[Schurkey]

The TH400 is "THE" easiest-to-rebuild-as-a-DIY-project 3-speed transmission. All the gears in the back, all the clutches in the front, no pistons in the case. A danged fine unit; similar to but more sophisticated than the Mopar 727 because in "Drive" the 400 never has to release a friction (band or clutch) to engage the next gear--it just adds another clutch on the upshift. Therefore, shift timing is easier.

1. I really liked the B&M rebuild instructions when I did my first 400 about a thousand years ago. Still have a photocopy of them in The Library downstairs. They make a big deal out of drilling various holes in the hard-parts "oversize"; when I've checked the 400s I've gone through, all those holes were already at the sizes specified. APPARENTLY, drilling those holes applies only to very early 400s. Mind you, I've got little use for B&M shift kits, but the rebuild instructions are excellent in terms of pictures and simplicity. They don't cover bushing replacement and other, more-in-depth items. And when it comes to shift kits, I'll take TransGo, thank you very much.

2. Bushings are a great idea...if you can install them properly. If you can't push 'em in correctly, you're better-off with the originals if they're in usable condition. Of course, any bushing that's obviously worn-out or damaged has to "go".

3. Not a big fan of the plastic thrust washers. Yeah, lots transmissions have them and they work just fine. Given a choice, I'd replace 'em.

4. Far as I know, rear end-play and front end play are the only end-play measurements to be concerned about. If there's more than that, I don't know about 'em. (That does not include "clutch clearances" of course.)

5. I've heard really good things about the 4L80E pistons with molded-on seals. Never use 'em personally. About the same with the solid Teflon sealing rings. I used the metal rings with the hook ends; I've used Teflon scarf-cut seals, (Put the scarf-cut seals on properly!) but "I'm told" the solid Teflon rings are the way to go...if you have the expander tools and the re-sizer tools; and virtually nobody in a D-I-Y capacity will have that.

6. There's "special tools" to select the proper pin-length for the band apply pins. Again, "good luck". Everyone I know just re-uses whatever pin came out of the transmission.

7. Get the center support properly loaded, and the bolt torqued to spec. I keep a socket JUST for that use, so that I don't wear out the socket points on other fasteners, and then wreck the wrenching surface of the center-support bolt. Most folks wouldn't bother with that. What I'm saying is "don't use a worn-out socket" on that bolt. This is a lesson I learned from 12-point Honda cylinder head bolts; if the socket isn't PERFECT, "good luck" getting the head bolts out without tearing up the wrenching surface, or rounding them off. Look for case wear from the center support moving around.

8. I put two lubricated O-rings on the filter tube where it shoves into the pump; and two lubricated O-rings on the modulator where it goes into the case.

9. Verify Governor operation, and tune the governor as needed to get the shift points where you want them. Don't dick with the modulator to tune the shift points, at least not excessively. If the original, huge, black OEM modulator isn't defective, I use that in preference to the small, compact, inexpensive aftermarket modulators. The OEM modulators are altitude-compensated (barometric pressure compensated).

10. MAKE SURE THE TRANS COOLER TUBES ARE NOT RESTRICTED, AND THE TRANS COOLER(S) ARE CLEAN AND NOT RESTRICTED. Given a choice, I'll use 3/8 cooler tubes instead of the more-common 5/16 tubes. Flush the cooler(s) to get debris out. If they're too bad, they'll have to be replaced. Restrictions in the cooler circuit can destroy the crankshaft thrust bearing/crank thrust surface. Some guys restrict the fluid passage into the converter to prevent over-pressurizing the converter leading to thrust problems; I don't know that that would be needed in your situation. Have a local trans shop flush a used torque converter, too. But I suppose you'll have a new or rebuilt converter for your assembly.

11. The TH400 world seems to revolve around the 34-element sprag clutch on the Direct drum. That's what I've used when I can get a suitable drum to use it with. HOWEVER, I have heard that the roller-clutch is just as good, and way cheaper. This is not something I've experimented with. If other folks tell you that the roller clutch is a troublemaker, I suppose you should believe them. IF you use a sprag clutch, or IF you're buying a core direct drum from some unknown transmission, MAKE SURE THE SPRAG/ROLLER CLUTCH SPINS THE PROPER DIRECTION. The rear-wheel-drive versions of the 400 (375, 400, 475) spin the clutch one direction, the front-wheel-drive TH425 has an identical drum generally with the extra clutch plate inside; but the clutch spins the opposite direction. DO NOT SCREW THIS UP.

12. Consider the ATSG manual, the Cliff Ruggles TH400 rebuild/modification book, and--if you can find it at a reasonable price--the out-of-print Ron Sessions TH400 rebuild/modification book.