Converter rebuilders want to know why some of the pistons have the orifice and some do not. The answer to this question begins with understanding why the orifice was used in the first place. The orifice was drilled through the piston to help cool the mass of the converter. The TCC-release passage is open to exhaust when the converter is in the TCC-apply mode. Metered oil, which is allowed to bypass the piston by way of the orifice, carries some heat away from the converter.
It may seem obvious initially, but when you’re talking about transmission orifices, size does matter. What may be surprising, though, is how significant a difference in vehicle performance small changes in orifice diameters can make. This becomes important when you’re considering modifications to separator-plate holes (see Figure 1) or cup-plug orifices (see Figure 2) during a rebuild.