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'81 GS650G: Aftermarket oversize pistons hit head

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    '81 GS650G: Aftermarket oversize pistons hit head

    Hello everyone,

    After a long abstinence here I'm looking for some knowledge and advise.
    I bought a cheap GS650G, lots of issues, came very close to quit, but actually made it my pandemic project and learning experience (along with toooo many $$$ spent, but hey, gained a lot of experience!)

    Here I was looking for oversize pistons...
    Here I was about to quit...

    Sooo a pandemic and gazillion years later here I am. Absolut total rebuild of the engine: balanced and polished crankshaft, all!! new bearings and gaskets, complete rebuild of the head, machined cylinders for oversize pistons and... the infamous cruzinimage 740ccm bigbore kit.

    Last weekend I finally wanted to put it all together and that's where i ran into issues where I'm looking for advices.

    Turns out the shape of the cruzinimage pistons are significantly different than the original ones. And thats where they hit the head. Here are some pictures to give you some idea. I probably need to clear around 5mm until the piston won't hit the head anymore....
    Collage.jpg

    Soooooo, what should/can I do???
    Vancouver, BC
    Suzuki GS 650G '81

    #2
    That's certainly not right .
    Depending on crown thickness you may be able to machine enough off to get the correct clearance and compression ratio or space the block but at a guess i'd say they sent you the wrong pistons.
    Last edited by zed1015; 05-13-2021, 05:22 PM.
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      #3
      IOM: Those are so out sized, they could never be machined to fit.
      "Only fe' collected the old way, has any value." from His Majesty O'Keefe (1954 film)
      1982 GS1100G- road bike, body, seat and suspension modded
      1990 GSX750F-(1127cc '92 GSXR engine) track bike, much re-engineered
      1987 Honda CBR600F Hurricane; hooligan bike, restored

      Comment


        #4
        Are you sure the pistons are hitting the head? Could the valves be mistimed causing them to hit the pistons? If it's the pistons you might want to put some clay on the piston top so you can figure out where it's hitting.
        Ed

        To measure is to know.

        Mikuni O-ring Kits For Sale...https://www.thegsresources.com/_foru...ts#post1703182

        Top Newbie Mistakes thread...http://www.thegsresources.com/_forum...d.php?t=171846

        Carb rebuild tutorial...https://gsarchive.bwringer.com/mtsac...d_Tutorial.pdf

        KZ750E Rebuild Thread...http://www.thegsresources.com/_forum...0-Resurrection

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          #5
          Thank you so far for your feedback.

          Originally posted by zed1015 View Post
          That's certainly not right .
          Depending on crown thickness you may be able to machine enough off to get the correct clearance and compression ratio or space the block but at a guess i'd say they sent you the wrong pistons.
          I'm not sure if I want to alter the pistons. Taking stuff off sounds kind of wrong.
          Unfortunately I bought these pistons a while ago. I will contact cruzinimage and see what they say.


          Originally posted by Buffalo Bill View Post
          IOM: Those are so out sized, they could never be machined to fit.
          Yes, I agree.

          Originally posted by Nessism View Post
          Are you sure the pistons are hitting the head? Could the valves be mistimed causing them to hit the pistons? If it's the pistons you might want to put some clay on the piston top so you can figure out where it's hitting.
          I'm 110% sure the pistons are hitting the head. I have not yet put in the cams therefor all the valves are fully in.
          Also no clay needed, the head leaves some tiny scratches on the piston. The problem is the big dome shaped "crown" of the pistons.

          https://www.thegsresources.com/_foru...38#post2603938 This post shows the difference where it becomes obvious.
          Vancouver, BC
          Suzuki GS 650G '81

          Comment


            #6
            We had a GS drag race guy who worked in our shop and he would often cut a spacer base gasket out of aluminium road sign material to lower compression for turbo applications.

            Could be worth a try if you don't want to relieve the head or machine the pistons straight up.
            Current:
            Z1300A5 Locomotive (swapped my Intruder for it), GS450 Cafe Project (might never finish it....), XT500 Commuter (I know - it's a Yamaha )

            Past:
            VL1500 Intruder (swapped for Z1300), ZX9R Streetfighter (lets face it - too fast....), 1984 GSX750EF, 1984 GSX1100EF (AKA GS1150)
            And a bunch of other crap Yamahas....

            Comment


              #7


              Originally posted by Berti View Post
              Well, I bought these pistons and this is the result...

              https://www.thegsresources.com/_foru...23#post2651323
              Awww man that SUCKS! They basically sold you a hemi head's domed piston, nearly identical to the stock 77-79 GS750 piston...
              I would DEFINITELY be contacting them to complain, at least so they pull them and don't screw over anyone else... Unless perhaps they actually did mix up and send you GS750 65mm stock hemi pistons vs big bore GS650 65mm closed chamber head pistons...
              MAJOR BUMMER...
              But... an 80+ 8v GS550 head may potentially work on that piston with minor dome machining around the perimeter. There's no way you could machine that big hemi dome down to the 650 dome shape/size, you'd cut through to the back side...
              '77 GS750 920cc heavily modded
              '97 Kawasaki KDX220R rugged terrain ripper!
              '99 Kawasaki KDX220R​ rebuild in progress
              '79 GS425stock
              PROJECTS:
              '77 Suzuki PE250 woods racer
              '77 GS550 740cc major mods
              '77 GS400 489cc racer build
              '76 Rickman CR1000 GS1000/1100
              '78 GS1000C/1100

              Comment


                #8
                So this isn't just a "stock photo" of a similar item... The dome design differences here is very plain to see, & CLEARLY will not work with the head designed to be used with the 650 piston...


                vs
                Last edited by Chuck78; 05-14-2021, 12:43 AM.
                '77 GS750 920cc heavily modded
                '97 Kawasaki KDX220R rugged terrain ripper!
                '99 Kawasaki KDX220R​ rebuild in progress
                '79 GS425stock
                PROJECTS:
                '77 Suzuki PE250 woods racer
                '77 GS550 740cc major mods
                '77 GS400 489cc racer build
                '76 Rickman CR1000 GS1000/1100
                '78 GS1000C/1100

                Comment


                  #9
                  Berti, do you have a good accurate set of calipers to measure the piston skirts on those?
                  You might be able to drop in a set of these Wiseco 740 pistons into those bores. I "might" possibly be conviced to sell one of my 2 spare sets. I was holding onto one for a friend who didn't have the $ to buy a set at the time, & has since had 2 kids and hasn't gotten around to putting the 650 top end on his 550... he's too busy with his kids, wife, job, PE175 Suzuki, & GS450E...
                  '77 GS750 920cc heavily modded
                  '97 Kawasaki KDX220R rugged terrain ripper!
                  '99 Kawasaki KDX220R​ rebuild in progress
                  '79 GS425stock
                  PROJECTS:
                  '77 Suzuki PE250 woods racer
                  '77 GS550 740cc major mods
                  '77 GS400 489cc racer build
                  '76 Rickman CR1000 GS1000/1100
                  '78 GS1000C/1100

                  Comment


                    #10
                    No, it's not a stock photo and the actually image. I sent cruzinimage an email explaining the situation. Let's see how they react. I'm not really happy with them.

                    Now that my cylinder is already bored oversize I don't have too many options here.

                    I don't actually want to go the road with extensive altering head/piston or even heads from different bikes. Just frustrated at the situation.

                    If you let me know what exact measurements you need, I have ways to precisely measure!
                    Vancouver, BC
                    Suzuki GS 650G '81

                    Comment


                      #11
                      What kind of compression ratio is that end up with anyway? In the old days we would double up on head gaskets, but those were simpler engines & desperate measures.

                      Comment


                        #12
                        I have no experience in fitting that kit but that looks to be something that cannot be resolved by fitting a thicker head gasket, the only other way would be to have the piston dome's machined to the same profile as the Oem ones.
                        Personally I would be contacting the supplier to check if they have sent the wrong ones as if extra machining or thicker head gasket (unlikely) is required that should surely be explained in the kit write up on the website.
                        The big guy up there rides a Suzuki (this I know)
                        1981 gs850gx

                        1999 RF900
                        past bikes. RF900
                        TL1000s
                        Hayabusa
                        gsx 750f x2
                        197cc Francis Barnett
                        various British nails

                        Comment


                          #13
                          Thank you all for your help!

                          I contacted the Manufacturer and so far no response...

                          To everybody who ever comes across this thread - i highly advise to not buy these specific pistons. May work for other models/manufacturers. The 740ccm big bore kit does not fit the original Suzuku GS650G head! I guess I paid for this lesson

                          ¯\_(ツ)_/¯
                          Vancouver, BC
                          Suzuki GS 650G '81

                          Comment


                            #14
                            From the cruzinimage site description for the GS650 - 740 BBK. Cruzinimage state the pistons are a different shape

                            81-83 SUZUKI GS650 740cc BIGBORE PISTONS KIT 65mm PISTON


                            -DESCRIPTION-◆10-GS650BB◆
                            *81-83 Suzuki GS650 Bigbore pistons kit with Head Gasket
                            Aftermarket
                            Piston size: 65mm( 3.00mm Oversize )
                            Head gasket: Bore size is 68mm
                            *Set includes: pistons, pins, clips,rings and head gasket
                            Note: Cylinder boring required
                            These pistons are aftermarket.different design from of the Suzuki genuine pistons
                            82 GS650E (Canadian), 83 XS650SK (Canadian), Main machines Running
                            Aussie, 74 TX650A, 80 XS650SG, 81XS650SH, 80XS850, in various states of repair/disrepair
                            Introduction and ongoing thread for myGS650Ez
                            Albums

                            Comment


                              #15
                              A bit of left field thinking............the GS650 barrels and top end is used on the 550 as a big bore kit............

                              Pics from cruzinimage show the standard size and up to 1mm oversized GS650 pistons as original shape No crown to speak of
                              Attached Files
                              82 GS650E (Canadian), 83 XS650SK (Canadian), Main machines Running
                              Aussie, 74 TX650A, 80 XS650SG, 81XS650SH, 80XS850, in various states of repair/disrepair
                              Introduction and ongoing thread for myGS650Ez
                              Albums

                              Comment

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