I am in the process of rebuilding my engine on a 1995 Polaris sportsman 400 2-stroke. I had to replace the oil pump side crankcase and all new piston kit due to the famous factory piston falling apart. Everything is going great up to the point where I am putting on the next case half that goes over the counterbalance and water pump housing. The crank shaft is turing very smooth even with that case sitting down in its place with the bearing and oil seal seated, but when I tighten all the bolts the thing locks up drastically. I've double checked everything down to the bearing seating. Everthing is very clean and all new gaskets and oil seal is new. Cant figure it out. Any help is much appreciated. Once this is all together I'm assuming it should still turn very easy and smooth.

Thanks, Jason

 

When outer case with bearing and seal and spacer installed,will tighten up a little,But should not lock up. ALSO is spacer washer behind 2 clutch side bearings installed and is crank centered? SOMETIMES I use a little LOVE tap with a DEADBLOW HAMMER to HELP align crank! After assembled and everything CORRECT,crank can bind especially if have installed a new rod.Hard to get crank EXACTLY same dimentional size of original rod and crank,can be a little snug. Tap one side,then another on leftside of crank,check if turning smoother,if not try right side. I KNOW a little crude,but have done so in the past. MANUAL gives exact dimensions for crank spacing,but have rebuilt quite a few and have had to do this. DONT know exactly what all you have done on rebuild,but hope this helps. Also is counterbalancer bearing correctly seated and locking plate correctly installed? Dots lined up on gears?

 

I didnt replace the bearings on the the replacment crankcase half, only the seal. I have not yet put the new pistin kit on or the head. I actually didnt even remove the crank shaft from the opposite side crankcase because I figured no need to go to any more work than nesesary if I dont need to replace that half. So with the new crankcase half put on and all tightened up it seems smooth as glass when turning. Never removed counterbalance bearing because I removed it from the other side. locking plate is installed correctly as well as dots line up. I know its hard just visualizing the problem without seeing it in person I just cant see why simply tightening the bolts on that gear cover causes it to basically lock up. Thanks for responding, this is very frustrating. Everything was going so well.

Thanks, Jason

 

Is NEW crankcase half installed because of damage? If new case then sometimes may not be machined exactly as original, can happen. Have had same problem with trans cases when replacing just one side! PROBABLY why KTM, HUSKY cases can only be bought as a machined pair,along with other brands. Fit critical on these machines and cases are etched number matched. BUT like you said if crank smooth when both cases tightened,then c/b case shouldnt cause problem,but apparently is binding somewhere.

 

yeah when the piston came apart it blew through the crank wall,so thats why it had to be replaced. I new that the fit of two halves was critical but in this situation since the two halves are mated up and evrything is smooth I didnt figure that had anything to do with it. Something is causing a the gear cover to bind on the crank shaft when it is tightened down.