Rapid RH® 4.0 EX Smart Sensors should be placed in holes drilled with a 4 fluted, carbide-tipped drill bit that falls within the specifications for an ANSI standard ¾” bit. This range is from 0.775” to 0.787”. A tight-fitting Rapid RH® 4.0 EX Smart Sensor is most likely caused by one of two reasons: The first reason could be that your drill bit has “worn” down with use and has lost some of its “diameter”. This will most likely become noticeable after about 60-70 holes drilled. The other reason could be that you have purchased a new drill bit that is on the lower end of the tolerance spectrum for diameter.