Forward, overrun, and 3-4 clutch bonded pistons for a 4L60E or 4L65E rebuild, seals molded on so they seat without fighting cut lip seals.
Rebuilding the input housing on a GM 4L60E or 4L65E out of a Silverado, Sierra, Tahoe, S10, Blazer or Camaro? This three-piece set, part number A74960EK, replaces the forward clutch piston, the overrun clutch piston and the 3-4 clutch piston with new pistons that have their rubber seals molded on. It is the part to reach for when a core comes apart with cracked pistons, hardened lip seals, or a forward or 3-4 pack that burned because apply oil was leaking past the piston instead of clamping the clutches.
All three clutches in this kit live in the 4L60E input housing, the drum splined to the turbine shaft. When the valve body sends apply oil into the housing, each piston is pushed against its clutch pack to lock it. The forward clutch is applied in every forward gear, the 3-4 clutch locks in for 3rd and 4th, and the overrun clutch applies to give engine braking when you select a manual range. A piston can only clamp as hard as its seal lets it, so any leak at the inner or outer seal bleeds off pressure and lets the pack slip and heat up. Bonded pistons put the seal on during manufacturing, so there is no separate lip seal to stretch, twist or nick on the way in, which is a common way a fresh rebuild ends up slipping on its first test drive.
Be careful about blaming the piston alone. Sonnax lists a cracked apply piston as one obvious leak in a 3-4 burnup, but warns it should not stop the search: pump cover cracks, input shaft sealing, the input housing checkball and the 3rd accumulator can all leak the same oil.
With the housing loaded, air check each clutch. A healthy pack applies with a clean thump and releases cleanly; a hiss or a weak apply points at a seal. Sonnax also recommends checking the input housing checkball, which should hold 25 in-Hg of vacuum. Our 4L60E problems guide covers how to tell a clutch apply leak from a pressure problem in the pump or valve body.
The listing fits the 1994-up GM 4L60E and 4L65E. It also retrofits 1993-1996 units when you update the forward clutch return spring at the same time, because the earlier spring design does not match. That covers the 4L60E and 4L65E in most 1994-2013 Chevrolet and GMC rear-wheel-drive trucks, SUVs and cars. Housings do vary, so compare the new pistons with your old ones before assembly, and send us your year and VIN if you want us to check. Every part we stock for this unit is on our 4L60E collection page.
For torque specs, clearances and the full input housing teardown, the ATSG 4L60E repair manual is worth having on the bench, and a new GM 2-4 band is cheap insurance while the unit is open.
| Type | Bonded clutch piston set, 3 piece |
| Fits | GM 4L60E, 4L65E (1994-up); retrofits 1993-1996 with updated return spring |
| Pistons included | Forward clutch, overrun clutch, 3-4 clutch |
| Seal type | Bonded (molded-on) rubber seals |
| Part number | A74960EK |
| Does not include | Return springs, frictions, steels |
Doing this repair right usually means replacing these at the same time. Same transmission, ships together.
It is listed for 1994-up 4L60E and 4L65E units. It retrofits 1993-1996 transmissions if you also install the updated forward clutch return spring.
No. It includes the forward, overrun and 3-4 clutch pistons only. The springs, frictions and steels come from a master or banner rebuild kit.
The seal is molded onto the piston, so there is no lip seal to roll, nick or install backward. That removes one of the most common causes of a forward or 3-4 clutch slipping right after a rebuild.
Only if the piston was the leak. Pump cover cracks, input shaft seal problems, the input housing checkball and the 3rd accumulator can cause the same failure, so check those before closing the unit up.
Air check each clutch with the housing assembled. You should hear a firm apply and a clean release with no hissing.
