The bore plug in 4L60-E units is held loosely in place with a coiled spring pin, allowing the opposing spring and oil pressure forces to stroke it within the bore.
This constant oscillation causes the plug to wear and allows PR oil at the low overrun valve and 3-4 clutch oil at the 3-2 downshift valve to exhaust at the abuse valves. This excessive oil loss can result in delayed apply of the low/reverse clutch and a tie-up of the 3-4 clutch and 2-4 band. Sonnax forward and reverse abuse bore plug 77754-21 resolves these problems.
This plug is used in two locations in the valve body: to separate the forward abuse and low overrun valves, and to separate the reverse abuse and 3-2 downshift valves.