700R4 Reaction Sun Gear (Rear)


This is the rear most sun gear in the 700R4 automatic transmission.  It’s official name is the reaction sun gear.


  • ’87-’93:  700R4/4L60.
  • ’93-up: 4L60E/4L65E/4L70E

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Sun Gear (700R4 reaction)
700R4 reaction sun gear.

New 700R4 Reaction Sun Gear. 

Surface Hardness VS Pitting:

When closely inspecting the reaction sun gear, it’s not uncommon to find pitting in the metal.  The pitting looks like spots.  It occurs on transmissions that have seen high mileage, poor maintenance, etc.  The metal on all gears have a surface hardness.  This provides a tough surface to withstand very high pressures.  This hardened surface is very thin, much like a skin.  When pitting is observed, the “skin” has been damaged.  Once this happens, the gears days are numbered…

Heads Up:

This sun gear meshes with the rear planetary carrier, which meshes with the output(rear) ring gear.  In the event that the reaction sun gear self-destructs, it will very likely damage these other components too.

Learn More:

If you’d like to learn more about inspecting the reaction sun gear, click on the video below.


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