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  1. #1
    PWCToday.com Is My Home Away From Home
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    what causes a flywheel key to shear??

    I haver to admit that this is not a ski it is an atv but i cannot find any atv forums with people who know anything about the mechanical workings of a 2 stroke. The engine is a polaris and it is very similar to the setup of older skis. I am replacing the flywheel as the key way is worn but i am worried that i am going to get the new flywheel and it is going to keep shearing them. I will put a key in and it will run fine for maybe 10 min and then at some point it shears. I do not have a head gasket leak (anymore) that is causing a hydro lock and the clutch doesn't grab at all at low rpms. Sometime it does it running and sometimes it does it during start up.

    Any things i should check or should i just assume that the damaged keyway is the problem and not a symptom. I guessed that the blown head gasket was prly what initially caused the sheared key but not sure as i bought it that way.
    Last edited by mcurcio1989; 02-13-2011 at 01:05 PM.
    Yes, i don't spell check or grammar check and i make frequent grammatical errors. If anyone wants to tell me that why don't you just bite your tounge. Were on here to share our knowledge of the great invention that is the jet watercrafts and lets keep it to that!!

  2. #2
    PWCToday.com Is My Home Away From Home WaveDemon's Avatar
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    Re: what causes a flywheel key to shear??

    The key is shearing because you're asking it to hold the flywheel in place while running. The key should only be for locating the flywheel correctly before bolting it on. The fly wheel should be held on to the crank by pressing it onto the snout of the crank. There are 2 possible problems. Either you're not torquing the flywheel bolt properly or the taper on the flywheel or crank is bad. If it's the taper you can try to fix it with some compound.
    Last edited by WaveDemon; 02-13-2011 at 01:20 PM.
    Me > You

    Stop asking stupid questions and just get a SuperJet...

  3. #3
    PWCToday.com Is My Home Away From Home delagem's Avatar
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    Re: what causes a flywheel key to shear??

    There's a couple of things to check.

    Are you sure it's the correct flywheel? A flywheel with the incorrect taper could be made to fit on, but will not "seat" properly and most of the holding will be being done by the key. A good way to check would be to smear prussian blue on the snout, install the flywheel, then remove. You'll be able to see where it touched, and didn't. If you don't have good contact all the way around, that's a big problem. If you don't have prussian blue, try any kind of dye, grease, just something that will smear and show where contact is occuring.

    Always use a new key. OEM factory, and new. People will post on here that they used an SAE equivalent key and it worked just fine. They got lucky. These are metric keys, if you live in a town like mine, you won't be able to buy one locally, except thru the dealer.

    You should always lap the flywheel to the snout, even if it's got a good fit. I use Clover Compound, but any decent automotive store will be able to sell you valve grinding paste. Put a smear on the snout, set the flywheel on without a key, and rock. Turn it 30 degrees, and rock some more. Do this for 10-15 minutes. Add more paste occasionally. Be sure to clean all the paste off of everything before final assembly.

    The snout and flywheel must be spotlessly clean before reassembly. Brake cleaner or straight ether is what I like to use. As mentioned above, make sure the bolt/nut is torqued properly, (although I never do this. I just zap it with a 1/2" impact gun till it feels good! )

    Are you using loctite on the snout? You should. Check your service manual to be sure, but I always put a touch of red loctite on (red is permanent). It will make it very difficult to remove, however. Red loctite is supposed to be heated to remove. But you can't use heat in this application, unless you are planning on junking the flywheel, stator, and changing the crank seal after you disassemble. I can get them apart without heat, but it takes a good puller, grade 8 bolts (not the crappy bolts they give you with the puller), and patience.

    Is the engine backfiring? That puts a tremendous stress on the flywheel as it reverses rotation.

    Whats your compression?

    I saw your comment on the mechanical aptitude of the ATV forum members, I had the same experience. Even better, go to a go-ped forum. Wow, just wow.
    Last edited by delagem; 02-13-2011 at 02:49 PM. Reason: I'm not wearing any pants!
    '58 Vincent Amanda Water Scooter, 0rpm, 0mph

    '84 SurfJet 236ss Freestyle, 5950rpm, 24.6 mph

    '91 Jetmate! SX-R 800,GroupK carbs,R&D 26cc head,R&D ProLok F/A, Coffman Rocket Pipe,Impros 13-20,76mm nozzle: 6630rpm and 38.1mph GPS

    '92 SL650/750, dry pipe,extended pump,trim,HotSeat Waterbox: 6320rpm, 50.1 mph

    '00 SLX,Tau Ceti F/A's,NuJet prop: 6670rpm, 63.1 mph

    '03 Octane, Watcon 2318 CDI

    '04 MSX140,Impros stage 2 Powertune 14/23,WetWolf,6deg wedge: 6750rpm, 60.5 mph

  4. #4
    PWCToday.com Is My Home Away From Home delagem's Avatar
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    Re: what causes a flywheel key to shear??

    Oh, and what model Polaris? Triple or twin? The older Polaris skis were known to hydrolock, and then twist their crank out of phase. The press fit of the cranks was much looser than on, say, a Kawi.
    '58 Vincent Amanda Water Scooter, 0rpm, 0mph

    '84 SurfJet 236ss Freestyle, 5950rpm, 24.6 mph

    '91 Jetmate! SX-R 800,GroupK carbs,R&D 26cc head,R&D ProLok F/A, Coffman Rocket Pipe,Impros 13-20,76mm nozzle: 6630rpm and 38.1mph GPS

    '92 SL650/750, dry pipe,extended pump,trim,HotSeat Waterbox: 6320rpm, 50.1 mph

    '00 SLX,Tau Ceti F/A's,NuJet prop: 6670rpm, 63.1 mph

    '03 Octane, Watcon 2318 CDI

    '04 MSX140,Impros stage 2 Powertune 14/23,WetWolf,6deg wedge: 6750rpm, 60.5 mph

  5. #5
    Top Dog js580's Avatar
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    Re: what causes a flywheel key to shear??

    Quote Originally Posted by delagem View Post
    Oh, and what model Polaris? Triple or twin? The older Polaris skis were known to hydrolock, and then twist their crank out of phase. The press fit of the cranks was much looser than on, say, a Kawi.
    it is not a ski he is working on a atv

  6. #6
    PWCToday.com Is My Home Away From Home delagem's Avatar
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    Re: what causes a flywheel key to shear??

    Quote Originally Posted by js580 View Post
    it is not a ski he is working on a atv
    The point is the same crank was likely used in both the ATV's and watercraft. Snowmobiles, too. People manage to twist their crank out of phase by holding the wrong piston down while unthreading their coupler, rope trick in the wrong cylinder, etc. It isn't specific to watercraft.

    I was pretty specific that I knew this was an ATV in post # 3.
    Last edited by delagem; 02-13-2011 at 06:39 PM.
    '58 Vincent Amanda Water Scooter, 0rpm, 0mph

    '84 SurfJet 236ss Freestyle, 5950rpm, 24.6 mph

    '91 Jetmate! SX-R 800,GroupK carbs,R&D 26cc head,R&D ProLok F/A, Coffman Rocket Pipe,Impros 13-20,76mm nozzle: 6630rpm and 38.1mph GPS

    '92 SL650/750, dry pipe,extended pump,trim,HotSeat Waterbox: 6320rpm, 50.1 mph

    '00 SLX,Tau Ceti F/A's,NuJet prop: 6670rpm, 63.1 mph

    '03 Octane, Watcon 2318 CDI

    '04 MSX140,Impros stage 2 Powertune 14/23,WetWolf,6deg wedge: 6750rpm, 60.5 mph

  7. #7
    PWCToday.com Is My Home Away From Home
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    Re: what causes a flywheel key to shear??

    it is a single 378cc cylinder. I think that the problem was that I was using a non oem key. I made marks and put it on without a key and it seems to be holding just fine now. I have a new flywheel and 3 (I they were so cheap mine as well) keys in the mail. The keys I was using were holding the flywheel from tightening up all the way. The other problem I see is that the nut they use is really small and i am scared to torque it to much. Thanks for the help so far guys. Good advice here. I really appreciate the lengthy response delagem! Lol those atv guys prly know how to hunt or snow plow but they know nothing about the mechanical workings of there machines haha.
    Yes, i don't spell check or grammar check and i make frequent grammatical errors. If anyone wants to tell me that why don't you just bite your tounge. Were on here to share our knowledge of the great invention that is the jet watercrafts and lets keep it to that!!

  8. #8
    PWCToday.com Is My Home Away From Home
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    Re: what causes a flywheel key to shear??

    I isn't backfiring it actually runs really smooth. It had 130 with a bad head gasket. For whatever reason i didn't check it after i put the new one on. I made the same assumption i am fairly certain they would use a very similar flywheel and crank on here as what they use on there ski engines.
    Last edited by mcurcio1989; 02-13-2011 at 09:00 PM.
    Yes, i don't spell check or grammar check and i make frequent grammatical errors. If anyone wants to tell me that why don't you just bite your tounge. Were on here to share our knowledge of the great invention that is the jet watercrafts and lets keep it to that!!

  9. #9
    Top Dog js580's Avatar
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    Re: what causes a flywheel key to shear??

    Quote Originally Posted by mcurcio1989 View Post
    I isn't backfiring it actually runs really smooth. It had 130 with a bad head gasket. For whatever reason i didn't check it after i put the new one on. I made the same assumption i am fairly certain they would use a very similar flywheel and crank on here as what they use on there ski engines.
    ahh i see.. glad its fixed now i never had trouble with flywheel key on any of my sled since i am always rebuilding one of my sled every 3 or 4th season so it never really occured to me or had trouble with flywheel keys so i am kinda in dark in that area but not dark in other area where it is prone to have trouble ie: piston shattering or hole in piston or blowing a rod and breaking crank in half or twisting them due to high horsepower that i am pushing on that motor

  10. #10
    PWCToday.com Is My Home Away From Home
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    Re: what causes a flywheel key to shear??

    okay so the thing is running pretty good now . . .lol aside from a pretty nice knock coming from the bottom end. The top end on this was recently rebuilt and there are no carbon build ups or anything on the cylinder that would be suspect. I suspect bearing failure in the bottom end. When you take the belt off the clutch and grab the clutch you can move it a significant amount axially. Maybe a couple hundredths of an inch (something like that) it is noticeable but not a total failure. Stator side is all good tho. The crankshaft turns without much resistance. I have never dealt with an issue like this. I have pulled cranks apart but never a bearing failure issue. Is it likely bearing damage or some kind of wear on the bearing to case fit. Or something else i am missing? The sound definitely seems to come from that area and with that play you can see why i suspect that. I can get all three of the crank bearings along with the seals shipped for $100 and my manual says i won't need to balance the crank to put it back in. So it isn't a bank breaker so long as that is all were looking at.

    Lol this thing was really beat on by a po. I have already replaced a lot of parts but i got it cheap enough that it isn't going to be to big of an issue. Its a good project to and will be a blast when its done so.
    Last edited by mcurcio1989; 02-16-2011 at 07:26 PM.
    Yes, i don't spell check or grammar check and i make frequent grammatical errors. If anyone wants to tell me that why don't you just bite your tounge. Were on here to share our knowledge of the great invention that is the jet watercrafts and lets keep it to that!!

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