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  1. #1

    Join Date
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    787 Cyl. Base Gasket Choices

    Hey All,
    Will I see any performance difference between using different thickness base gaskets? I can use any thickness I want because I have not had the head modified yet. I know that raising ports (generally) increases rpm and lowers lowend torque, but, there is only .6mm difference between the thinest (.2mm) to the thickest (.8mm). Is that really enough to make a difference?
    Thanks for your advice!
    Robert V


  2. #2

    Join Date
    Jul 2005
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    Ohio
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    If the engine is still together, you need to take a measurement of the squish, that will tell you if you can drop down to a thinner gasket.

    What are you trying to acheive?

    Is this a rebuild

    What ski, your 95xp?

    TLR

  3. #3
    96XPSS's Avatar
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    Warner Robins, GA Home of the 2007 All-Star Little League World Champions!
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    Hey Robert! The 5 hole was optimum, but is no longer available. I would go with a 6 hole base gasket.

  4. #4

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    I would try cometic gaskets company for a 5 holer. or post here, I bet someone has a few laying around?

    TLR

  5. #5

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    Jul 2008
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    Southeast Texas
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    Hey Tim & Robert!
    Good to hear from you guys, I was hoping you 2 might jump in on this.
    Actually this is a stock motor that I picked up with the top-end disassembled. I don't know what the original base gasket was. It was missing. I have a stock head (did not come with this engine) that I plan to have PPG cut for more compression/tighter squish, but, I need to decide what base gasket would be optimum. They (PPG) needs to know what base gasket that I plan to use before they modify the head. It sounds like you guys recommend a .6mm (6 hole). That seems to be the most common since the .5mm are no longer available from BRP. I know the last several Rotax remanufactured 787s that I bought came with 6 hole base gaskets. I just didn't know if there was really any performance advantage by going thinner or thicker on the base gasket. It only raises or lowers the port height a tiny bit anyway.
    Tim,
    Do you plan to come to Texas to ride this year? If you do, could you please bring us some water. The highland lakes (Hill Country) are having the worst drought ever. Just about all the ramps are closed on Travis and Canyon.
    Thanks Again guys,
    Robert V

  6. #6

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    Location
    Ohio
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    Been already to DFW area in July & August. They have tons of rain there, prob the best summer rain in a few years, he he.

    Cometic Gasket, Inc.
    8090 Auburn Road
    Concord, OH 44077
    (800) 752-9850Call Toll Free(440) 354-0777Order / Tech Line(440) 354-0350Fax

    I've seen a ton of 800's with 5 holes as taking apart and when replaced with the 6 it does drop the performance a bit. I had that happen on our 97xp.


    TLR

  7. #7

    Join Date
    Jul 2009
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    Fort Worth, Texas
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    I am currently rebuilding my top end on my 785. It came with a 4 hole from the factory. If you know what gasket it left the factory with, Shouldn't you replace it with the same thickness? I would also like to know if you install the gasket dry or use some type of sealant on it.

  8. #8
    96XPSS's Avatar
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    Yes, replace with the 4 hole as it is available. BRP builds them dry, but I use a thin coat of Three Bond 1211 on both sides. It seals better, and makes removal and cleanup much easier the next time it gets disassembled.

  9. #9

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    Jul 2009
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    96,
    You are a great source of info. I ran into a problem yesterday putting my top end together on my 97 XP. I used some Hondabond HT because my 1211 had gotten old. While trying to hook up the two cylinder drain lines on the back side of the jugs, I tore the base gasket. Oh well, pulled the jugs back off and what I nightmare trying to clean the HT $hit off the cases. I wish that I had put the gasket on dry because I don't think the gasket would have torn if it had not been glued down. So my 4 hole got torn, but my kit has a three and a six..... Group K did my machine work and I told them that I use a four hole, so I would think porting and squish settings would like to see a 4 hole used again, what do you think?

    I have built many engines, but this baby is really getting on my nerves. Heck, I thought those Rotax circlips on the wrist pins were the hardest thing that I have ever come across!

    The motor is in the boat and I left the exhaust in tack. I am going to move the drain line for the cylinders over to under the carbs by using a longer pigtail coming off the jugs so that I dont have to screw with them while putting the cylinders on. Any suggestions appreciated on the top end rebuild.

  10. #10
    96XPSS's Avatar
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    On the cylinder drains, never use a zip tie on the barbs that attach to the brass elbows on the cylinders. As they connect straight into eachother, they can never blow off. I've never had ties on mine, and have run it many years without a leak.

    When I assemble, I install the front cylinder first. Then install the drain tee on that barb. Then, after you get the rear piston in the cylinder sleeve, while sliding it down, bend up the rubber drain at an angle, reaching around the back with your left hand, to "catch" the barb on the rear cylinder as you drop it down. Slightly twist the cylinder too, to aid in getting it on the barb. While dropping it, the base gasket should be sitting on the case. Be real careful that last inch to make sure the skirt of the sleeve doesn't cut or bind against the gasket, and slips past it.

    Yes, it is a trick to keep from breaking the gasket. Also try to use the sealer as thin as possible, this makes it tacky quicker, and less likely to slip around the case. I actually apply the thin coat to the bottom side of the gasket, then put in place, then apply the second light coat on it while in place. This serves two purposes, it allows the bottom to be pressed in place by your fingers applying the top coat, and the time it takes to do the top, allows the bottom to get tacky enough so that it stays in place when through.

    I'd use the 3 hole, just check your squish and make sure it is not less than .038" when done. If you have any questions, let me know.

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