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  1. #1
    TDR's Avatar
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    GPR Big Bore Build

    Hello guys

    well i have just ripped out all the things ready for a big bore build. Only thing left it carbs to pull out will be doing that on the weekend. Also got the piston heads off tonight with my dad, one clip feel into the crank case but luckly we got it out, thank god!!

    Brought the ski couple weeks ago had 100psi right through all the cylinders so i thought needs a rebuild. But instead of being normal im gona be a RXP killer and go big bore hehe.

    Now this is going to be a slow build but will get done and cant wait

    This is going to be the setup:
    -Rebore 86.5mm (1390cc)
    -86.5 Riva pistons
    -Riva or R&D Head
    -Top end gaskets
    -Riva flame arrestors
    -VFIII Reeds
    -Prop ??/?? need help with pitch
    -Pump tunnel reinforcement
    -Free Flow
    -Jims riva FF plate
    -1200 riva grate
    -Riva sponsens
    -Riva PPK
    -Rejet carbs- need help with settings please?

    If i am missing anything could yous please fill me in because i have never done such a big build before, this will be my first.

    Here are some pics:
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  2. #2
    If it were me I would be yanking that whole engine out, T/W the crank. Pressure test the whole set up when done, looks like you have the old style pump. I would get an 05 pump for all that power as well. Who is cutting your PV's?

  3. #3
    alex29573's Avatar
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    87mm would actually be 1390cc
    You can get 87mm pistons from group k and they are forged Wiseco's.

    Also, I have a brand new head gasket for an 87mm bore I am selling in the classifieds.


    Osidebill can help you with jetting specs, and I would get the stock head milled and forget about riva/r&d head

  4. #4
    mudslanger's Avatar
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    I would pull the motor separate the cases then clearance them for the 87mm pistons. then install new seals on the crank. it would be nice to have someone port the cylinders wile you got them off but not just any shade tree porter.
    Good Luck With The Build

  5. #5
    TDR's Avatar
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    Quote Originally Posted by scittb View Post
    If it were me I would be yanking that whole engine out, T/W the crank. Pressure test the whole set up when done, looks like you have the old style pump. I would get an 05 pump for all that power as well. Who is cutting your PV's?
    That is a good idea, because i don't want to have headaches when i get everything together then wondering whats wrong and it could be the crank. Is it a OEM 05 pump? and i forgot to add im getting the R&D power valves.

    Quote Originally Posted by alex29573 View Post
    87mm would actually be 1390cc
    You can get 87mm pistons from group k and they are forged Wiseco's.

    Also, I have a brand new head gasket for an 87mm bore I am selling in the classifieds.


    Osidebill can help you with jetting specs, and I would get the stock head milled and forget about riva/r&d head
    I thought 86.5mm is 1390cc saw it in a post. Anyway im going to go 86.5 because im getting in the riva pistons. Thanks anyway. I will get a hold of Osidebill, once i get the carbs off, and does any machine shop mill the head?

    Quote Originally Posted by mudslanger View Post
    I would pull the motor separate the cases then clearance them for the 87mm pistons. then install new seals on the crank. it would be nice to have someone port the cylinders wile you got them off but not just any shade tree porter.
    Good Luck With The Build
    I will probably get the cylinders ported while there off. The place were i get them rebored will hopefully do porting. Whats the price for a port and polish? for the gpr cylinders and how much will i be looking for it to get bored out 6.5mm

  6. #6
    TDR's Avatar
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    Is it necessary to clearance the case for the bigger pistons? if so i will be pulling the motor and is there a thread on how to do it?

  7. #7
    mudslanger's Avatar
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    Quote Originally Posted by TDR View Post
    Is it necessary to clearance the case for the bigger pistons? if so i will be pulling the motor and is there a thread on how to do it?
    YES if you dont the piston skirt will hit the case on the exhaust side above the crank. there is a little ledge that will need to be ground back. if you put the new replated 1390 cylinder on the case and look down the exhaust side of the crlinder you can see the edge of the case sticking out and when the piston comes down it will hit the ledge. I ground mine back about 20 thousands from the inside of the cylinder. there should be a thread with pics on here if this doesnt help.

    As far as the porting there should be someone on here that does it. Rocketman K.H. done mine but i dont think he is doing it for the public anymore but not sure about that. if someone doesnt chime in you could start a post asking who can.

  8. #8
    TDR's Avatar
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    Im stil tossing up to go big bore or not, but im going to port the cylinders. Today i was trying to get the carbs off but they are seized on the 4 hex bolts and ive only got 1 off. Is there a easy way to get them off?

  9. #9
    Gus13letter's Avatar
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    Looking forward to your results! Lookin good!

  10. #10
    800AMSOIL4U's Avatar
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    See how much Water Woody will sell you his whole tripple pipe big bore set up. I think he had it going 86-87MPH at 4,200 feet elevation.

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