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Discussion Starter · #1 ·
I'm doing a top end for a guy locally and i checked his compression and i came up with 44...yes 44psi and i read that at least 105 is needed for it to run and factory is 120-130.So obviously this 44 psi would be reason why his bike wouldn't run any more.Anyways, I pulled the top end and the piston (stock) is just completely destroyed.There is no piston skirt/windows on either side of it left..just blown to shreds.I was surprised to see that the cylinder is actually still in pretty good condition and the head isn't even pitted.It could be thrown back together (new piston obviously) and ran how it is but i'm gonna have it bored anyways.My question is what would be the reason for it be be shredded this much? Maybe just excessive riding on a bad top end eventually led to this or what? It's still oil injected and from what i could tell when i pulled the oil line off oil came out so i assume the pump is still working but am still definitely gonna double check that.

Anyone got an idea of what's up with it...?
 

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That's a new 2 stroke slipper style piston, they're trying to copy the 4 strokes. No skirts on them, very light weight, you must have the prototype....



under the crown is black as black can be. I'd bet was worn out, got WAY hot and self destructed.

Check the bore clearance with the new piston...I'd be taking apart the bottom end too. That material had to go somewhere, and inside the crank bearings is NOT where you want it.
 

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Discussion Starter · #3 ·
QUOTE (dajogejr @ Jul 7 2009, 06:28 AM) index.php?act=findpost&pid=49511That's a new 2 stroke slipper style piston, they're trying to copy the 4 strokes. No skirts on them, very light weight, you must have the prototype....



under the crown is black as black can be. I'd bet was worn out, got WAY hot and self destructed.

Check the bore clearance with the new piston...I'd be taking apart the bottom end too. That material had to go somewhere, and inside the crank bearings is NOT where you want it.

Haha That thought crossed my mind too...I was gonna see if i could get these prototypes in a 75mm bore for my cylinders


Thanks for the input Dave
 

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Those stock cast pistons do it all the time on those 400s. They are made of glass it seems, but they have tough heavy cranks and only turn 6200 rpms or something stock. Thats very common on a polaris 400 with a worn bore especially of they have a pipe and clutch kit that makes them turn 6800 rpms or more. Even stock ones have piston slapping noise from the factory cause the piston ends up oval shaped very quickly after new. They are thin, cast pistons.
 

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Yep, once you put a good forged piston in it then you wont have that problem anymore.

Radar and myself have a ton of experience with these 400's, and he still has one. Shoot us a question if you need any help.
 
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