Rebuilding my first 4L60E

Rebuilding my first 4L60E

By in Mechanic Tag , ,

Building a automatic transmission was on my bucket list. It’s one of those things every person should do at some point in the path to becoming a master mechanic. I purchased a broken 4L60E core off craigslist back in 2022 to do just that. There was one nuance with the transmission I never could sort out but other than that the build turned out awesome. It’s 2026 now and I finally blew it up so we are going to tear it down and see what happened then rebuild it again.

The original build

The hardest part of the build was figuring out what parts to use. I ordered my first rebuild kit only to discard most of the parts. The quality so was poor I couldn’t bring myself to put them in the transmission. Most of the parts discarded were shift kit parts.

The custom transmission shops I spoke with wanted to sell me a kit because this requires minimal if any transmission tuning. I planned to tune my transmission so I didn’t want to mess with any more modifications to the 4L60E than necessary.

Many people who build up trucks like mine give up on the 4L60E and go with a 4L80E. However, those take more horsepower as well as some modifications to the chassis. I wanted to stick with factory fitment stuff, especially if I could conserve some power.

How the trans broke

I have also have a built up 6.0L I put in the truck. I don’t ever drive the truck hard. However, a good song was on the radio and I got that itch. I turned onto a county highway and put the pedal down. Yup the truck is very fast.

First gear as well as reverse started slipping on my drive home. Everything else worked fine. I parked the truck and noticed a leak in my trans cooling system! The ATF was so low it barely wet the stick! Adding fluid seemed to cure the problem at first but the symptoms returned within 3 miles of driving.

Shortcomings of 4L60E Design

With some research and talking to other experienced builders, I came up with a set of 4L60E weaknesses to address in the build.

It’s worth noting many people go down the valve body rabbit hole. They delete the TCC partial apply valve and or modify the valve body. I did not do this. I chose to keep the valve body in it’s original configuration.

The guys I spoke with had built hundreds of these transmissions. They felt those issues were a bit exaggerated on the internet and they didn’t feel it was a good idea to modify the TCC apply circuit which is what I ultimately did.

Hydraulic leaks The hydraulic system eventually leaks hydraulic pressure which can lead to clutches not locking up and burning up in process. The plan was to address these with Sonnax brand (factory certified parts) design with this shortcoming in mind.

Valve Body most factory transmissions prefer to use the steel check balls in the valve body. These check balls hammer their way through the transmission (separator plate) ultimately leaking pressure and burning up clutch packs. Utilize plastic check balls.

Clutch Packs some of the clutch packs are a bit weak in the factory transmission. When possible upgrade those with better clutches. Including the band clutch.

Sun shell some of the factory sun shells have a tendency to break at the splines, use a heat treated one, specifically the Sonnax sun shell.

Input Drum given age of these transmissions (the rebuildable cores) most all the input drum / input shafts leak. Press out the shaft and reseal it. Then reinforce the input drum with the Sonnax collar they sell. Money wasn’t permitting in this project but ideally a modified, 4L79E input drum would be desirable.

Here is the link to my original parts list from my first build.

Drilling the seaparator plate

This was a problem with my original build. I sourced parts from two places. The first customized them to a shift kit I ultimately didn’t use. I got rid of most of those parts but kept the separator plate which was drilled for the shift kit. The transmission despite multiple tuners and myself fiddling with it couldn’t ever smooth out the 1->2 shift which always was a clunk. I think this may have been due to the plate having been drilled incorrectly.

Installed separator plate specs, drilled for a shift kit

I’m told by Nick’s Transmissions that the following configuration would have been more appropriate considering I omitted the shift kit. The gentleman who helped me build the transmission has a recommended configuration as well.

Recommended NickRecommended TravisOriginal Plate
1 to 20.070″ to 0.076″0.082″0.086″
2 to 30.093″0.110″0.110″
3 to 40.093″Leave alone0.110″
Band Release0.093″
Lo Reverse0.086″
Note: this is with a aftermarket torque converter.

More to come… I’ve yet to tear down the transmission.

Matthew Jeschke

I've been hard on jeans since childhood when my single most important job was to stuff my Fisher Price camera in my pocket and set out to explore. My mission was to photograph animals I wanted to make pets such as squirrels, chipmunks, and rabbits. A sense of adventure propelled me deeper into the grassy fields of rural Nebraska. My trusty sidekick Domino (pet dog) was a great companion always by my side. An old Schwinn bicycle served my iron horse. We set our sights set on the distant horizon over which, somewhere, the rolling hills of Nebraska had to turn into mountains. Years later I landed on that distant horizon asked to make aerospace parts whatever those must be. I was a long ways from those squirrels and endless cornfields in Nebraska. I'm also few boot sizes bigger from those days, but still armed with my cameras and and a sense of adventure. Only this time I have a vast desert and dozens of mountain ranges to explore. The wilderness here hasn't much changed since the original Spanish colonists arrived nearly 500 years ago. Heck I imagine this place is not much different than when God created it. What has changed is the internet. Nobody grew up dreaming of being an internet sensation, rotary phones were still the big thing! The only viral videos to be found were on Saturday morning of Wile E Coyote chasing the roadrunner.

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