1990 kx250 cylinder fitment on a 91 kdx 250 ?

8 replies to this topic
  • Arl9890

Posted November 10, 2010 - 03:59 PM


Hey guys, just recently picked up a 91 kdx 250 and it turned out to be pretty beat. Bought it in Non running condition, cylinder was toast with a 1/4 deep scrape on the intake side all the way up.

Seems as if the bottom ring bumped out at high rpm and jumped up on top of the ring positioning pin. The cylinder was in terrible need of replacement, the piston skirts were curved in from skirt slap. I can only imagine the racket that it made when it was running.

Anyhow, tore it down completely and cleaned out the gobs of metal filings from the bottom end, noticed also the large amount of crank walk which in turn ate out the big end on the connecting rod.

After diagnosing everything and adding up the cost I branched out and began a parts list. And am pretty much at a stand still with the cylinder, I know I do not want to send it out to get re-coated and have been leaning towards a wiseco sleeve. Found a used cylinder on ebay from a 1990 kx 250, it seems to be similar to the kdx 91 cylinder. It's already been sleeved and needs a piston.

My question is WILL IT FIT on the kdx cases and have the same stroke. I've tried looking for a used Kdx cylinder but the do not seem to exist. It's either buy this used kx 250 cylinder or a wiseco sleeve. Figured I'd run it by you guys for good measure. Thanks for any answers.

  • The Spanky

Posted November 11, 2010 - 06:06 PM


The '89-'91 KX cylinder and head will bolt right on, but you need the KX power valves as well. The bore and stroke of the KX changed in '92.

I would have the cylinder repaired and re-plated before I would install a sleeved cylinder on it, but that's just my personal preference.

I ran a '91 KX250 cylinder on my '91 KDX250 for awhile. The KX porting makes for a much more aggressive and harder-hitting power curve that wasn't well suited to the tight gnarly mountain trails that I rode in, so I had the KDX cylinder ported and started using it again. The KDX cylinder seems to be happier with the KDX ignition curve, and it has much better low-end and a smoother and torquier midrange. Properly set up, the KDX250 has so much low-end it will climb anything. It would probably climb a tree if you could hang on.

  • The Spanky

Posted November 11, 2010 - 06:08 PM


What the hell did that piston in your avatar come out of??? That thing is scary looking!

  • Jeekinz

Posted November 12, 2010 - 06:12 AM


Didn't the KDX's get out dated KX parts, early 90's?

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  • Arl9890

Posted November 12, 2010 - 04:24 PM


Yea this is going to be the back up bike for me, got a torquey yz400E That I take out often for a rip. Been stuck in the 70's lately with air cooled technology and getting this kdx back on the dirt is going to be a treat. Modern suspension, Disc Brakes !!! can't wait. Hey man, really appreciate the answers you gave me. well put about the porting difference. Thanks again.

  • The Spanky

Posted November 12, 2010 - 05:27 PM


Didn't the KDX's get out dated KX parts, early 90's?

The '91-'94 KDX 250 uses a modified '89 KX250 engine. In it's debut year ('91) the KDX 250 was using the same basic engine that the '91 KX 250 was using. The KDX did get a different frame (cheaper and heavier), a shorter swingarm, and much cheaper suspension components, but some parts were straight off the KX, such as the brakes and radiators.

  • Arl9890

Posted November 14, 2010 - 03:39 PM


Hey Chokey, swapping out the kdx cdi with a KX would better suit the unhappy ignition curve (going with the kx porting) that you spoke about, right? Do the Kx and kdx stators produce the same amperage?

  • The Spanky

Posted November 14, 2010 - 04:30 PM


Hey Chokey, swapping out the kdx cdi with a KX would better suit the unhappy ignition curve (going with the kx porting) that you spoke about, right? Do the Kx and kdx stators produce the same amperage?

If I'm not mistaken you would have to use the entire stator, harness and CDI module. Which means losing the lighting capability and the heavy flywheel, since the KX is an inner rotor and the KDX is an outer rotor.

It's not that the ignition curves were incompatible, it just seemed that I could never quite make the jetting completely happy with the KX cylinder, but it was perfect with the KDX cylinder. The only thing I could conclude was it was the difference in ignition curves.

  • KDXGarage

Posted November 15, 2010 - 04:32 AM


Keep looking on eBay. One will turn up eventually. I have bought one or two KDX250 cylinders off there in the past (and forks, carbs, several shocks, glide plates, etc., etc.).

Congratlations on getting yours going.

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