ATC185S and ATC200X Top End Swapable Parts

My testing was with a 1982 ATC185S (has blue over spray in the photos) and a 1985 ATC200X.


Stroke is the same, so the crank does not need to be changed. The bore size is the difference between them and piston of course.


The pistons will swap as well for the cylinder swap. You will have to match piston cc with cylinder though.


They appear to be exactly the same, however the 185s intake boot is angled differently, so keep the 185s intake boot on a 185s machine, and a 200x on a 200x machine. See notes about the head cover, or stick with the same model of head cover for the head. Exhaust should bolt up fine, but I didn’t confirm.

Head Cover

They are slightly different shape for the sealing surface, but it isn’t enough to give any problems sealing it up like normal.


The cams will interchange, but not sure how much of a change it would make on the engine. The 200x should be the more aggressive cam for better top end output.


The carburetor will swap, but I’m unsure if the air boxes are the same size inlet or not though (currently). Remember that jetting is different between the models, the 200x on a 185 will be too rich, and the reverse jetting for the other way.

Intake Boot

The intake boot does swap and bolt on the head fine, but they are at different angles so the air boxes will not line up. Shouldn’t be a problem if you don’t use an air box and have an air filter such as a K&N on the carburetor.


1982 185s cylinder and head on 1985 ATC200X bottom end

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1982 ATC185S head cover on ATC200X cylinder and cam



Intechange by Part Numbers

1982 ATC185S Head (PN: 12000-427-305)

  • 1980 ATC185
  • 1981-1983 ATC185S
  • 1981-1983 ATC200
  • 1982-1983 ATC200E Big Red
  • 1984 ATC200ES Big Red
  • 1984-1986 ATC200S
  • 1984-1985 ATC200M
  • 1984 TRX200
  • 1979-1983 XL185S
  • 1979 XR185
  • 1980-1982 XR200

1982 ATC185S Cylinder (PN: 12100-427-010) – Note: These all are in the list above

  • 1980 ATC185
  • 1981-1983 ATC185S
  • 1979-1983 XL185S
  • 1979 XL185

1985 ATC200X Head (PN: 12002-965-505) and Cylinder (PN: 12100-965-010)

  • 1983-1985 ATC200X

 Ending Notes

Using the ATC185S head cover would be ideal for the Auto X builds for the compression release option since the pull starter has the hardware to make it function. The ATC200X cam should be an upgrade option for an ATC185.

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