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Reapairing Crankshafts

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  • Reapairing Crankshafts

    Has anyone ever pressed apart snowmobile crankshafts and rebuilt them? I have a 1979 Yamaha exiter that needs some attention. Rebuilt crankshaft required. How hard is this really? Thanx Guys madman

  • #2
    With the correct tools, it's not a bad job.
    I have done 1, 2, and 3 cylinder bike cranks.


    Rex

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    • #3
      I have done single cyl bike and atv cranks- Need a hydr press, big brass hammer for adjustments, dial indicator and you can use a lathe with live centers on both sides to hold the crank while you rotate it by hand and check it with your dial indicator to check it for true- then take it out and adjust with BFH then recheck and repeat as needed.

      Shop manual for your engine would be a great idea- you need the clearances and total thickness specs.

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      • #4
        Madman,

        Getting them apart is easy , Its putting them back together that is the problem. Ok, now that I got the smart a$$ out of my system on to the problem... I have been involved in rebuilding cranks for Artic Cat machines, I think they are basically the same. The way we did it was to carefully press the journal out of the outer counter weight. The next part is the tricky part. You will need to heat the outer counterweight to expand the journal pin hole. We did it in a crock-pot of oil set on high temp for several hours to expand the hole. Chilling the journal pin to shrink it will help also. Get everything ready to reassemble, new bearings and con rod if needed. We never had a jig but if you have the materials to make one it would be much better for the reassembly. To reassemble you just slip the heated counterweight on the journal pin and try your best to keep it in alignment, if you are off a large brass hammer will move it while it is hot. Now for the bad part... after this type of repair the journal / counterweight press fit is weakened and will likely twist eventually binding the crank and possibly damaging the crankcase. We finally gave up on these repairs as the failure rate was nearly 100% in less than 3 seasons. A new crank assembly is actually a better investment after you get over the sticker shock . This was many years ago so I don't know how the new locktite formulas would work or if they would improve the reliability. I really think I would abandon the repair due to the unpredictability of the accuracy of the alignment and the investment in time and the potential for more damage if the repair fails.

        Robin
        Robin

        Happily working on my second million Gave up on the first

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        • #5
          I have done 2. Parts are quite cheep to do the whole thing. The rod kits are the most at around 150 per. Bearings you can get at you normal brg. place.

          I use a cheep 20t. press. You will need to make or somehow get a hold of a C bracket that will support the counter weight, as you press the big end rod pin out. I made mine out of a 1/2 plate. I hang the crank in a piece of large pipe, my press is too small to hang it through the press bed.

          Pressing things back together is a little easier press wise. But you have to have the 2 counter waits lined up as you press it back together. Press a little check, press more check ect.

          I put the crank between centers in the lathe to check run out, V blocks could be used. Check it before and check each individual piece for run out when it is apart.

          A large lead or brass hammer works well for tweaking as you press and align as it is going back together. Scribe lines can help as well (before pressing apart).
          This is my self thought teck. I would like to hear others as well.
          Good luck
          Stephen

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          • #6
            Sled cranks are unreal expensive... AC580...$1500 plus tax. 600 triple..$2300 plus tax. The triples are like hand grenades at the best of times. Buy a new crank for a triple for the $2300 and you now have a sled that's worth at LEAST $1500
            Peter (Brockely 1) does quite a bit of work on sled motors...not real sure about cranks tho..
            I have tools I don't even know I own...

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            • #7
              I have done a lot of M/C crankshaft work, my first suggestion is to do some research, some Japanese crankshafts a not repairable. Other crankshafts require different presses. A crankshaft for a 250 OSSA requires 40T press,
              anything less aint' gonna work. I would also suggest manufacturing a fixture
              that fits between the crank webs and con rod to support it and avoid trashing the crank with pressing.
              Non, je ne regrette rien.

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              • #8
                I can never understand the stupidity exhibited by manufacturers when it comes to spare part charges. We all know how much you would expect to pay to have a "One off" manufactured, but this isn't the case as once the go button is pressed, there are dozens on the floor. I know it costs to store and the guy that has to pick it off a shelf has to have a wage but shelves have more on them than the bit I want and the picker doesn't sit waiting for my order. It's difficult when you manufacture components in small quantities to a contract specific design, but the major manufacturers don't. Our throw away society is driven purely by greed.

                Regards Ian.
                Last edited by Circlip; 01-14-2009, 07:37 AM.
                You might not like what I say,but that doesn't mean I'm wrong.

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                • #9
                  Yamaha single throw:
                  If the crank has not been damaged too much;

                  1) then carfully measure around the two throws and record (just in case you do not have the factory specs)
                  2) blue crank and put scribe marks using a machinist square across each throw.
                  3) remove all bearings*. make sure you record what side each was on (PTO or Flywheel) Note any pins,dowel hole etc and what side they are facing.
                  Record any numbers on the connecting rod and what direction they face
                  ie Towards the PTO side or the Flywheel side.
                  (A drawing aways helps)
                  4) Set up the crank in your Press (You will need 30 to 50t)
                  You will need plates, etc to hold. BE CAREFULL
                  5)With your crank supported via plates/jig press out the pin from one side
                  of the throw.
                  6) turn over and press pin out of other throw.
                  7) Clean up crank parts that are to be reused.
                  DO NOT REUSE any rod bearings,pins or thrust washers
                  8)Set one side of the throw on the press, lube the new pin and press in.
                  9)With the new bearings, washers and rod in place, put other side on top of the throw with the pin already in it.
                  and using a machinist square line up the scribed marks.
                  10) press on about 1/3 the way down.
                  11) take the crank out of the press and using the square and a large brass hammer line up the scribed marks.
                  12) put back in press and press together , stopping from time to time to check the distances between throws. Match the measurement you recorded or the shop manual specs.
                  13) Using your square against the side of your crank check to see that the throws are concentric. use your brass hammer to adjust.
                  14) Put your bearings back on the crank. (bath of hot oil works best)
                  15)Set up crank on Vee blocks and using a dial recheck to see if the throws are concentric and parallel. Adjust with brass hammer. TIR of 0.001" or better.
                  DO NOT put the crank between centers to check.Most center holes get damaged during removal of flywheels and clutches.Always do the final check on vee blocks with the new crank bearings installed.

                  * Be supper careful removing crank bearings.If you press them off, wrap then
                  in rags just in case then break. I prefer to split them with a reinforced Dremel cut off wheel.

                  eddie
                  Last edited by motorworks; 01-14-2009, 07:33 AM.
                  please visit my webpage:
                  http://motorworks88.webs.com/

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                  • #10
                    And to re-emphasize, hardened bearing outers and inners break like shrapnel, and a cut from one seems to bleed forever.

                    Regards Ian.
                    You might not like what I say,but that doesn't mean I'm wrong.

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                    • #11
                      Motorworks, as fine of "How To" I've ever seen. Thanks for the fine write-up.

                      That's the way I learned it from a HD trained mechanic in the '60's without your improvements re scribe mark and v blocks. Took a 200 twin Suzuki to an excellent machine shop, Luther-Shelton, in Kansas City. Instead of the support plates and press he simply placed 6 bolts with long nuts (rod couplers)screwed on them between the disks and started unscrewing the nuts off the bolts to press the disks apart. No special tools and done in no time. If anyone doesn't understand my narrative I'll post a drawing. It's a wonderful trick to know.

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                      • #12
                        Reading Motorworks description seemed to follow what I used to do with twins and tripples. It's fussy and time consuming but considering the cost of a new bottom end it's worth it. Have never had one come back. Peter
                        The difficult done right away. the impossible takes a little time.

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                        • #13
                          Here's a sketch of using miniature "jacks" made of bolts and long nuts to push crank wheels apart. Space several between the crank disks and start wrenching the nuts off a 1/6 turn at a time.

                          http://img.photobucket.com/albums/v4...n/P2100013.jpg
                          Last edited by GKman; 02-10-2011, 11:37 PM.

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                          • #14
                            There must be a fold in the Time-Space fabric at least this one isn't seven years old like the ''pin'' thread earlier.

                            Steve

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                            • #15
                              Sounds cheaper and easier to drive your pick up truck to the bar.....what?, you mean not everyone that rides snowmobiles does it just to get to the bar

                              Good luck with your rebuild......post some pics it sounds interesting.
                              Obviously I never look at the original post date,DOH!
                              Last edited by BigMike782; 02-11-2011, 08:59 AM.

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