I have an old 96 400 xplorer,
recently the engine case got cracked below the carb , i can see the crank thru the hole.
Can i patch this?
what do you recommend?

 

I would want to know the reason for the crack, In a non stress area, I have patched cases with JB Weld, just clean the area well before applying. Usually crankcase pressure is not that great, so any product which will seal it will work. It all depends on what cased the crack, and where it is located structurally.

 

Your piston probably broke(350's and 400;s did that a lot) Pull your carb and reed cage and look at the piston.
I have had that happen on customer machines in the past and actually found the part that broke out. I had a local welding shop weld it back in and then carefully filed and sanded it down to try and make a perfect fit to the other crankcase half. Then used a little extra sealer all over that area during reassembly to prevent air leaks. \
I have 3 400's that all had that problem happen(no huge holes, just cracked cases, it's how I ended up with them, cust didn't want to fix, so i bought them and fixed them) and all 3 run perfectly.
If you do rebuild it, use a wiseco piston, not a polaris piston.