8L90 Transmission Sealing Ring Expander and Resizer Kit

This is the expander and resizer pair you need to get the Teflon sealing rings back onto a GM 8L90 eight-speed without ruining them. The 8L90 uses solid (non-scarf-cut) Teflon rings on the clutch drums and shafts, and those rings have to be stretched over the groove land to go on at all. Stretch them by hand or with a screwdriver and they stay oversized, which means the drum will not slide into its bore, or it goes in and shaves the ring on the way, and you have a pressure leak you cannot see once the case is closed.

The kit works in two steps on the bench. The expander cone slips the ring up and over the shaft or drum land and drops it into its groove without nicking the ring or rolling it. Then the resizer sleeve goes over the ring and squeezes it back down to bore diameter and holds it there for a minute so it takes a set. After resizing, the drum drops into the case bore by hand the way it is supposed to. Do the expanding and the resizing on every ring in the unit, not just the ones that look tight.

The 8L90 is the GM eight-speed used behind the 5.3L and 6.2L in 2015-up Silverado and Sierra, the Camaro SS, the Corvette C7, and the Cadillac CTS-V and Escalade. It is a transmission with a reputation for shudder and harsh 2-3 and 4-5 shifts, and a chunk of that reputation comes from sealing ring and fluid problems rather than from hard part failure. If you are already in the unit for a torque converter or a valve body, replacing the sealing rings with the correct tooling is cheap insurance against going back in.

Bench note: warm the rings slightly before expanding if the shop is cold, and never leave a ring sitting expanded while you go do something else. Expand, seat, resize, and move on to the next one. A ring left stretched for an hour will not come back down to size.