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  1. #1
    PWCToday Regular
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    Jul 2007
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    New York
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    85

    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 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 throttle bodies 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.
    Last edited by grumpy_steven; 07-28-2010 at 10:18 PM.

  2. #2
    Top Dog kawiski's Avatar
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    Jul 2004
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    So Cal L.A.
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    1,241

    Re: Difference between 12F and 15F motor???

    You seem to know the BEST about the subject what you are asking around here. Since the two cylinders have two different P/N and Kawi does not mix them using between 12 and 15, maybe they have a good reason why they don't do that.
    Last edited by kawiski; 07-30-2010 at 04:02 PM.

  3. #3
    PWCToday Guru
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    Sep 2007
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    Shuswap Lake, Canada
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    Re: Difference between 12F and 15F motor???

    Good thread. I have one of each: 05 12f and a 2010 15f. Both are running extremely well, but I am interested in interchangability.
    Thanks

  4. #4
    PWCToday Regular
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    Re: Difference between 12F and 15F motor???

    Quote Originally Posted by kawiski View Post
    You seem to know the BEST about the subject what you are asking around here. Since the two cylinders have two different P/N and Kawi does not mix them using between 12 and 15, maybe they have a good reason why they don't do that.

    I don't know everything about these motors, and I'm sure there are plenty of people who know more than me... which is why I posted. I thought maybe other members would share what they know.

    As for the cylinders, I was hoping someone would post the info I requested, so that I could understand the difference. I was hoping to find out if the shorter stroke means the cylinders are shorter or if they are the same size, but the pistons use less of the cylinder. Just because they have different part numbers doesn't mean that they can't be adapted for use on the other motor.

    The point of the post is to share what I know and to learn something new... which is what I thought this site was about. I'm sure other members will find this information to be worthwhile.
    Last edited by grumpy_steven; 07-30-2010 at 09:17 PM.

  5. #5
    PWCToday Guru
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    Re: Difference between 12F and 15F motor???

    [QUOTE= I'm sure other members will find this information to be worthwhile.[/QUOTE]

    I'll find it interesting!

  6. #6
    PWCToday Regular
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    Nov 2008
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    Cecil Co. Md
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    Re: Difference between 12F and 15F motor???

    I would venture to say the blocks may be the same with the same bore size. The bigger motor would need a different crank for the stroke. Rods can be the same length, so all they would need to do is change the compression height of the piston to make it taller and there is your stroked motor. This goes for any motor. Now if Kawasaki did it this way or not, I am not sure. You can also have a longer rod and a shorter lighter piston in a stroked motor as well, taking weight away. Just figured I would explain the different ways to do pistons and rods for a stroked motor to help clear that up some for you. I come from the car world and this is how I setup race motors that get some added stroke to them.
    Scott
    old ski- 08 stx15f
    new ski- 2012 seadoo gtxs 155

  7. #7
    PWCToday Regular
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    Re: Difference between 12F and 15F motor???

    Quote Originally Posted by 08stx15f View Post
    I would venture to say the blocks may be the same with the same bore size. The bigger motor would need a different crank for the stroke. Rods can be the same length, so all they would need to do is change the compression height of the piston to make it taller and there is your stroked motor. This goes for any motor. Now if Kawasaki did it this way or not, I am not sure. You can also have a longer rod and a shorter lighter piston in a stroked motor as well, taking weight away. Just figured I would explain the different ways to do pistons and rods for a stroked motor to help clear that up some for you. I come from the car world and this is how I setup race motors that get some added stroke to them.
    Thanks. I do understand the various ways to change the displacement on a motor. I was looking to get some definite answers on how Kawasaki did it with this motor.

    The bore is definitely the same on both motors, verified by the fact that they use the exact same ring sets.

    The crankshaft is definitely different... I had both parts side-by-side and could see that the journals are bigger on the 15F, which accounts for the longer stroke.

    If you put the 12F crankshaft into a 15F motor, obviously the pistons won't reach the tops of the cylinders. For the piston to reach the top, you would have to do one of several things... shorten the cylinder, lengthen the con-rod, or increase the height of the piston. I'm still waiting for someone to measure the cylinders and the con-rods, then post the measurements here for comparison with the measurements I posted for the 15F.

  8. #8
    PWCToday Newbie
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    Re: Difference between 12F and 15F motor???

    Quote Originally Posted by grumpy_steven View Post
    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 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 throttle bodies 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.

    The 15f rods are SHORTER! With a longer stoke you need a shorter rod other wise the piston would rise above the cylinders. I am doing a conversion now I'll post all the pics with the differences. I am putting the 15F motor in a stand up ski so I am going to try and use the Motorcycle ZX12 cams to get a little more RPM out of the motor. I will keep you guys posted.

  9. #9
    PWCToday Newbie
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    Aug 2017
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    Southern NY
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    13

    Re: Difference between 12F and 15F motor???

    Glad I found this post. Just bought a 2007 STX 12f with a bad motor. The motor is disassembled and in boxes. Just checked the motor SN and it's a JTT50AE0 15xxx. That be a 1500 cc. WooHoo...Has the 1500 intake and an aftermarket ECU that's been setup in the ski. Crankcase spun # 3 main bearing due to pump bearing failing and pushing back on crankshaft (crankshaft wiped, too). Rest of motor (head, valves, pistons and rods) looks in great shape.

    Now need to find a 15f crankcase and crankshaft.
    Last edited by 04fxdwgi25; 08-06-2017 at 07:47 AM.

  10. #10
    PWCToday Newbie
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    Jul 2017
    Location
    VA
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    9

    Re: Difference between 12F and 15F motor???

    I have my 12F motor apart right now, what measurements are you looking for on the cylinder? Looks like an 85mm bore sure would have unshrouded those valves better. Also seems that the jug is plenty thick to do it, only drawback being the expense. It is a shame that no one put more r&d into these motors back in the day.

    One thing of note is that my crankcase is notched, certainly not needed for the 12F so the cases must also be used in an application with a MUCH more severe rod angle. The 15F stroke is almost 15mm more but unless the rod's big end is huge on the 15 I couldn't even see where notching would be necessary with that. Even the 15's 69.2mm stroke is very tame for a motor that only turns 7K.

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