69' 400 THD work with 427/390HP Rebuild

[Don W.]

I wanted to get some thoughts on my auto trans 400 THD while the engine is out and being rebuilt. My initial thoughts would be to have it removed, gone through for a complete check out and re-sealed. Any other thoughts? Don

[Jim T.]

Re: 69' 400 THD work with 427/390HP Rebuild

Don I am the original owner of my 1970 350/300 turbo 400. Only thing I have done is change the fluid and filter as regular maint. I did take the turbo 400 out once and replaced the front and rear seals. I replaced the torque converter because the front seal had worn a ring on the snout and with a new front seal I did not want any leakage. It was leaking just a little when I did the replacement. I also replaced the seal for the shift selector shaft. Have over 160,000 miles on it now. Last time I tried it, it would still chirp the tires going WOT shifting from first to second.
If the turbo 400 can handle the torque and horsepower of a L88 or a LS6, my 70 SB will never hurt it.

[Steven B.]

Re: 69' 400 THD work with 427/390HP Rebuild

Don, I don't know how you will use the car but assume on the street desiring a little extra performance. Years ago I rebuilt my TH400 for autocrossing and track days (Mid Ohio, IRP, etc.) and along with GM parts (seals, converter, etc.) I installed a B&M shift kit. The performance increase is noticeable and improved times. If you aren't going to do much serious driving I would suggest a rebuild with GM parts and forget about it for the rest of your life--it will last that long. Mine has as much as I have beat it.

Steve

[Terry M.]

Re: 69' 400 THD work with 427/390HP Rebuild

If the turbo 400 can handle the torque and horsepower of a L88 or a LS6, my 70 SB will never hurt it.

Jim, The TH400 that is in the L88 and LS6 (and the L71) is not quite the same TH400 that is in your 350/300. That said the TH400 in all its Corvette variants, is a strong transmission. It does not suffer abuse (nothing does), but properly maintained it will last a long time.

Don: How many miles are on the TH400 in your Corvette, and do you know what kind of driving was done with it? Has it had regular maintenance?

[Rich W.]

Re: 69' 400 THD work with 427/390HP Rebuild

If it were mine I would rebuild it..but that's me since I can myself..really though if it worked well before a simple reseal would be an easy thing to do..a shift kit would be a nice touch but not really needed..it would be nice to have some line pressure readings before the motor came out..stall rpm and such..the more difficult thing is you are used to how the trans shifts..have you been in one where the trans was stock but new or rebuilt? I notice many times driving different cars that the people who own them don't realize that something is worn out..or getting there..untill it stops working or close to it..has there been any fluid and filter changes in its life? That's usually all they need..

[John D.]

Re: 69' 400 THD work with 427/390HP Rebuild

How would a TH400 with a noticeably non stock shift feel from a shift kit fare in PV judging ?

[Don W.]

Re: 69' 400 THD work with 427/390HP Rebuild

Hi Terry...the car has about 67,000 miles on it. The prior owner had it for the 1st 65,000 and put a 4.11 rear in it and it is now back to the correct 3:08 rear. I assume it has been abused with the 4:11 rear...Don

[Terry M.]

Re: 69' 400 THD work with 427/390HP Rebuild

Then Don I would rebuild the transmission while the engine is out. A lot of the R&R labor is already done, and you will get a fresh start with it.

I know that is easy for me to say, because like Don I could do it myself -- but even if I had to have someone else do it I would. See if you can find someone who will reuse the good parts from your transmission, rather than mixing parts with other transmissions. There probably isn't much danger of that now days because TH400s are rather "old school" anymore, but it is a base you want to touch with whoever does the work. It would also be to your advantage to find someone, likely with gray hair or little hair, who has experience with them.

[Ray K.]

Re: 69' 400 THD work with 427/390HP Rebuild

I also agree with Rich and Terry, I would rebuild it, and this also would be something I could do myself. I would look at the clutch packs and replace the lined discs, maybe the steel plates, and look at the various bushings in the unit. If nothing else shows up as abnormal wear or abuse while it is dis-assembled then it should not be to costly. New seals, gaskets, filter, and fluid will get you going.

Also check the trans yoke for bushing wear on the O.D. surface.

Ray

[Terry M.]

Re: 69' 400 THD work with 427/390HP Rebuild

I haven't bought a TH400 kit since I got rid of the Orange Suburban. I just gave away the TH400 that was stored in school to one of the students. I don't think I will ever own one again. Last kit I bought was in the $300 range, but that could have been 15 years ago.

[Tom L.]

Re: 69' 400 THD work with 427/390HP Rebuild

Don, several years ago a month after I bought my vette on a cool november morning the car wouldn't move. I bought a B&M rebuild kit and did it myself. I've never done one and it wasn't that bad, the directions were clear and specific. It wasn't that bad, you just need to be organized. There's a lot of parts in those things. If you want I can look up the exact kit I used and give you the number. Good Luck!!!

[Tom L.]

Re: 69' 400 THD work with 427/390HP Rebuild

I don't remember noticing any torn seals but the clutches were worn. I measured them, don't remember what the measurement was, and compared them to the new ones. They were quite worn. I assume (hate to do that) that the cool weather didn't allow everything to expand enough for it to work. Only a few days earlier it seemed to work OK. Like amany things in life, these cars are a steep learning curve.

[Terry B.]

Re: 69' 400 THD work with 427/390HP Rebuild

Hi Don,

I would also suggest like the others, to have it rebuilt. One thing I did was have the transmission case cleaned too and lost the paint on my transmission label. Try to have it protected if you have the case cleaned too. I think they dipped it in something. I should have asked more questions before the process. Good luck.

Terry

[Terry M.]

Re: 69' 400 THD work with 427/390HP Rebuild

The case tag can be removed -- and you should before cleaning if this feature matters to you. It is held on with drive rivets, but the C1 guys have lots of experience with these on generator tags.

[Clem Z.]

Re: 69' 400 THD work with 427/390HP Rebuild

i ran a stock 400 THM behind a 482 cu in 500+ HP BBC in my pickup for 5 years without a problem. this included many red light drag races with either dirt bikes or a snowmobile in the bed for traction.