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  1. #1
    PWCToday Guru Jim_ii's Avatar
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    Aug 2011
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    Compression difference between 1998 and 1999 Sxi Pros with Group K head modification

    Hello,

    I own two 750 Sxi Pros, one is a 1998 (brand new to me) and the other is a 1999 (owned for 10 years). Both skis have the stock heads that have both been modified by Group K. Both have stock cylinders, cases, cranks, etc... The only engine work has been the head modifications.

    When I do a compression test (air filters off, spark plugs out of both cylinders, WOT, cold engines) I get a 20 psi difference between the 1998 and 1999. The 1999 with stock pistons and rings gives the higher reading. Should the two skis be about the same assuming good piston to cylinder clearance and good rings?

    Is there anything different between the motors from 1998 to 1999 from the factory that would account for this difference in compression?

    The readings between cylinders on a given ski are within a few psi of each other.

    The guy I bought the 1998 from said he honed the cylinders (did not bore) and replaced the stock pistons with Wiseco pistons and rings. I'm wondering if there's too much clearance between the pistons and cylinders with the Wiseco setup resulting in lower than expected compression readings?

    Any other thoughts?

    Thanks for the help!

  2. #2
    PWCToday Guru kawipilot's Avatar
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    Re: Compression difference between 1998 and 1999 Sxi Pros with Group K head modificat

    That is not a fair assessment. You cannot determine what compression should be based on what info you are giving. When you send a head to group K, they modify it based on "stock parameters". There is a lot of variance on "Stock" set ups. If you want precise measurements, you need to measure (on each engine) the distance from the top of the exhaust port, to the top of the cylinder, distance from top of piston at TDC to top of cylinder, head gasket thickness, and then measure both heads (deck, volume, etc.). Ten thousandths difference between the two motors will show ten psi compression difference on a gauge, so it is not unreasonable to expect 15 thousandths difference in tolerance between the two motors (per Kawasaki, there can be as much as 18 thousandths tolerance). If you were to give the measurements to Harry (group K) when you send off the heads to have them machined, he would modify them individually, (which is what he does when you send him the complete motor). Hope this helps.

  3. #3
    PWCToday Guru Jim_ii's Avatar
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    Re: Compression difference between 1998 and 1999 Sxi Pros with Group K head modificat

    Quote Originally Posted by kawipilot View Post
    That is not a fair assessment. You cannot determine what compression should be based on what info you are giving. When you send a head to group K, they modify it based on "stock parameters". There is a lot of variance on "Stock" set ups. If you want precise measurements, you need to measure (on each engine) the distance from the top of the exhaust port, to the top of the cylinder, distance from top of piston at TDC to top of cylinder, head gasket thickness, and then measure both heads (deck, volume, etc.). Ten thousandths difference between the two motors will show ten psi compression difference on a gauge, so it is not unreasonable to expect 15 thousandths difference in tolerance between the two motors (per Kawasaki, there can be as much as 18 thousandths tolerance). If you were to give the measurements to Harry (group K) when you send off the heads to have them machined, he would modify them individually, (which is what he does when you send him the complete motor). Hope this helps.

    Great info, thanks!

    Any thoughts on the previous owner swapping out the stock pistons for Wiseco ones without boring the cylinders?

  4. #4
    I dream skis JC-SuperJet's Avatar
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    Jul 2013
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    645

    Re: Compression difference between 1998 and 1999 Sxi Pros with Group K head modificat

    +1 kawipilot

    FYI If there is too much clearance between the Wiseco pistons and the cylinder bores, there is a good chance that the piston skirts will break off and earn you a complete engine rebuild...

    Wiseco forged pistons have different clearance requirements than OEM cast pistons.

    Also make sure both batteries are fully charged so that the starter motors can compress the air at about the same rpm's, then you can compare apples to apples.

  5. #5
    PWCToday Guru Jim_ii's Avatar
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    Re: Compression difference between 1998 and 1999 Sxi Pros with Group K head modificat

    Thanks for the input guys.

    And just for clarification I reread my original post and I guess you could take my comments as though I was calling into question the work done on the heads by GroupK. That's not at all the case.

    In fact GroupK has done work for me on all 5 of my previous skis and I've always been 100% impressed with what they do. No complaints at all about their services.

    The real question was whether installing the Wiseco pistons in the stock bores could have a negative effect on compression. JC-SuperJet brings up a valid concern with regards to clearance between the pistons and bores. Looks like I will be doing a teardown over the winter just to be safe. Oh well, winters in MI suck without a project!

    Thanks again for the input.

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