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  1. #1
    Dave in DE's Avatar
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    milling head to bump up compression

    I have a basically stock 02 GP800r I'm looking to boost compression slightly nothing radical. I have a good spare head and was going to mill about 8/10 thousands off of it. my concern is the squish rate will change but will it change enough to be a bother or cause issues. I'm sure someone else has done this. what was your out come?
    Any comments or concerns you see?
    Last edited by Dave in DE; 03-12-2017 at 02:56 PM.

  2. #2

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    .008 to .010 might get you 6 psi. .017 on my 1200 gave me 10 psi, with no problems

  3. #3
    Dave in DE's Avatar
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    Crazypirate, thanks, figured someone had done this. based on your example I guess I'll start at taking 15 off and go from there.

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    Dave, you need to fit the spare head and measure your squish on each cylinder first.

    Get some thick solder wire and bend into an L shape, hold it down the plug holes so it touches the cylinder wall and turn the engine over by hand. Measure it where the piston has squashed it or get the head skimming place to measure.

    If you don't do this then it's impossible to say how much you can remove from your head, it could have already been cut by a previous owner. To tight squish will give you detonation and a blown motor


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    Dave in DE's Avatar
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    Raider, good point. Never thought of solder idea. Have plenty of it and will check it that way before going forward. Thanks

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    Raider has a valid point, or you can just take the original head off with known original PSI (115 to 120?) Remove one layer of the head gasket, like a lot of the 1200 boys do, (but 1200 are three layers,) some run one, some run two, some all three. I just measured a stock 800 gasket, each layer is .013. As long as there is a lip left on the head, you are going to be fine 99% of the time. The original question was .006 to .010.

  7. #7
    Dave in DE's Avatar
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    Yes the head gasket is a 2 layer steel gasket. I could remove one layer. the spare head I have looks to be uncut. it still has a pretty deep lip at the edge of the cyl. that's not to say that a couple thousand hasn't been pealed off at one time.

    the current head/motor combination has 145 psi on both cyl. I've always been told it should be around 120? I think. I'm doing compression testing with a snap-on gauge and I don't question the gauge. I also know the motor is untouched. I bought it from the original owner and it only has 32 hrs. he says he's only winterized and changed plugs. this thing is a pristine piece of history. found it on a very sm lake in NY. about 3/4 mile in diameter.

    I'm going to start in morning and i'll solder test the original head and then change to the test head to verify before cutting head down.I need to pull top end off to clean out power valves and install wave eater clips, and R&D billet power valves.

    do either of you know for sure what the 800 compression should be?

  8. #8

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    145psi is perfect for a rec ski...there is no reason to add anymore, because you won't even notice it...There is 8 mph (always has been, always will be) difference between my 800 and 1200 with its 30 hp advantage...They both have increased about 9mph, sofar (what is tested on one, has same results on the other,..same hull xl vs xlt)...but that all came from playing with the bottom of the ski, not the motor...love working in the engine compartment of the 800, so much room compared to the 1200, except for PVs. There the 1200 has the advantage, because you don't have to pull the exhaust manifold to clean PV's..kev
    Last edited by Crazypirate; 03-27-2017 at 08:01 AM.

  9. #9

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    As far as stock 800 original PSI...I always heard around 115/120...Which is what mine had when I first got it...BUT that was with a SBT motor, hours unknown, a lent out pre-mixed ski filled by a kid with no oil to the gas, missing aft PV...After replacing PTO and Mag bearings, WSM Platinum top end kit, PV, re-installing oil injection , and an ebay cylinder head...145/150.
    Hey it was free, because it was a premix ski�� Same way I got 1200 for free, but it took a new 1300 crank, then the oil injection system was reinstalled and clamped. Just because one has the abilities to change things, it is not allways towards the better.

  10. #10
    Dave in DE's Avatar
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    Solder test shows me 77 thousands at squish ring. This is ski number 2. Also have an 01 gp800r w/about 115hrs. the 01 I bought w/blown motor. There was at least a quart of oil in bottom of the hull from a broken line. Rebuilt motor and some pump/plate work and I was getting 58/59 mph on gps. Would like to squeeze a bit more out of this one if I can.

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