I've been building various automatic transmissions since 1958, when I sat in on a class my father was teaching for local mechanics on the Dual Range Hydramatic used in mid-50's GM cars. The Turbo 400 is one of the toughest transmissions out there, surpassed only by the Torqueflite 727. A good rebuilder will clean and inspect all the parts, and replace whatever is necessary to get it back into good running shape. On a high-mileage trans, replacing bushings is a good idea. Also, inspect the pump gears and housing for wear. Lots of whining noises can come from a worn pump, before the hydraulic pressure is affected. End play is important, determined by the thickness of the thrust washers used during assembly. One of the advantages of the 400 over other transmissions is the center support which locks into the case with a snap ring. Turbo 350's, 200's, etc, don't have that feature. It makes for a much stronger assembly. The 4L60 is the overdrive version of the 400, and it's even better.
Jerry
