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  1. #1

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    Difference between 12F and 15F motor???

    I'm trying to learn the differences between the two motors, to figure out what parts are interchangeable. I've never had a 12F motor, but I'm re-building my second 15F. Knowing what parts can be interchanged would expand the selection of used parts.

    The two motors use the same crankcase, but a different crankshaft. The journals on the 12F crankshaft are smaller, which makes for the shorter stroke. I found this out when I bought a 12F crankshaft on ebay. I sold it at a nice profit, so it was not an expensive lesson.

    The head is the same... different cams.

    The pistons are completely different.

    From what I can gather, the cylinders are the same, except that the 15F cylinders are honed differently to account for the longer stroke. Can anybody verify this? Since the same head and crankcase are used, obviously, the cooling passages and bolt holes must line up. The cylinder depth on the 15F is about 4-5/16" and the measurement between the gasket surfaces (head to crankcase) is 3-7/16". Does anybody have a loose 12F cylinder that they can measure and post the measurements?

    Has anybody ever honed a 12F cylinder and used it on a 15F?

    If the cylinders are the same, except for the honed area, then the con-rods (or the pistons) on the 12F must be longer. The 15F con-rods are 6-1/8" overall and 4-1/8" from where it meets the crankshaft to the top (above the wrist pin). Anybody have a loose 12F con-rod to check?

    The intake manifolds are totally different. Both motors used the same fuel injectors, and probably most of the same sensors.

    I didn't check every single part number, but the exhaust seems to be the same, from the exhaust manifold all the way back to the hull outlet.


  2. #2
    My name is Sean and I am addicted to STXs smokeysevin's Avatar
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    the cylinder do not interchange, the waterjacket on the 12f cylinders is much shorter and they are not finish honed as far down the cylinder walls, I want to say the con rods are the same length as a zx12r but thats just an educated guess based on motor geometry. The manifolds fit but do not interchange. You are correct about the exhaust. The fuel system is the same, the 12f adds less fuel in the ecu.

    its less than you would expect

    Sean

  3. #3

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    Quote Originally Posted by smokeysevin View Post
    the cylinder do not interchange, the waterjacket on the 12f cylinders is much shorter and they are not finish honed as far down the cylinder walls, I want to say the con rods are the same length as a zx12r but thats just an educated guess based on motor geometry. The manifolds fit but do not interchange. You are correct about the exhaust. The fuel system is the same, the 12f adds less fuel in the ecu.

    its less than you would expect

    Sean
    OK... shorter cylinders. That's one way to shorten the stroke.

    Would you happen to know the dimensions of the 12F cylinders (depth of cylinder and depth of water jacket)?

  4. #4
    My name is Sean and I am addicted to STXs smokeysevin's Avatar
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    the cylinders are the same dimensions but the waterjacket on the 15f does all the way down the cylinder but on the 12 it ends about half way or so.

    Sean

  5. #5
    cerebral's Avatar
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    I have a 03' 12F and 2009 15F....

    I have a 03' 12F and 09' 15F on my trialer, I also have KAWAI service manuals for both, where do you want to start

    I am in the process of taking the emissions crap off my 12F this season finally, I always thought it was useless but the dealers warned of dire results, BAH!

    I do agree the 15F engine is taller and most of the top of the engine is different, the bottom end seems to be the same.

    Cerebral

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