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Discussion Starter #1
Hey everyone I've a 1993 350cc Polaris 4X4 it's has new piston and rings, had the head sized for the new piston and rings. I tried a compression test on it today and it's running 100 psi and I'm just wondering what could be the problem for it running so low? Any advice would be great cheers :)

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Crankcase seal leaking? Piston in 180 degrees off? Rings not seated?


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There should be an arrow on piston top. It should point towards the exhaust port in the cylinder which is the front of the machine


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Yes sir. Any time


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Discussion Starter #7 (Edited)
the arrow is the right but it's got a gap between the back wall and piston could that be the problem

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That would be a big problem.


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If you have a dial caliper check diameter of bore front to back and side to side.


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Discussion Starter #11
From front to back is 75.80 and from side to side is 75.65
The guy i bought it off said his mate bore it out

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The piston to wall clearance should be pretty consistent the entire circumference. If it's more in the back than the front I would assume the rings are worn.


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Discussion Starter #13
Cheers guys, any reason why the rings would worn since they haven't been worn in and only been bore out a week or so before I bought it

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Seems to me it was a honed cylinder after a ring failure. That is too large of a difference in diameter. It could also be undersized piston but the bore should be more true than that.


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130 is good. Below that is considered low


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