I tried this, I clone the subtool then masking the area I want to keep I make polygroups from masking. Hide what I want to cut and delete hidden geometry. The problem is I am left with a one sided mesh at a very High subdiv level. I have tried to use reconstruct to no avail.( I made sure everything was unmasked when I tried the operation). If some one could show me a work around to this issue it would be greatly apreciated as my current sculpt would benefit greatly from such a process.