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  1. #1

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    Why Do Gpr Tunnels Crack?

    I have read and seen alot of post on this subject over the last year or so and now that i have experienced striping the entire pump area out of a gpr i cant really figure out why it cracks where it cracks at.
    there dont seem to be any stress in that area from the engine or drive line,,the only thing that bolts to that area is the pump shoe,,so this tells me that there is a lot of flexing at the pump housing,,i did notice when working on it that there is no pump support at all except the 4 bolts that hold it to the transom plate!!!!!!!!
    unlike kawasaki`s which bolt to the transom plate and also bolts that go through the ride plate, then bolt to the pump,,i bet if the pump area on these gpr`s was bolted to the ride plate, there would be no more pump shoe issues with cracking.so my thinking is the pump tunnels are not really weak its just that there at the middle of the flexing point,,,,if you would hold a long stick out at each end and bend it where does it usually break?? THE MIDDLE that just happens to be where the pump tunnel is at.the pump is where the real support is needed.
    Last edited by jeff007; 02-01-2007 at 08:18 PM.


  2. #2
    Drink My Dust's Avatar
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    When you start modding your GPR..........you usually end up pushing more water through the pump tunnel. More water through the pump tunnel = more pressure in the pump tunnel. Result..............cracking pump tunnels.

    You will find here repeated attempts to NOT re-enforce this area on modded GPR's only to find them later dealing with cracks.

    Spend the little extra $$ and support the tunnel with the tunnel re-inforcement kits..........you will never have to worry again.

  3. #3
    Chrisrokc's Avatar
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    Quote Originally Posted by Drink My Dust View Post
    When you start modding your GPR..........you usually end up pushing more water through the pump tunnel. More water through the pump tunnel = more pressure in the pump tunnel. Result..............cracking pump tunnels.

    You will find here repeated attempts to NOT re-enforce this area on modded GPR's only to find them later dealing with cracks.

    Spend the little extra $$ and support the tunnel with the tunnel re-inforcement kits..........you will never have to worry again.
    Yep! He is right. I am thinking about reinforcing mine soon but I think I am going to let it go as is till next winter. When you get over 70mph you need to start thinking about it soon.

    Also something I have learned on here is that the 1200 grates put more stress on the pump tunnel than the 800 grates. The 1200 pushes water more into the pump tunnel wall, where as the 800 grate pushes it more so into the pump area.

  4. #4
    Happily Self-Employed WFO's Avatar
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    Quote Originally Posted by Chrisrokc View Post
    Also something I have learned on here is that the 1200 grates put more stress on the pump tunnel than the 800 grates. The 1200 pushes water more into the pump tunnel wall, where as the 800 grate pushes it more so into the pump area.


    I just recently reinforced my tunnel after 77 hard hours. Been through several top and bottom ends, and a couple of pumps.

    I mainly ride with the 800 grate.

    On any customers boat that I modify, I strongly insist on reinforcing the tunnel area.

  5. #5

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    Use a pointer lazer or a straight edge on the rake and the scoop of the 1200 and 800 grates while they are mounted onto the hull. Line the lazer to match the same angle of the scoop and grate. You can see where the water hits the tunnel wall (on the 1200) and the impeller (on the 800).

  6. #6
    Moderator RX951's Avatar
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    Quote Originally Posted by salty View Post
    Use a pointer lazer or a straight edge on the rake and the scoop of the 1200 and 800 grates while they are mounted onto the hull. Line the lazer to match the same angle of the scoop and grate. You can see where the water hits the tunnel wall (on the 1200) and the impeller (on the 800).
    yup, Salty showed me that in my garage in person ! That 1200 grate is hard on the tunnel.

  7. #7
    Mr Bill's Avatar
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    Interesting! For those that had their tunnel crack... were you runing a 800 or a 1200 grate?

  8. #8
    mtrager's Avatar
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    Quote Originally Posted by Mr Bill View Post
    Interesting! For those that had their tunnel crack... were you runing a 800 or a 1200 grate?
    Single , 1200 grate.

    Triple, 1200 grate. Twice.

  9. #9
    Mr Bill's Avatar
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    Anyone else?

  10. #10
    YudLuz2's Avatar
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    The 1200 grate will put 10 times the pressure on the pump tunnel
    compared to the 800 grate. Having the pump shoe sealed completely
    will also help the shoe stay in the hull. The silcone will help support
    the shoe in place, just not the 4 bolts. A lot of people just run
    the pump seal kit without doing the full seal job, this will also
    put more pressure on the pump tunnel. Its much better on your hull
    to run the 800 grate, but it also takes more toll on your pump. You
    will have a increase of wear in this area due to increase prop spin.
    I would much rather replace a pump stator than having to deal with
    a hull issue.

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