Will flat top pistons work in GS450?

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  • safwan.khan
    • Jan 2026
    #1

    Will flat top pistons work in GS450?

    Hi all,
    I live outside US and do not have access to spare parts easily. I am rebuilding a GS that I bought a few months back and it has a cracked piston. After looking around for alternate and measuring sizes, I have come to know that Lancer (1985-1986) pistons are similar in dimensions to the original dome head GS pistons however they have a flat top.

    Now my question is, if I replace pistons on my bike with identical flat top pistons, how will it affect the performance and fuel economy? I have read about it a bit on the forums and figured out that flat top pistons will reduce the compression ratio in the cylinders a little bit but I am no racer so can compromise a little bit here, but not too much.
    Tags: None
    • tkent02
      Forum LongTimer
      Past Site Supporter
      • Jan 2006
      • 35571
      • Near South Park
      #2
      What is a lancer?
      http://img.photobucket.com/albums/v5...tatesMap-1.jpg

      Life is too short to ride an L.

        Comment

        • salty_monk
          Forum LongTimer
          GSResource Superstar
          Past Site Supporter
          Super Site Supporter
          • Oct 2006
          • 14001
          • London, UK to Redondo Beach, California
          #3
          Suspect he means Mitsubishi Lancer...
          1980 GS1000G - Sold
          1978 GS1000E - Finished!
          1980 GS550E - Fixed & given to a friend
          1983 GS750ES Special - Sold
          2009 KLR 650 - Sold - gone to TX!
          1982 GS1100G - Rebuilt and finished. - Sold
          2009 TE610 - Dual Sporting around dreaming of Dakar..... - FOR SALE!

          www.parasiticsanalytics.com

          TWINPOT BRAKE UPGRADE LINKY: http://www.thegsresources.com/_forum...e-on-78-Skunk/

            Comment

            • GregT
              Forum Sage
              • Jul 2009
              • 3541
              • New Zealand
              #4
              Originally posted by salty_monk
              Suspect he means Mitsubishi Lancer...
              I seem to remember there's something even worse called a lancer in India...

              The answer you're looking for is that if it'll get your 450 running again, do it.
              The drop in compression will lose performance, yes, but there's not a lot there anyway - and you'll be able to use low octane petrol too.

                Comment

                • safwan.khan
                  #5
                  Thanks for the helpful advice friends. I managed to get my hands on a slightly used gs450 piston from a local junkyard mechanic which seems to make an OK combination with the working one that I already have. So it looks like I am good to go with the stock pistons for now

                  P.S., I love this website!!

                    Comment

                    • celesboy1
                      #6
                      as far as piston diameter and the pin have same size....then it will be fine....

                      i live in indonesia and and many of bike using car piston to bore up displacement....including me...
                      i'm have gs250 with KIA s515 piston inside...and made new custom liner (bore)....its more safely to have thicker bore if u gonna do boreup diaplacement

                      but if i have much money....then i'll go to wiseco...and order custom forging piston online....it can make my bike accelerate much faster
                      Last edited by Guest; 02-13-2014, 03:15 AM.

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                        • safwan.khan
                          #7
                          Can you please tell me a bit in more detail about it with links to the piston that you are referring to? It seems interesting and I might end up trying it out

                          Originally posted by GregT
                          I seem to remember there's something even worse called a lancer in India...

                          The answer you're looking for is that if it'll get your 450 running again, do it.
                          The drop in compression will lose performance, yes, but there's not a lot there anyway - and you'll be able to use low octane petrol too.
                          Originally posted by celesboy1
                          as far as piston diameter and the pin have same size....then it will be fine....

                          i live in indonesia and and many of bike using car piston to bore up displacement....including me...
                          i'm have gs250 with KIA s515 piston inside...and made new custom liner (bore)....its more safely to have thicker bore if u gonna do boreup diaplacement

                          but if i have much money....then i'll go to wiseco...and order custom forging piston online....it can make my bike accelerate much faster

                            Comment

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