BSPT vs NPT on Kawasaki 1100
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  1. #1
    PWCToday Guru Mentzel's Avatar
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    BSPT vs NPT on Kawasaki 1100

    Just ran across an interesting leak on an 1100 I am working on. It appears an additional 3/8Ē npt cooling port was added on the exh manifold then the OEM fitting was screwed back into the new port with npt threads. Over time it leaked and eroded out the manifold threads making it no longer useable. The OEM brass fittings are BSPT not NPT like some other OEMs. The difference is a 55 degree angle on the thread vs 60 degree NPT. They are not metric and still use imperial diameters 1/8, 1/4, 1/2 etc. So a BSPT will partially thread into npt but they wonít seal properly. I see a lot of reference articles on PWCToday using npt threads on Kawasaki. Watch out for this... It destroyed this manifold.




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    Last edited by Mentzel; 09-03-2019 at 02:52 AM.

  2. #2
    Resident Guru JC-SuperJet's Avatar
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    Re: BSPT vs NPT on Kawasaki 1100

    Was this additional Orifice drilled into an existing raised Boss or just thru the Exhaust Manifold regular wall?

    If it was a hole drilled thru regular wall, which is barely 1/8 inch thick, the leak will happen regardless of what thread pitch is used, right or wrong.

    Reason being that the 1/8 thickness only allows about 3 threads of engagement, not enough for a proper water-tight fit.

    Some people build-up the area with metallic epoxy, but this a temporary solution because the epoxy will eventually break off due to thermal stress and vibration.

    Best solution is to weld an aluminum thread-o-let over the hole. Make sure the fitting will clear the Starter...

  3. #3
    PWCToday Guru Mentzel's Avatar
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    Re: BSPT vs NPT on Kawasaki 1100

    Quote Originally Posted by JC-SuperJet View Post
    Was this additional Orifice drilled into an existing raised Boss or just thru the Exhaust Manifold regular wall?

    If it was a hole drilled thru regular wall, which is barely 1/8 inch thick, the leak will happen regardless of what thread pitch is used, right or wrong.

    Reason being that the 1/8 thickness only allows about 3 threads of engagement, not enough for a proper water-tight fit.

    Some people build-up the area with metallic epoxy, but this a temporary solution because the epoxy will eventually break off due to thermal stress and vibration.

    Best solution is to weld an aluminum thread-o-let over the hole. Make sure the fitting will clear the Starter...
    Thanks. The port was built up weld then machined flat and tapped with NPT. The original tee fitting was threaded in. The weld filler wasnít perfect which made me really examine the fitting. Then I noticed the thread difference in the brand new muffler and head pipe I bought. Not sure how people are using NPT fittings on these ports with success. I guess you could tap all then go with NPT. But once you do you canít go back.

    Having problems posting pics.. not sure whatís up.


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  4. #4
    Resident Guru JC-SuperJet's Avatar
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    Last edited by JC-SuperJet; 09-03-2019 at 12:54 PM.

  5. #5
    PWCToday.com Is My Home Away From Home theVetteman3's Avatar
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    Re: BSPT vs NPT on Kawasaki 1100

    Interesting. How did you discover this on the new head pipe? You tried the two back to back and the BSPT fit better?

    I think pretty much every pipe thread on every ski I've modified I have re-tapped. To clean up the threads or adjust how far the fitting threads in / angle of orientation. I'm interested to know how well a BSPT can be re-tapped to NPT. I can't say I personally have seen any leaks on my skis. This is really good to keep in mind. Next time I have an OEM fitting to compare I'll check myself.
    1985 JS550/800 "Pretty Red": Newmiller 800, Rhaas 750 pump conv. [56.3mph gps]
    2009 1100 SJ:
    Kawi 1100 build in progress
    2009 SJ MINT:
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    1987 WJ650:
    6M6, JD Intake Grate, 17/20, cheater pipe [47.8mph gps]
    1994 XiR: 830cc Superstock, PJS side draft, 13/18 swirl [53.7mph gps]
    1995 SJ MINT:
    [TBD]



  6. #6
    PWCToday Guru Mentzel's Avatar
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    Re: BSPT vs NPT on Kawasaki 1100

    Quote Originally Posted by theVetteman3 View Post
    Interesting. How did you discover this on the new head pipe? You tried the two back to back and the BSPT fit better?

    I think pretty much every pipe thread on every ski I've modified I have re-tapped. To clean up the threads or adjust how far the fitting threads in / angle of orientation. I'm interested to know how well a BSPT can be re-tapped to NPT. I can't say I personally have seen any leaks on my skis. This is really good to keep in mind. Next time I have an OEM fitting to compare I'll check myself.
    I first noticed this on a brand new head pipe where brand new NPT fittings would barely (one thread) screw in. The I started looking at my manifold closely and noticed a very slight difference in the taper angle. The BSPT fittings (OEM Kawasaki) are flatter (more straight) and the threads are less aggressive with a 55 degree angle. But they have same pitch as NPT. So the BSPT screw in farther. If you tap the holes with NPT threads then use NPT fittings, I think you are safe as the tap will change the angle and make the grooves deeper.

    In my case someone had tapped the new manifold port with NPT then put the OEM fitting back into the port. So it snugged up but did not seal properly and eroded out the aluminum over time.


  7. #7
    I dream skis bird's Avatar
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    Re: BSPT vs NPT on Kawasaki 1100

    Quote Originally Posted by Mentzel View Post
    I first noticed this on a brand new head pipe where brand new NPT fittings would barely (one thread) screw in. The I started looking at my manifold closely and noticed a very slight difference in the taper angle. The BSPT fittings (OEM Kawasaki) are flatter (more straight) and the threads are less aggressive with a 55 degree angle. But they have same pitch as NPT. So the BSPT screw in farther. If you tap the holes with NPT threads then use NPT fittings, I think you are safe as the tap will change the angle and make the grooves deeper.

    In my case someone had tapped the new manifold port with NPT then put the OEM fitting back into the port. So it snugged up but did not seal properly and eroded out the aluminum over time.

    I'm no professional welder, but that is poopty! Reminds me of my first solder joints, age 9. The more the better!

    They should have used bar stock, pre-drilled and cut at the proper angle. The mountain of porous filler rod was bound to fail. Especially on a cast piece like that.
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  8. #8
    PWCToday Guru Mentzel's Avatar
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    Re: BSPT vs NPT on Kawasaki 1100

    Agreed on poor welding and using a weld bung. Using the wrong threads didnít help.

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