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My 4L60E rebuild kit supplied a approx 1/4" cup plug. Is that to plug the LO/1st(accumulator bleed) in the trans case. There is a captured ball in this location now. For picture reference--The captured ball is illustrated but not labeled in the following link. Go to the bottom illustration "transmission case". The item is the one on the right. http://www.artcarr.com/instructions/documents/17050.pdf
This will eliminate the hydraulic cushioning effect that the 1-2 accumulator has on the apply of the 2nd gear servo piston. In other words it makes the 1st to 2nd gear shift much harder.
Would I recommend installing it? No! It results in a very hard, harsh shift all the time (part throttle included).
There is a much better way to adjust the 1-2 shift firmness: Find some washers that have an ID that will fit over the accumulator piston shaft (pin). Vary the amount of washers to limit how much the 1-2 accumulator piston can move. You know can adjust the 1-2 shift firmness by changing the washer stack height.
I didn't install the supplied plug anywhere. I still don't know what it was for, I was only assuming it went in the case. I would still like to know for future reference.