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How do you know if another motorbike engine fits your motorbike?

I have a 1980 Yamaha RX125 with a non functioning motor. If i am going to get another motor, does it have to be a RX125 engine or can the others fit as well? How do i know if the other engines fit? Which engine would fit, affordable ( This is my school project), and easy to get? I can show the photos of my motorbike, just sent me an email. Cheers
I cannot afford to repair the current engien as the workshop say it will cost around 500 bucks.

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6 Responses to “How do you know if another motorbike engine fits your motorbike?”

  1. racefanwfo said :

    it has to be the same motor.
    why don’t you just rebuild the motor that is in the bike.

  2. 4wheeler nut said :

    You can put anything in the hole if it fits or you fab it to fit. I have seen 550 to 1000 cc motors put in 4wheelers. I would go to the motorcycle junk yard and fid one that will fit. Make sure that it has the same side drive sproket. You will have to fab up the engine mounts. Try the yellow pages for the Yard. They might have the 125 motor that you are looking for. I would look for a YZ125 motor and put it together it will take some work If your not scared easyly then take it by the horns and make what you want.

  3. ShuggieMac said :

    I am afriad the first responder is wrong – it does not have to be the same motor at all. It purely depends just how much work you are willing to do to get another engine to fit.

    The RX125 uses one of Yamaha’s long running series of 125 engines so the first thing to do is a little research and find out what other models used that motor. If that fails then the next logical step would be to look at other Yamaha 125 models of a similar vintage but perhaps with different engines and do some measurements to see what will physically fit inside the frame. The mounting points may well be different so that is where you will need to get creative in the workshop and start making up mounting plates and so on. The reality is that if you are willing to put the effort in then you could squeeze many motors into that bike of even bigger engine capacity, though that is not reccomended as the frame and brakes of the RX125 would struggle to handle much more in the way of power.

    The fact of the matter is that if you can source an RX motor then of course it would be easiest but in any case it would appear that you are working on a tight budget and 500 bucks is not a lot of cash, so if that is a problem then getting a second hand motor will not guarantee you that it will be any better. So what is the problem with the engine that you have got already? If it is a school project then I assume you have access to a work shop, so why not fix it yourself? Email me if you want to describe what are the issues and I’ll be happy to try and help you – from here on the other side of the world.

  4. Philip P said :

    The RX 125 is a single cylinder 2 stroke egine, it was used in the DT 125 in a different level of tune and on a number of other japanese 125’s ( not just by Yamaha)
    The Yb series of engines is similar but will be difficult to fit to your existing frame.
    the later watercooled engine could be considered but the addition of brackets for the radiator nd cooling system might prohibit it’s use.
    to Be Honest I’d seriously consider rebuilding the engine that you have.
    If it is just seized or ran out of lub then replacing every bearing seal and having the engine rebored is going to be a lot cheaper and easier than trying to rig another engine to fit.
    At the same time you could “BLUEPRINT” the engine to exact specs and improve it over even yamaha’s tollerances, thus you would learn the art of tuning and performance enhancements..

  5. warren_ddd said :

    a dt 125 should fit with no problems and with very little work you could shoe horn a dt 250 lump in too.

  6. motorrobin said :

    Erm you look at our engine and look at Yamaha range. On a simple swap you could use a RXS100 RS125.
    to me the things you have to look for are
    1.. Engine will bolt in the frame (width where it bolts into frame, hieght of engine) without having to chop the frame. Making some adaptor brackets or stand-offs/ washers is fine.
    2. tank will drop on with out probs
    3. output sprocket will align with the rear wheel sprocket
    4. engines generator and ignition system is the same as original (unless you wana rewire the bike!)
    5. you can mount the carb without it conflicting with anything.


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