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Thread: Taking two and making one ATC250 ES and ATC250SX

  1. #1
    Join Date
    Nov 2019
    Southwest Nebraska

    Taking two and making one ATC250 ES and ATC250SX

    I recently acquired two free three wheelers, a Honda ATC250ES (Big Red) and a very rough ATC250SX. After getting them home and doing a more thorough inspection, I discovered Big Red had some serious damage to the right side case of the engine. So much so that the end of the crankshaft was broken off. The SX had no plastics other than the seat pan and someone had nearly destroyed the wiring harness for reasons unknown to me.

    First order of business was getting the engine in the SX to run. I cleaned the carburetor and got enough of the wiring fixed to get it running, and it sounded pretty good. The charging system worked and it ran through all the gears while I had the machine up on jackstands. This bit of initial success started my plan of putting the SX powerplant in the ES chassis.

    Having stumbled onto this site, I read enough to figure out that the engines were identical, so one afternoon the broken ES engine was removed and the running SX engine replaced it. After a few shakedown runs in the alley behind my house, I loaded it up and rode it for a couple hours. The longer it ran the better it ran. My only problem was 5th gear was broken, but all the rest of the gears were fine.

    Big Red is really in pretty good shape. It is missing the two plastic side pieces and some of the front fender. The wiring harness looks to be untouched and all the lights work as they should. I have nearly completed a snow blade for Big Red so I can use it for something. I remember reading that the rear axle ratios were different between the ES and the SX, but I didn't realize until later that the transmissions were also geared differently as well.

    While there is not a huge difference numerically between the two, I am still considering moving the gears from the broken ES case to the intact SX case. I would definitely tear down the ES transmission first to make sure all the gears are good in it before tearing into the SX case. Before I go to this trouble, I want to make sure the transmission part of the powertrain cases are the same. Can anyone enlighten me in this regard?

  2. #2
    newby200x's Avatar
    newby200x is offline At The Back Of The Pack Arm chair racerAt the back of the pack
    Join Date
    Sep 2012
    I'm not 100% certain that the transmissions are identical, but I believe they are. A look at Partzilla and confirming part #'s would be my first step. I have some cases left over from a 1985 250SX which you might be able to use for your Big Red if you are interested.

  3. #3
    Join Date
    Apr 2019
    South Dakota
    Sounds like my current situation. have 3 BRs. 2 85s and an 87, trying to make one ride-able machine. I believe the actual transmission gearing is the same, but the final drive at the output shaft and the differential are different.
    Currently owned:
    1987 250es - Hooptie Deluxe
    1987 250es - sold, wish I still had it...
    1987 250es sold
    1985 250es (parts)- sold!
    1985 250es (parts) sold!
    1984 200es- named Ruby Red
    1983 200e (parts)
    1980 CT70
    1995 Foreman 400
    1980 ytm 125- parts
    1971 AT1 125- Sold!
    196? Suzuki B100p- Sold!
    1986 lt125 (rod knockin on heavens door)
    1976 KZ400D
    1979 Kawasaki G5 100

  4. #4
    Join Date
    Apr 2012
    MI, USA
    I grabbed the specs from the service manuals, the transmissions are not the same gearing, the actual gear sets might be able to be swapped as a whole unit, but besides primary reduction (crank to clutch basket) nothing is the same. Taking into account the highest gear 0.906 vs 0.848 is about 7% higher gearing in the transmission. The final reduction (rear diff) says it's 5.684 vs 4.969 which is another about 14% increased gearing. The stock tire size is 25in vs 22in, which is about 13.6% difference so overall the 250sx is a little higher geared by roughly 7% for travel distance vs engine rpm. I've never ran the numbers before, so this is new for me. If I messed up anywhere in the math, let me know I'm just using the raw numbers in the service manual so maybe I'm overlooking another detail. In theory with all the right numbers + math, you can estimate engine rpm vs speed (mph) for a given transmission gear/tire size. I think it would be a neat utility to program on my site, maybe I'll do that some day and pre-populate the 3 wheeler models and such.



  5. #5
    Join Date
    Apr 2012
    MI, USA
    To follow up, I made a quick and "dirty" page for this feature. It's not the most user friendly yet, but the numbers seem to make sense. I picked 5000 rpm for the screen shots but you can slide to whatever you think is a logical number (500-10,000rpm). Let me know if the math is wrong, java is a bit of a pain to work with decimals. In theory you can calculate max speed with this too, you just have to estimate the max rpm the engine can do in a given gear. Also can change it so you can slide every 100rpm, figured 500rpm would be a pretty solid step value.




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