Three new bonded pistons for the 4L60E input drum: 3-4, forward and overrun. Fits 1994-up GM units and stops apply-pressure leaks at the piston lip.
A Silverado or Tahoe that slips into third, a Camaro that flares on the 2-3 under throttle, or an S10 that came apart with burned 3-4 frictions all share a common question: why did the pack burn? Very often part of the answer is in the input housing, where three pistons share one drum. This set replaces all three, the 3-4, forward and overrun pistons, with new bonded parts for the 1994-up 4L60E input housing. It also fits the 4L65E and 4L70E.
Laid on the bench, they are easy to sort. The 3-4 piston is the smallest diameter of the three and applies the clutch pack that carries third and fourth gear. The forward piston is large and has drive teeth around its outside edge. The overrun piston is also large and carries a check ball, which lets the apply oil exhaust when the clutch releases. On these pistons the rubber lip is bonded to the steel, so the old step of fitting separate lip seals, and the chance of rolling one, is gone.
Clamping force on a clutch pack comes from oil pressure acting on the piston, and a lip that has gone hard or taken a set lets some of that oil escape. Sonnax names leaks in the third-gear oil circuit and low line pressure as the root of most 4L60E 3-4 slipping, which is why a builder who puts new frictions behind an old 3-4 piston often sees the same burn again. The overrun piston matters too. Its check ball has to seal for the overrun clutch to hold in manual ranges and exhaust cleanly when it releases. Our 4L60E overrun clutch guide explains what that clutch does and what it feels like when it fails.
A worn piston is not the only cause of these symptoms. A worn pump, a leaking third accumulator or a stripped sun shell can produce similar complaints, so inspect the whole unit. With the housing apart, look for scored bores, a cracked housing and worn sealing rings. Sonnax describes vacuum testing the input housing circuits and the checkball, which shows a leak you might miss by eye. Before you buy anything else, our 4L60E common problems guide sorts out which symptom points to which part.
These are the current-design pistons for the 1994-up 4L60E input housing and also fit 4L65E and 4L70E housings. They can go into a 1982-1993 700R4 or early 4L60 housing only with the updated return spring assembly, GM 24206085, which is not in this set. If your housing is from the changeover years or has been swapped before, compare the old pistons to the new ones before you start. Send us the year and VIN if you are unsure. Find more internal parts on our 4L60E parts page.
A 4L60E rebuild tool kit covers the spring compressor work, and if the frictions are burned, a high friction deluxe rebuild kit pairs well with these pistons.
| Pieces | |
| Includes | |
| Fits | |
| Early 700R4 / 4L60 (1982-1993) | |
| Weight |
Doing this repair right usually means replacing these at the same time. Same transmission, ships together.
They are the 1994-up design and fit 4L60E, 4L65E and 4L70E input housings. A 1982-1993 700R4 or early 4L60 housing needs the updated return spring assembly, GM 24206085, which is sold separately.
Replace all three while the housing is apart. The lips age together from the same heat, and a leaking forward or overrun piston will cause its own engagement problems after the rebuild.
You need a clutch spring compressor to remove the return spring and snap ring, and a lip tool or smooth thin wire helps the lip go in without folding. An air nozzle is needed to check each circuit afterward.
Only if the piston is the leak. Third-gear slipping can also come from low line pressure, a worn pump or a leaking accumulator. Check those too, and replace the burned 3-4 frictions, or the slip will come back.
GM specifies Dexron VI for the 4L60E family. Use it to lube the pistons during assembly and to fill the unit.
