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If you are rebuilding a 4L60E transmission, get a good manual, either a factory service manual, or the ATSG 4L60E guide.
Below is a list of the weak points in a 4L60E transmission. These need an upgrade when putting more than ~300 horsepower through a 4L60E:
By the way, when General Motors upgraded the 4L60E from a torque rating of 360 ft·lb to 380+ ft·lb to make it the 4L65E, they made the following changes:
Here are some areas you should consider when rebuilding a 4L60E:
Pump: Pumps for the 4L60E can be found in 10 vane and 13 vane versions. Use the 13 vane pump. The 13 vane pump does not have a higher pump volume than the 10 vane pump. Instead, the number of vanes was changed to alter harmonics that caused the rotors to break up. You may also want to get the heavier duty 'unbreakable' pump rings from TransGo. If you use a teflon pump bushing, always apply Locktite® to it so that it cannot move forward. This is a common leakage problem with the 4L60E. Always replace the rear stator bushing since it is the sealing point for lubrication. On a performance build, you should replace all the bushings. It is inexpensive to do (if you have the tools). You also want to make sure that the splines on the stator are in good shape. Replace the boost valve with a Sonnax valve or TransGo, as these are frequently worn out.
Pressure regulator & reverse boost valve and sleeve: The reverse boost valve and sleeve in the pump should be replaced every 40,000 miles (65,000km) to avoid damage to the transmission. Damage can occur when wear in the boost sleeve causes insufficient line pressure in reverse. Many major repairs of the 4L60E at around 50,000 plus miles are because of the boost valve and sleeve. The valve and sleeve can be replaced easily during a routine service of the transmission for around $50. This should be considered a necessary procedure if you are transplanting one into your car. Oversized (0.490" versus the stock 0.470" or 0.420") reverse boost valves are available that provide a moderate increase in pressure, and are a good addition to any 4L60E.
As well, the pressure regulator valve (located under the reverse boost valve) should have two small 'flats' (~3/16" wide) ground on the second land - counting the lands from the end opposite that spring seat (or alternatively it should be replaced with an updated valve that has these flats). See the ATSG manual for more information.
Reverse drum: Put a straight edge across the band surface and make sure it is absolutely straight. The drum tends to become dished over time, which doesn't allow the band to make full contact over the full width. The center will be really low and the two outside edges high. A quick glance at the band will show you where it burned on the edges and the middle is still new. If your reverse drum is not perfectly straight, replace it.
Bands: Do not use Kevlar bands. Kevlar, the material bullet proof vest are made from, is very hard material which does not have the holding capacity of other, more conventional materials. If you are making a lot of horsepower, you will have excessive slippage on the 1-2 shift with a Kevlar band. Instead of Kevlar, use a performance band made by ALTO. The band is a wider and provides much better holding capacity than the Kevlar band. Shifts are crisper and the durability is very good with the ALTO band.
Input drum: This is the weakest component of the 4L60E transmission. The 3-4 clutches tend to burn up. Buy a 3-4 clutch set called the Raybestos Max Pac. Don't use Kevlar (see above) and don't use the Raybestos blue plate clutch set. The Max Pac is an 8 clutch set (instead of 6 in the stock clutch).
Some companies sell a 9 clutch set. You will get more clutch area, but with reduced the thickness of the steel plates. The thin steel plates will distort and burn in high power situations, so stay away from them.
Also, the Max Pac instruction require you to drill a 0.035" (0.9mm) hole at the back of the drum. This hole prevents any possibility of a centrifugal apply of the clutches.
On the 3-4 clutches, clutch clearance should be kept to a minimum, aim for 0.010" (0.25mm). This allows for faster 2-3 shifts and no burnt clutches.
You can get a hardened input shaft, but it is rarely necessary. Broken input shafts are very unusual in the 4L60E, but the input shaft can strip out the aluminum shell. Art Carr racing sells a shell that can help keep it from stripping.
Forward clutches: The forward clutches apply ONLY when you shift from neutral to drive. Then they remain on throughout the entire time you are driving forward. These clutches do NOT come on and off while you are driving, and as a result they experience minimal wear. You should focus on the clutches that come on while at high torque is delivered from the engine, as those are the clutches that get burned up. So use stock clutches (such as the Borg Warner brand) for the forward and over run sections. Keep the original front planet and ring gear, they have proven to be reliable in high power applications
The sun shell: The sun shell is a very weak point in the 4L60E transmission. This weakness must be addressed in high power applications. Always replace the sun shell. The best shell is the "BEAST" from SPX, they are nearly unbreakable.
For hard parts, the Beast or the GM #24221190 revised sun shell kit has worked in most cases, however the sun shell is always going to be a concern. The revised GM sun shell rarely fails and if it does, it's most likely to strip the teeth off the sun shell.
5 Pinion planetary gears are great. You can use the original GM from the 4L65E or you can use some aftermarket ones. Either way, these help by spreading the load among more pinions. And since you can't replace the Torrington bearing that is inside the planetary gears, so you might as well replace the planetary gear.
Low/reverse clutches: The stock clutches are fine here, but keep a loose clearance on these.
Low/reverse piston: This is the large piston at the rear of the transmission's internal components. Never reuse casting number 8681725, as it is known to have durability problems. Replace it with one that has casting number 8685550 (GM part number 8685549), which is a much stronger piece.
3/4 Load release spring assemblies: Remove and discard the 3/4 load release spring assemblies (5 small rectangular spring assemblies installed into the input housing, on the outside of the clutch pack). This really improves the 2-3 shift.
Since you are inside the transmission, replace all the electrical components (wiring, solenoids and switches) inside, especially the variable force motor solenoid (pressure control).
Lastly, install the largest transmission cooler you can find and fit. Get one like a car's radiator, called a 'plate' type cooler. Keep the transmission cool (maximum life occurs around 170°F) and it will last a long time.