We recommend .0015" of clearance between the apex seal and the rotor slot for all applications, boosted or naturally aspirated. 

3 mm 12A or 13B Goopy Apex Seals are .115" thick, and you should be able to slide a .0015" feeler gauge into the rotor apex slot with a new apex seal. 

Our oversized 12A 3 mm seals measure .118" (+ / - .001"). If your rotor slot measures .018" - .020", you may benefit from our oversized 2 mm seals which will allow you to re-gain the .0015" suggested clearance.

 Our 2 mm apex seals are .075" thick and you should be able to slide a .0015" feeler gauge into the rotor apex slot with a new apex seal in place.Goopy Performance 13B 2 mm oversized apex seals measure .078" (+/- .001"). If your rotor slot measures .077 - .078", then you may benefit from our oversized 2 mm seals which will allow you to re-gain the .0015 suggested clearance.  

If your gap with the seal in place measures .002-.005" it is still usable, but keep in mind that tighter clearances means greater sealing characteristics. 

Take note that 12A and 13B 3 mm apex seal slots suffer the most when it comes to rotor slots over-tolerance. We have measured many used rotor slots and have found very few that still had the stock widths.