When a piston gets "scored," some of the aluminum it is comprised of transfers to the cylinder wall. This reduces the diameter of the piston. In a piston port engine (virtually all saws now) The skirt of the piston also functions as your intake "valve." The port is uncovered at the appropriate time, and fuel charge goes into the bottom end. If the clearance between the piston and the wall is large enough, some of the fuel charge will spit back through after the port is closed. Also, exessive piston to cylinder clearance causes piston slap, which is the piston rocking in the bore. Piston slap, in turn causes more wear on the skirt. In the end, the very thin piston skirt will break, and trash the entire top and bottom end.
Running a scored piston is never desirable. Sometimes you must for one reason or another. Saving money is not usually a good reason. This is false economy. Once that piston breaks, it will trash a whole bunch of parts, other than just the $35 piston.
With that said....... If the ring grooves are free of scoring, and it is not totally wasted, here is a basic test. Take the rings off the piston, and put it in the bore you intend to run it in. Make sure it is dry (no oil) for the test. Put your thumb over the spark plug hole, and turn the cylinder over. If the piston slowly drops out in maybe 5 or more seconds, it is probably tight enough. If it flops right out, it is way too loose. I have seen folks here say to sand down the scoring. This only increases tolerances. If you must use a scored one, just knock off the high spots, and then do the previously mentioned test.
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