4L80E Molded Clutch Piston Kit, Hi-Per Blue 34088

This is the Seal Aftermarket Products Hi-Per Blue piston kit, part number 34088, for the GM 4L80E and 4L85E. The package is labeled for 1991-on units and contains two molded clutch pistons. Per the label, it does not include the forward clutch sleeve piston, so reuse yours or order it separately if it is damaged.

A molded piston has the sealing lips bonded directly to the piston body instead of using separate loose lip seals. That means no rolling or pinching a lip seal during assembly, and the bonded rubber holds its shape and sealing pressure longer than an old seal that has taken a set from years of heat.

When to replace them: slipping or delayed engagement after the unit is hot, a flare on the shift into the affected clutch, or burnt frictions on teardown are all signs the clutch is not getting full apply pressure. Leaking piston seals are a common cause. If you are already doing frictions and steels on a 4L80E, put new pistons in while the drums are apart.

On the bench: compress the return spring with a clutch spring compressor, remove the snap ring and spring assembly, then lift the old piston out with air pressure through the apply circuit. Clean the drum bore and check it for scoring. Lube the new piston lips with assembly lube or ATF, press it squarely into the drum by hand with an even rocking motion until it seats, then reinstall the spring and snap ring. Air check the clutch before it goes back in the case.