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Discussion Starter #1
So I have a 2015 sportsman 570 EFI and recently rebuilt both the bottom and top end of the engine (new crankshaft, piston, etc.) When we tried it out we eventually heard a loud metal rattling or slapping noise on acceleration. Removed the valve cover and found the timing chain had slipped. Short version is after a couple more tries we figured the tensioner was failing so we replaced with a manual one. Same sound but chain is fine. Sound really seems to be coming from lower right engine area but am at a complete loss as to what it could be. Oil pick up possibly? Oil pump seems to be fine and no visible damage around that area. Any thoughts??
 

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Could be clutch noise - remove the clutch and start the engine if noise is still there then make sure the flywheel is tight - did you install new main bearings in the crankcase - it might be spark knock (since you only hear it on acceleration), try using 93 octane fuel or it might be piston slap - what was the piston to cylinder clearance when the new piston was installed? - finally, the lower rod bearing might be getting loose - or it might be something else, let us know what you find as the source of the noise.
 

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Gotcha. The clutch is not exactly in perfect condition but we did replace the weights on it with no change but not ruling it out. I was thinking a rod bearing too actually. Not sure what the piston clearance was. At any rate my brother (who actually installed the new parts) and I are going to pull the engine today and check on these things if we can. Will let u know what we find
 

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So after pulling the engine and splitting the case we found that the balance shaft wasn’t positioned exactly right causing the piston rod to knock against it every so often. Repositioned it and made sure that they were missing each other. Pretty satisfied that was the culprit but unfortunately we found the plain bearing that goes inside the crankcase was worn down so we’ll have to wait on getting a new one before we can try it out again. I won’t say anything about how the only way to get that bearing is by getting the crankcase it comes with but whatever
 
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