Cartridge heater fit is measured as the difference between the inside diameter (I.D.) of the bore and the outside dimension (O.D.) of the heater.

The suggested fit for a Watt-Flex Cartridge Heater is .007" greater than the nominal Watt-Flex heater sheath diameter.

For example, the ideal bore diameter for a 1/2" diameter Watt-Flex heater would be 0.502 --> 0.505". At the maximum tolerance dimension of heater and the minimum bore diameter, the heater would have a 0.005" fit, allowing ease of insertion and removal.

Generally, smaller diameter heaters benefit from a tighter fit and fit can be loosened for larger diameters.