400's are a little tricky to rebuild. A couple of the clutch packs have a very tight-tolerance fit between the piston and its bore, so getting the clutch assembled without damaging the outer piston seal is a challenge. One sneaky trick is to leave the piston, with seals installed, in a refrigerator overnight to shrink the seal a little, then install it quickly with LOTS of Vaseline as assembly lube before it warms up. There are also two identical diameter lock rings associated with the center support and intermediate clutch, one with a chamfer, and one square-shouldered one.

DO NOT INSTALL THE TAPERED RING IN THE GROOVE IN THE CASE!

If you do, the transmission will make one 1-2 shift, and the pressure of the clutch discs against the tapered ring will dislodge it. You'll have to tear everything down and swap positions with the lock rings. The square-shouldered ring MUST be installed in the case groove ahead of the intermediate clutch! Don't ask me how I know that!
Jerry

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