5R55S Reaction Shell, remanufactured and ready to install. Fits the 5R55S. Ships fast from the USA.
A 5R55S that bangs and then loses drive, growls in some ranges but not others, or leaves steel flakes on the magnet in the pan has often stopped being a valve body problem and become a gear train problem. That is the point where rebuilders working on 2004-and-later Explorers and Mountaineers, 2005-2010 Mustang V6 cars, and 2003-and-later Lincoln LS and Thunderbird models start pricing a reaction shell. This listing is a remanufactured 5R55S reaction shell: original Ford hard parts that have been cleaned, inspected and made ready to go back in the case, so you are not trying to reuse a shell with cracked welds or worn lugs.
Ford's service information describes the 5R55W/S as a five-speed with three compound planetary gear sets, three bands (overdrive, intermediate and low/reverse), three multi-plate clutches (coast, direct and forward) and two one-way clutches. Every gear other than direct needs one member of a planetary set held still so the other members have something to push against. That held member is the reaction member, and the reaction shell is the stamped steel link that carries that holding force between the gear train and the band or one-way clutch doing the holding. It never gets a rest: it is loaded in the low gears and in reverse, and it spins with the gear train the rest of the time. When the shell cracks at the hub, its splines round off or its lugs wear, the gear it supports slips or disappears, and pieces of it go through every other part in the case.
Most 5R55S slip and engagement complaints are hydraulic. Sonnax traces many of them to valve body bore wear and to solenoid problems tied to early Mercon V fluid, and servo pin bore wear in the case is another common cause of a slipping band. Start with the fluid level check at the plug, pull codes, and work through our 5R55S no movement diagnosis and the P0730 gear ratio guide. If the unit still has good pressure but a gear is missing, or you have a noise that follows road speed plus metal in the pan, the case has to come apart. A shell is the right part when you can see the crack, the rounded splines or the chewed lugs on the bench.
This shell is listed for the 5R55S. Per Sonnax, the S replaced the 5R55N in the 2003 Lincoln LS and Thunderbird and replaced the 5R55W in the 2004-2005 Explorer and Mountaineer, and the S also went into the 2005-2010 Mustang V6. The S and W share a lot, but Sonnax warns that only a few 5R55N parts carry over to the W and S, and the two later units use different overdrive sun gears (38 teeth in the S, 24 in the W). This listing does not give a Ford part number, so lay your old shell next to the photo and match the spline count, the lug pattern and the overall depth before you install it, or send us your year and VIN and we will check it for you. It is not a part for the 5R55E used in Rangers. Everything for this family is on our 5R55 transmission parts page.
If the gear train came apart, the frictions and seals were cooked along with it. Pair the shell with a 5R55S master rebuild kit and a fresh filter and pan gasket kit so the new hard part is not living with old debris.
| Fits | 5R55S |
| Part type | Reaction Shell |
| Condition | Remanufactured |
| Brand | OEM |
| Ships | From the USA |
| Warranty | 1-year |
Doing this repair right usually means replacing these at the same time. Same transmission, ships together.
It is listed for the 5R55S. Many W and S hard parts are close, but they are not all the same, so compare your old shell's splines, lugs and depth to the photo or send us your year and VIN before ordering.
No. The 5R55E is a different gear train descended from the A4LD. Shop the 5R55E listings on our 5R55 page instead.
Hydraulic faults usually give you every gear with bad shift feel, or a slip that comes and goes with temperature. A cracked or stripped shell gives you a bang, a gear that is simply gone, or metal in the pan. Confirm with codes, a fluid check and a teardown inspection.
Thrust washers and bearings, the filter, and usually a full rebuild kit, because the debris from a failed shell damages frictions and bushings. Inspect the low/reverse band, one-way clutches and mating splines while the unit is open, and flush the cooler.
Mercon V. The unit has no dipstick. Ford's procedure checks the level at the fluid level indicator plug in the pan with the engine running and fluid temperature between 81 and 120 degrees F.
Yes. Clear codes and perform an adaptive relearn drive so the PCM resets the shift pressures it learned while the old parts were failing.
