Mirrors can come in Motorbike(10mm) and Scooter (8mm) sizes, it may pay to whip them off your PCX and check before you buy, and some do come with specific left and right threads as well - trust me that ratty runner I'm sorting is throwing all sorts of surprises at me.
i thought they came in different thread sizes, i looked in to a few sites and they had different thread sizes listed.
i'll have to whip mine of when i have a spare hour at work
I had a quick check this morning against the mirrors on the PCX and the thread is 10mm.
That said you can buy an adaptor insert that brings 8mm mirrors to fit 10mm holes.