Originally Posted by GoldDot40
Some things are just going to fail regardless of how well you try to keep them serviced, maintained and 'babied'.
The weakest links in a GM 700R4/4L60 (and newer variants) are the input drum and sun gear shell assembly. The sun gear shell is a flawed design because they are a two-piece design. The upgraded aftermarket units are beefier and one-piece. The center spline ring will break and you essentially lose 2nd, 4th, and reverse. I know old grandpa's who've had this things fail...never pulled a trailer or hauled anything over a few hundred pounds in the back.
Another good upgrade is the input sprag...especially for towing. I will also echo the recommendation for a larger transmission cooler.
OE Reaction Sun Shell's were never a 2-piece design, Though the early versions did break into 2 pieces when they failed
, Then GM decided to reduce the heat treat on the splined area.....That led to stripped splines. Around 2001, They got the heat treating down pretty good, But can still eventually strip out.
Sonnax has a upgraded sun shell, They take a OE 4L65E shell & further heat treat the splines & weld a reinforcement around the splined collar to prevent breakage.
The "Beast" sun shell was the first successful attempt at completely eliminating sun shell failure, But SPX-Filtran sold the Patent to Pro-Select & manufacturing was moved to Taiwan. While still very strong....They didn't balance them & 2 out of every 3 were bent or otherwise had a ton of runout.
A bit of attention has to be paid to the Input Drum! I first check that the Input Shaft is not loose in the drum. Put it in my hot water cleaning machine at 250° for about an hour. While steaming hot I try my hardest to pull the shaft out of the drum. If I can pull it out by hand.....The unit gets a new OE drum!!!!
If the shaft doesn't pull out, I put it back in the machine for @ 10 minutes, Take it out & immediately press the shaft out with a arbor press.
Using a wire hanger to suspend the drum by one of the machined holes. Then take a screwdriver & lightly strike the drum, IF a crack is present it will have a dull short ring to it. IF no crack is present it will have a long crisp ring to it.
I apply Loctite high temp retaining compound to the shaft splines & press the shaft back into the drum & let it sit for 24 hours. Then temporarily install the old 3/4, Forward, & Overrun pistons, Friction & steels back in the drum.
With the drum sitting upright it the parts washer, Fill the cavity around the Input shaft with solvent, Place a finger over the 3/4 clutch bleed hole, & apply air to the 3/4 clutch feed hole in the Input shaft.
There shouldn't be ANY leakage from the spline interface or the check ball capsule, The Capsule is replaceable if leaking!
I then install Sonnax 77733-51K Input Drum reinforcement kit, And assemble the drum with new pistons & clutches.
The 3/4 Clutch is another item that needs it's share of attention! I start by leveling the 3/4 apply housing "Fingers". You got to use a T-Mic to check the distance variation between the apply plate & backing plate & then sanding down the taller housing fingers 'til it's level within .002".
I use 7 .080" Borg Warner High Energy frictions & 6 .077" Alto Kolene treated steels.....I adjust the clearance with GM selectable backing plates to .020"-.025"
In high performance/high RPM applications.....Use 700R4 Overrun return springs with the 4L60E bottom spring retainer & new Sonnax 3/4 Boost Springs 77763-01
The French made SFK Input/Forward Sprag used in the later units isn't so bad & I've seen them take some abuse. It's the Brass Races/Thrusts that wear out & allow the sprag to run cock-eyed.
A Borg Warner 29 element, Dual Cage, Steel Race sprag is the gold standard & what I use on all builds.
GM really missed the mark on calibrating the truck & fullsize passenger car 4L60E's!
They used a "553" 2nd apply servo piston & a large ratio Accumulator pressure control Bushing & Valve. Causing the 2/4 band to not have adequate holding power, But firm up the 1-2 shift with higher Accumulator pressures.
It's backwards!
Use a "093" 2nd apply piston (Known as a Corvette servo) for better 2nd apply holding power, And leave the bumper spring out of the Accumulator Valve......Or better yet, Find a "A" or "B" code Accumulator bushing & Valve & reuse the original spring. 700R4's are a good source for these. Most truck 4L60E's come with a "D/DX" code.
Be sure to add the secondary servo cushion spring GM# 8681195 when installing a 093 2nd apply piston!
Transgo has 5 different weight springs in their shift kits & a special pin that allows removal of the Bushing/Valve/Spring without dropping the valve body. This allows fine tuning of the 1-2 shift without much hassle.
Drill the 2nd apply orifice to .070"-.073", Don't fret.....You can adjust 1-2 firmness with the Accumulator Bushing/Valve/Spring! It's a lot tougher to pull the valve body & replace the plate because you followed some "Boy Racer" advise on the internet & drill the feed hole out to 1/16".
Set the 2/4 Band clearance to .060", Yes this IS tight. This can only be accomplished with a longer apply pin......Use Sonnax 77787-02K. Never shim the piston like Transgo states in their shift kit instructions!
Use a Borg Warner High Energy Band & a New GM Reverse Input Drum (What the band rides on). Resist the urge for Alto Red & Kevlar bands.
There's a lot more to tech share on this unit, But I think I'll stop here. This thread doesn't even concern 4L60E's, I guess they're the 6.0L PSD of the transmission world? But when built correctly....They can be REALLY good unit & actually prefer them over a 4L80E in performance applications! They can downshift straight from 4th to 1st in a instant & put more power to the ground!
If you're towing heavy or making more than 500HP/TQ &/or using power adders.....A 4L80E will be more durable!
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