DT / MX Motor Swap Fail

Last year I met an older guy at Ride 2 Skate. Ed and I had rode our dirt bikes down to the show and this guy was pumped on them. He said he used to build and race flat track with the DT’s back in the day. He explained the hot setup was to use the earlier model DT’s with dual rear shocks and swap in a motor from a later model mono shock DT / MX and that it was a direct swap.

The benefits are:
– Lighter engine
– 6 speeds vs 5 speeds
– Overall more solid engine
– Parts availability are better (Wiseco makes piston kits for $100)

Well this got me pumped so at some point last year I picked up this 1981 MX 175 cheap with a solid motor, no title, and a bit rough around the edges. It was perfect candidate for the motor swap. However, after a few hours of disassembly and attempted reassembly the motor doesn’t swap over clean. Motor mounts kinda line up but more importantly the chain line is about an 1/8″ off so basically it would need all new mounts to correct everything. So the swap is possible but currently more work than I’m looking to invest in it. I had to try it for myself and this post is for anyone (probably few but..) who has thought about doing this.

DtMx Conversion-4
DtMx Conversion-3
DtMx Conversion-5
DtMx Conversion-2

MX motor piston is still standard bore which is good.
DtMx Conversion-6

MX cylinder head
DtMx Conversion-7

Motors side by side. DT175 left and MX175 right
DtMx Conversion-8

MX motor in the DT with just the bottom right bolt engine bolt.
DtMx Conversion-9

DT motor in the DT
DtMx Conversion-10

DT will stay stockish for now. I have an early model MX175 exhaust that I’m going to try and fit on. Supposedly a power upgrade but we’ll see.
DtMx Conversion-11

7 responses to “DT / MX Motor Swap Fail”

  1. Frank says:

    Which way is the sproket off? You might be able to weld up an offset sprocket for only an 1/8″.
    Looks like it would be a great upgrade though.

  2. Adam says:

    Hey Frank, the sprocket needs to sit closer to the engine to take up the 1/8″. If there was clearance and the chain doesn’t hit the motor, how would you go about making an offset sprocket?

  3. Frank says:

    It’s a little bit involved, but if you have access to a lathe ( I didn’t see one in your shop pics) it can be fairly simple.
    You find two sprockets, one with the gearing and chain size you plan on running, the other is to match the splin on the MX engine.
    Then, carefully anneal the centers of both so they can be cut in the lathe. Some aftermarket sprockets may not be quite as hard as the factory ones so you may be able to cut them without softening.

    Cut the center out of the outside sprocket (#1) you are using. Then outer off the spline portion of the other (#2). They should be cut to clearance of slip fit, so you can ajust for runout. Slip #1 over #2 with the amount of offset you require.
    Set up on a flat surface, check runout, weld. Tacking first.
    Making sure you have clearance with the case is good to check before welding.
    The welding process should generate enough heat to get some “hardening” back if you quench it in water immediately after welding.

    Drag racers in the 70’s would do this for running wider slicks.

  4. Carson says:

    If you ever think of going the route of a custom pipe. I know a dude down here that builds all flat track bikes and motors in our area, dude really knows his shit, he builds custom 2-stroke pipes for racing and shit. He’ll make you an awesome pipe.

    have you put the boysen dual stage reeds in that thing yet? I’ve got em in my DT400 and you can definitely tell the difference on the bottom end in the power band.

  5. ken says:

    That looks like a neat swap,everything is close enough to make work. A direct fit would be a 75 mx 175 if you can find one, way more power due to its head, porting, and pipe which will also fit the dt chassis. No sixth gear though,that was the year before the first monoshock yz. Raced one of those for a season, pretty good machine.

  6. Amigo a caixa de macha da mx 175 se encaixa na RT180?
    caixa da RT180 MUITO RUIM

  7. m latham says:

    Been there dun it dt175 2k4 fits into 1g1 frame or 1f9 frame easy. My bike was UAB 6R. engine chain cover needs to be modified. rear wheel and sprocket needs to be changed to mx wheel or make spacers for new sprocket.

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