Century Stage III 4L60E Automatic Transmission Rebuild - Forward Shifting

And sure, installing a heads-and-cam package in an LT1 or LS1 F-body is like sentencing your 4L60E to death row. Worse yet, in heavier machines like Impalas and GTOs, simple bolt-ons are enough to burn this notorious slushbox up. Granted it's tough to shake such a miserable track record, but recent innovations have made it possible to build a 4L60E capable of handling in excess of 600 rear-wheel hp.

While we can't blame you for involuntarily convulsing in laughter after reading that last line, Mike Kurtz of Century Transmission is about to make good on that claim. With GM's recent updates to the 4L60E, building one that can survive behind a brutal street/strip motor is an entirely feasible proposition. In 2001, GM upgraded the 4L60E's four-pinion front and rear planetary gearsets to more rugged five-pinion units, and renamed it the 4L65E. Other updates included stronger bushings and bearings. To meet the demands of the LS2-powered Trailblazer SS in 2006, GM added a stronger heat-treated output shaft and a robust steel reaction gear to create the 4L70E. When combining many of these new factory internals with the best of the aftermarket, the once-feeble 4L60E can now be transformed into a stud. "The 4L60E certainly has its limits, but they're much higher now than just a few years ago," Mike explains. "I wouldn't recommend one in a 9-second drag car, but when built correctly they work great in 10-second street/strip applications, and have a very deep 3.06:1 First gear."

Of course, a popular course of action by late-model GM fans up to this point has been ditching overdrive altogether in favor of the venerable TH350 and TH400. While there's nothing wrong with sticking with these battle-proven three-speed stalwarts, losing a gear in a modern EFI platform does seem a bit regressive. Besides, who wants to have their motors buzzin' on the freeway while the Pro Touring Chevelle or first-gen Camaro in the other lane is cruising comfortably at 1,800 rpm in overdrive? Furthermore, since the 4L60E is nothing more than a glorified 700-R4, the same tweaks presented in this buildup also apply to third-gen F-bodies and early C4 Corvettes as well. Without further delay, follow along as Century Transmission shows you how to build a bulletproof 4L60E.

Easy there, Turbo. Your frustrations...        read full caption Easy there, Turbo. Your frustrations are well justified, but don't give up on that 4L60E just yet. Yes, the GM automatic in question has let more than a few enthusiasts down in its day.	Just like inside an engine,...        read full caption Just like inside an engine, removing all traces of grit and grime is imperative when rebuilding a transmission. To accomplish this, Century loaded the 4L60E's case and tailshaft housing into a trans parts washer. To get things squeaky clean, the washer blasts the parts with a soapy water solution via high-pressure jets for an hour while rotating everything on a turntable.	In addition to spraying the...        read full caption In addition to spraying the case with a fresh coat of paint, the bottom side of the trans housing was resurfaced on a sanding block to ensure flatness. Like cylinder heads that can warp over time, the case casting can settle and create high and low spots where it mates with the valvebody. Smoothing the surface flat helps prevent leaks.
Century's basic overhaul kit...        read full caption Century's basic overhaul kit used in all of its 4L60E rebuilds include BorgWarner heavy-duty clutches and steels, and all necessary seals and gaskets. Since 4L60Es built prior to 1998 came with aluminum pistons that have a tendency to crack, transmissions of that vintage are upgraded with steel-bonded pistons that came as standard equipment in most LS1-powered cars.	The first step in the assembly...        read full caption The first step in the assembly process is tapping in a new bushing into the bottom of the case, which provides support for the output shaft.	Mike says that the stock output...        read full caption Mike says that the stock output shaft (left) is sufficient up to 450 rear-wheel hp. The cryogenically treated and shot-peened shaft (center) used in our Stage III buildup will handle just about anything a naturally aspirated street/strip machine can throw at it. For trucks and heavy vehicles like Impalas and GTOs that are running power adders, Century offers an optional 300M billet steel output shaft (right).