1st Winner of Guess the Code

First winner of TCRA “Guess the Code” contest announced

ImageImage A few months ago we appealed to you to help us crack the code – the GM code – and come up with a verifiable way to figure out the damper load rating or style, the type of friction material, or any other variable characteristic that may be identifiable from the exterior of a GM converter. We offered whoever could solve at least a piece of the puzzle the undying gratitude of your fellow rebuilders.

We have a winner! Robert T. Holt & Sons, a well-established transmission shop in Mount Pleasant, Texas, with an in-house converter rebuilding department, has come up with a virtually fool-proof way to tell the difference between a 400-style converter with metric threads and one with standard threads.

When we first announced our “Help Break the GM Code” contest, Gary Rogers, the manager of Holt’s converter rebuilding, had this response: “We should win this contest because we have been breaking GM’s codes for years.” We just had to put them to the test.

Do you know how to tell the difference between a 400-style converter with metric threads in the mounting pads from one with standard threads – without checking the threads? Without revealing their secret to us, the folks at Holt proceeded to show us how to correctly identify the proper threads on numerous 400-style converters.

Their secret (also GM’s secret for many years) is this: There is an identification groove just above the welds that attach the mounting pads to the cover on all 400-style torque converters with metric threads. (See Figure 1.) A typical converter with standard threads will look like Figure 2. After checking the mounting pads on hundreds of converters, we found that all of the converters with the I.D. groove did have metric threads. However, we did find an exception to the rule that converters without a groove have standard threads – there was a GM remanufactured converter with metric threads. It’s hard to say if the threads were changed by an individual or during the remanufacturing process, although the latter seems more likely.

The information Gary has gleaned about the 400-style converters may well apply to other converter styles as GM tends to stick with processes once developed. His tip may help someone else solve yet another piece of the puzzle.

The purpose of the “Help Break the GM Code” contest is to share knowledge of what is inside certain 245, 258, 298 and 300mm converters, from a visible identification on the outside of the converter. The 13” 400-style converter is unquestionably a GM converter. It was first introduced by Buick in 1964, became the flagship of the GM converter line and remained a staple until nearly the end of the century. Knowledge about this converter most certainly can be gained from visible markings on the outside of the converter, and it should prove helpful to many torque converter rebuilders and transmission technicians.

As we thought this information about one of the most popular converters ever produced was especially valuable, we decided to give the first “Help Break the GM Code” winner’s certificate to Gary Rogers at Robert Holt & Sons of Mount Pleasant, Texas.

– ©2005 Sonnax Industries, Inc.


– June 2006 Newsletter