One of the more frequently asked torque converter questions is about the .028″ orifice found in some General Motors 298mm and 300MM TCC pistons with carbon weave lining.
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 by-pass the piston by way of the orifice, carries some heat away from the converter.
The Original Equipment Manufacturer (OEM) later discovered that the loss of clamping force, caused by the leakage through orifice, was not worth the extra cooling gain. For this reason the orifice is being phased out of production and will not be in new models. The TCC pistons in some foreign converters also had a similar orifice, but only in a limited number of applications. Today, the best approach for a rebuilder is to close the orifice with a small tig weld.
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Article No.: TCTIP-08-09
Author: Ed Lee
Total Pages: 2
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