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  1. #1
    jetpilot785's Avatar
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    Stock cylinder height on ultra 150/ stx-r 1200

    Hello, I am working on a 2002 STX-R 1200 that appears to have a skiworx level 2 kit in it with flat top pistons, 30.5cc domes and aluminum base shims in it. The aluminum shims are .065” thick. The engine burned a hole in one cylinder and broke rings in another. It has 160lbs if cranking compression on the middle cylinder as it was the only one to survive. The installed cc was 34.8 for a 12.26:1 static ratio. Typically 12-12.5:1 is safe enough for pump gas but that also depends on port heights, ignition timing etc. The 160lbs has me thinking this is more of a 100+ octane level as stock is typically around 120-125 and according to skiworxs’ site, the 100 octane dimes typically see 170lbs. My burning question is, anyone have a stock unmolested cylinder they could measure the total height of the cylinder in inches so I see how much they were cut so I can revert back to stock pistons and pump gas compression and have the port timing close to stock. Thanks in advance for any info!!


  2. #2
    Quote Originally Posted by jetpilot785 View Post
    Hello, I am working on a 2002 STX-R 1200 that appears to have a skiworx level 2 kit in it with flat top pistons, 30.5cc domes and aluminum base shims in it. The aluminum shims are .065” thick. The engine burned a hole in one cylinder and broke rings in another. It has 160lbs if cranking compression on the middle cylinder as it was the only one to survive. The installed cc was 34.8 for a 12.26:1 static ratio. Typically 12-12.5:1 is safe enough for pump gas but that also depends on port heights, ignition timing etc. The 160lbs has me thinking this is more of a 100+ octane level as stock is typically around 120-125 and according to skiworxs’ site, the 100 octane dimes typically see 170lbs. My burning question is, anyone have a stock unmolested cylinder they could measure the total height of the cylinder in inches so I see how much they were cut so I can revert back to stock pistons and pump gas compression and have the port timing close to stock. Thanks in advance for any info!!
    As far as I know the cylinders themselves are not cut. I could be wrong but I believe only the heads are cut or modified. I have a couple stock cylinders I could measure for you if really needed. I just need to dig them out. Let me know.

  3. #3
    steve45's Avatar
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    5.090"

  4. #4
    Let me out of here......I need to go on Greenhulk!
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    They are probably cut right around .060. Check your squish I think most of those setups had ~ .040 You could add a stock base gasket or head gasket and check your cranking compression. Like you said with stock ignition 150-155 psi cranking compression is pushing it on these with pump gas. You wont be able to go back to stock port height unless you change out to stock cylinders

    If you are set on using stock pistons could try the cylinders with a 36cc head which is a fairly common dome. may be a little lazy on the bottom.

  5. #5
    jetpilot785's Avatar
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    That would be goofy if they cut the cylinders .060 and then stuck a .060-.065 base gasket shim underneath it. Would defeat the purpose as port timing would still be the same.


    Quote Originally Posted by TMiller View Post
    They are probably cut right around .060. Check your squish I think most of those setups had ~ .040 You could add a stock base gasket or head gasket and check your cranking compression. Like you said with stock ignition 150-155 psi cranking compression is pushing it on these with pump gas. You wont be able to go back to stock port height unless you change out to stock cylinders

    If you are set on using stock pistons could try the cylinders with a 36cc head which is a fairly common dome. may be a little lazy on the bottom.

  6. #6
    jetpilot785's Avatar
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    Thank you sir! These measure 5.035”. So they literally decked the cylinders .055” and stuck a .065” shim under it. Why not just sell a .010” shim and leave cylinders stock height. Sounds like a money making scam to me lol. I can see if they lowered the transfer ports a little and raised the exhaust with some porting work but to raise both up a lousy .010” and go through all the trouble of cutting cylinders down is goofy.

    Quote Originally Posted by steve45 View Post
    5.090"
    Last edited by jetpilot785; 08-11-2020 at 10:29 PM.

  7. #7
    Just a FYI, here is a pic from ADA racing for their different dome sizes for this engine.

    Click image for larger version. 

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    On my freshly rebuilt engine with 42cc domes I got between 150-152 psi. I tested with plugs installed, cold engine and WOT. Mine is on the higher side but that is the recommend highest safe compression for 92/93 octane pump gas. Anything higher and higher octane/race gas is recommended.

  8. #8
    what im thinking is whatever they cut off the top of the cylinder is what they added to the bottom with the shim,.065 raised the ports .065 ? why they did it idk. im thinking all that does is raises the power band up higher,it doesn't make more, Kinda strange.

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