff though. Comparable to what the RM125 had 1996-2008 (& KX250F 2006+), but the RM valving was a bit more firm but easily altered. The RM's ran Showa Twin Chamber forks, YZ had KYB's nearly identical version of those, the S.S.S. Closed Chamber forks. I've got several sets of 49mm 1998 RM125 forks here, the pinnacle of right side up fork technology. 99-00 went inverted but still 49mm, and were too stiff both structurally, and valving wise, which was completely completely garbage vs the 96, 97, and best 98 stock valving. 2001-2008 saw them go down th 47mm Showa inverted twini chamber forks for just the right amount of flex for pro motocross supercross racers. 2005 was a really good redesign in the Showa forks also, 30mm valving pistons vs 20mm (slightly larger but that's the size of the largest valving shims, the 98 RM125 piston is 23mm diameter vs the KDX fork base valve pistons at 20mm overall with smaller shims). FYI RM250 was similar but went to KYB suspension in 2001-2004, but back to Showa Twin Chamber 47mm inverted forks in 2005-2008.

One other thing that you're surely able to find would be a 2004-2005 KX250F, they can also make great donors. Those years still had KYB Open Chamber forks though. Open Chambers are a bit easier to valve plush for technical rocky rough woods riding, and easier to service, but with more attention to detail, closed chamber or twin chamber forks (same thing) can also be valved quite plush. the damping chamber has an Internal Compensator Spring that does the same thing that the nitrogen charge in our rear shocks does, vs open chamber forks which lack this feature entirely and succumb to oil cavitation under hard use in washboard sections and racing in general, making the damping get very fast when the oil cavitates with the air in the fork.
The 2006+ KX250F is an aluminum frame, so a bit harder to work with for most unless you're a skilled TIG welder. They do have the best suspension, especially in my opinion the 2011-2016 models with single sided forks, one side fork sprinig, one side damping. Best to get plush feel out of a closed chamber fork in my opinion.

Join some Facebook MX bikes for sale type groups to find a 99-02 or 03-05 KX125 roller or basketcase if you haven't given up on a KX125 yet!

Best of luck, keep us posted
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Re: KX250/YZ250 Donor Chassis

Post by SS109 »

Great post by Chuck.

Only thing I'll add is a '91 KX250 is, honestly. not much of an upgrade over the KDX chassis and suspension plus a lot harder to find plastics for than the KDX. I would stick with a chassis that is at least '96 and newer.

Oh, I'll also second the '04-'05 Kawazuki, both RM250F and KX250F, bikes as the bikes handled great and they are easy to work on steel frames. Their only real negative was the horrible Suzuki engine that was in them.
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Re: KX250/YZ250 Donor Chassis

Post by Chuck78 »

SS109 wrote: 01:43 am Feb 02 2024

Oh, I'll also second the '04-'05 Kawazuki, both RM250F and KX250F, bikes as the bikes handled great and they are easy to work on steel frames. Their only real negative was the horrible Suzuki engine that was in them.

RM-Z 250 is what the Suzuki 4-stroke MX bikes are called, not RM250F

I personally have been considering & keeping an eye out for a cheap 99-02 KX125 chassis for a hybrid build, for the stylings over the 03-05 (USA, 03-08 Internationally) RM125/250. Really I like the 94-98 stylings the best, but the comfy seating position like the KDX is one of the biggest things '99+ KX/KDX hybrid builders cite as a major difference, 99+ feels like you're on top of the bike, vs KDX & pre-99 KX, you feel as if you're sitting "into" the bike. 99+ seats are taller and flat, + thicker foam. The thicker foam adds a pound or two of additional weight, and the KDX larger fuel capacity + perimeter frame fuel tank placement, + thicker foam, + steel seat subframe add several pounds up top (mostly fuel weight but the rest also is significantly noticed). Just a few pounds extra up high on the bike really give it a noticeably heavier feel.
The KX's keep their fuel weight lower mostly by carrying LESS fuel, 2.2 or 2.3 gallons is standard on them as well as modern KTM branded enduro bikes.

The 99+ chassis is slimmer in the shrouds area near your knees slightly, but the 03+ slimmed down more noticeably for a much more sleek feel that people really notice. 94-98 & 99-02 125's maintain nearly identical chassis geometry to the KDX, 03 is similar but a more raked out steering angle for high speed stability, and 04-05 were same but longer swingarms. Each year 03-04-05 had it's own proprietary swingarm and linkage setups, FYI. 03 was the last year of the conventional pull rod linkage, 04-05+ got the newer style.
There's tons of aftermarket support for the 99-02, and although they don't look as classic as the 94-98, they still favor those stylings vs the more modern 03+ pushing the envelope for the edgy bold jagged lines of modern machines.
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Re: KX250/YZ250 Donor Chassis

Post by KDXGarage »

I would not go through all the welding and measuring to be riding a 1991 KX250 with a KDX engine.

A KX125 is going to feel short to you. I own a 2005 KX125 and a 2005 KX250. I can tell the 125 is thinner, easier to move the 125.

Go sit on a few 125's and 250's before buying ANYTHING! Good luck.
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Re: KX250/YZ250 Donor Chassis

Post by Chuck78 »

The 1994-1998/1999-2002 KX125 have almost identical wheelbase and geometry to the H series KDX. 2003 KX125 has a similar wheelbase, but the steering rake is a bit more mild for better high speed stability. 2004 - 2005 got a longer swingarm than 2003, and each year of that era '03-'05 (or '08 internationally), had completely different suspension linkages for the first 3 years. 06-07-08 International models I believe basically the same as the 2005.
The '03-'05+ KX125 were designed to be slimmer than all prior models.
The taller slimmer flat seat is a big difference from the KDX seating position and the '94-'98 KX, and you feel as if you are sitting on top of the bike rather than sitting into the bike like a nice comfortable sofa cushion.

Generally I believe the frames are almost identical aside from the engine cradle areas 125 versus 250 on all 94-08 model 2 strokes. The 250 is usually have a longer swing arm than the 125. Acadia x200 engines generally Mount better in the 125 frames, as the 250 frames have the cradle tubes a bit further forward which cause problems with the exhaust unless you are grafting the entire engine cradle from the KDX frame onto it.
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'99 KDX220R project - '98/'01 RM125 suspension, Titanium hardware, Lectron Billetron Pro, Tubliss
'77 Suzuki PE250 & '83 Suzuki PE175 Full Floater - restomod projects
'77 Suzuki GS750-844cc, '77 GS400/489cc & '77 GS550/740cc projects
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Re: KX250/YZ250 Donor Chassis

Post by KDX-thusiast »

Thank you all for the replies!! I’m now thinking a 99+ KX125 frame would be best, as the swap is very thoroughly documented, and I would prefer the ‘higher’ feeling seat position. Plus, I am a decent welder, but don’t trust myself enough to mess with the actual frame of a KXF or RMZ. Also read somewhere on here that the YZ swing arms are hollow and would need much more modification to fit the KDX case. I think the first step for me should be to actually go and sit on a KX125 and see how I like it. Then I’ll just have to stay patient and wait for a good KX125 roller to pop up in my area. Rather take the time and do it right. It’s not like a couple more months of riding on good ole reliable is gonna be an issue for me :supz: :supz: :supz:
I will keep you guys updated when I take the plunge. A good friend of mine is convincing me to enter an enduro race with him in September, so the KDX Hybrid may very well be my Summer project!!!
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Re: KX250/YZ250 Donor Chassis

Post by Chuck78 »

I've found that the 99-02 seem to have the widest variety of aftermarket parts availability, especially with oversized tanks. 94-98 not much asude from maybe 3 taks, and OEM shrouds need trimmed substantially for the oversized 94-98 tanks.
Stilll plenty of aftermarket available for 03-08, but 99-02 seemed to have the most. I also prefer the stylings of the 99-02 better, modern but slightly more classic looking, although not as classy looking as the 96-98, which I'm a sucker for. Both those generations have handles modled into the side covers which I also really like, although you can just buy an over-the-seat grab handle strap that mounts off the seat bolts.
'97 KDX220R - purple/green! - KLX forks, Lectron, FMF, Tubliss
'99 KDX220R project - '98/'01 RM125 suspension, Titanium hardware, Lectron Billetron Pro, Tubliss
'77 Suzuki PE250 & '83 Suzuki PE175 Full Floater - restomod projects
'77 Suzuki GS750-844cc, '77 GS400/489cc & '77 GS550/740cc projects
'62 GMC 1000 Panel Truck
'88 Suzuki Samurai TDI/Toyota swaps
'88 Toyota 4x4 pickup
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