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  1. #1
    Canadian Beaver Inspector jkindt's Avatar
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    Piston deck height

    I have spent the last half hour searching and searching, I even checked the Polaris manual and I still cannot find out what the factory piston deck height is supposed to be on an SL750. I understand that this can be modified by using different base gaskets, but what is the factory height supposed to be? Also, what effect would using a thicker base gasket have on performance (due to raised port heights) assuming a proper head gasket was used to keep squish clearance within spec?


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    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    What do you mean piston deck height? CrazyA has an article listed in the tech section for checking some sort of deck height, not sure if that's what you want......

    Changing port heights will either improve bottom or top end depending on which way you move them. Starflight has an article in the tech section about porting. Have you checked that out?

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    Canadian Beaver Inspector jkindt's Avatar
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    Quote Originally Posted by xlint89 View Post
    What do you mean piston deck height?
    At TDC is the edge of the piston flush with the edge of the cylinder or is it above or below the cylinder? Different base gaskets will move the cylinder up or down in relation to piston TDC (also the ports in relation to the piston as well).

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    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    Pretty sure the piston edges are just below the top of the cyl.

    If I remember correctly, I had to special order 40 thou head gaskets to get a 60 thou squish. ALTHOUGH, I believe my case was milled at one time, which would reduce the squish and alter port timing.

    I can't tell you what spec is for sure.

    And if I'm not mistaken, I remember being told that WISECO pistons (at least the set I had) were designed with a tiny bit less material on the edge. Giving it SLIGHTLY more port timing,

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    Canadian Beaver Inspector jkindt's Avatar
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    Quote Originally Posted by xlint89 View Post
    ... I had to special order 40 thou head gaskets to get a 60 thou squish. ...
    Why would you want a 60 thou squish? Wouldn't you have been better off using 30 thou head gaskets and having a 50 thou squish? Too wide leads to detonation.

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    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    Quote Originally Posted by jkindt View Post
    Too wide leads to detonation.
    ??????? What ????????????

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    Canadian Beaver Inspector jkindt's Avatar
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    Too wide on the squish band leads to detonation. The squish band, also referred to as quench band, at the optimum clearance has detonation minimizing properties. It keeps the flame front in the center of the head and allows the outer part of the head to keep cool due to the 'quenching' of this area. Do a Google search on squish band and detonation and check out some of the two-stroke sites that come up, they discuss this in much greater detail than I can explain. Basically to sum it up, if the squish band is too wide, the flame is allowed to get too far towards the edges, heats them up, and leads to detonation.

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    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    And if the squish it too close, the air fuel will pre-ignite before the spark occurs.

    Also keep in mind, as you decrease squish, you're increasing compression. Which is why engine builders reshape your domes when they machine you're heads for 92 octane or higher.


    Factory squish is about 50-55 thou.

    I was recommended to run 60 thou for safety sake. My PRO is set up with a fuzz under 60 and "her new ride" is set up with 60. No detonation issues on either one, and both have been kicking butt on the water.

  9. #9
    Canadian Beaver Inspector jkindt's Avatar
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    Quote Originally Posted by xlint89 View Post
    And if the squish it too close, the air fuel will pre-ignite before the spark occurs.
    If the squish is too close, the piston hits the head. Being too close doesn't cause pre-ignition as long as there is a proper angle to the squish band, the fuel is just forced into the combustion chamber.

    Quote Originally Posted by xlint
    Also keep in mind, as you decrease squish, you're increasing compression. Which is why engine builders reshape your domes when they machine you're heads for 92 octane or higher.
    Yes, this is very true. Reducing squish band clearance reduces effective head volume leading to increased compression. Not always a bad thing, but go too far and you screw yourself in the end.

    Quote Originally Posted by xlint
    Factory squish is about 50-55 thou.

    I was recommended to run 60 thou for safety sake. My PRO is set up with a fuzz under 60 and "her new ride" is set up with 60. No detonation issues on either one, and both have been kicking butt on the water.
    I would reconsider that recommendation. The tighter the squish band clearance, the more efficient the combustion. If you have the time, check out the article on cylinder porting by A. Graham Bell. Chapter 2 goes into great detail about head design and squish bands in particular. Some engines run very tight squish, as little as 25 thousandths. Tighter clearances lead to cooler temps and increased power, until the piston hits the head.

  10. #10
    Canadian Beaver Inspector jkindt's Avatar
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    Here's a link to the tech article by Graham Bell that I was referring to. Check out Chapter 2, it explains this very well.

    Do we actually know what the proper squish clearance is supposed to be on these engines??? Anyone??? I tried to measure the squish on my wife's 650 but it is greater than the 0.062" solder that I had to test with so I know it is too wide. Could explain the low compression! Could be time for different gaskets as well as rings.

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