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4R70W Rebuild Diary - Part 9 - Reverse Shell and Sun Gear


EF Tranny Guru
Moderator Emeritus
February 8, 2003
Reaction score
City, State
Sacramento, CA 95827
Year, Model & Trim Level
1992 XLT
Part 9 - Reverse Shell and Sun Gear

It is called various things in various transmissions, but present in all of them. It is the outer shell into which usually some set of clutches or a planetary gearset, and the sun gear in the shell engages the planetaries, either in the shell, or on the other side or both.

Ford calls it a "Gear and Shell" and it is FORD part no. F4SZ-7A019-A

This is the part where the "dog ears" on the reverse drum fit and as such it is a wearing part because of that fact. When you mate it to the reverse drum the max play should be about 1/16 inch. My advice? replace. Likely Both.

Here is a reposting of pictures from the first thread about this issue:


I'll post a few more pictures to illustrate this


can you make out the wear? Here is a dog ear close up... this is not "terrible" wear, but evident.


and here is a look at the input (drive) shell where the dog ear engages


Well compare it to a new one...


And the lug that was worn (I have one much worse than the one I showed before).... well here is a reman'd one...


I have been told that they repair these by mig welding hard steel onto them and grinding them, that they are even better once done that way as the wearing surfaces are harder... here is evidence of the truth of the method, anyway...



The proof is in the pudding, as they say, and once I fit the new sun shell onto the reverse drum lugs... the fit is ***PERFECT***... no slack whatsoever!


I suppose that the sun shell could be fixed by welding too, but I do not have a MIG welder [Edit, just got one, still learning to use it]. Anyway the new sun gear shell was $80. And it IS new.

(more to come here picture wise - this is an important area deserving careful attention)
Through the sun gear comes the other (forward-direct connecting) stub shaft. This is a busy area !

(pics coming)

not much else to say really. I think this is likely a "replace in any quality rebuild" item, along with the reverse drum.

Next thread is Part 10 - Forward Drum