TH400 Information Thread

[DoubleDingo]

@NickTransmissions Holy Moly! Just watched the video you provided and the assembly video. I don't have the tools or the spot to do this work. Sucks, because I would like do it myself for the experience. If I can't find a reputable place in town I will be reaching out to see if you'd be willing to take mine on. I'm about a 5 hour drive away.

 

[KCKKen]

Yes, there are for sure different dipsticks. Not sure how many, there are many though since they're intended for many different model cars, trucks, vans, RV's etc

There is a deep pan and a shallow pan, and the difference is very obvious. One is I'd say an inch deepl the other usually found on trucks is close to 3 inches deep. So if yours that you compared looked the same, then they are the same.

Awesome for you and the variable pitch Th400. I had a VP Th400 out of a 67 Olds that was behind an Olds 425. I wanted to use it too just for the cool factor of having 2 different stall speeds. I wanted to make a street/strip truck and use a toggle switch for close to stock stall, and hit the switch and go into about a 3000 stall. Well, as many awesome awesome projects and opportunities I've had with cool parts, that I had to sell to make ends meet, those parts got sold on eBay for about 5 times what I paid for them, not to mention all the other spare parts I had too. But yeah, get that done and let us know how it goes. I had trouble finding someone that build my converter the way I wanted it built and I knew it would be expensive if I did find someone, so that was part of the decision in selling the parts to someone who may have already had the converter they wanted.

What is sittting for an extended period? You know an older transmission that sits for several months or years will tend to harden the seals in the clutch drums and such and it won't move. After warming up a bit, you might get a little action out of it as the seals soften just a bit. If that's the case, I've had some real good luck in the past by adding a trans additive. I know I know, I too had additives to anything, engine, trans, anything, but I have to say, it's been more than once or twice where I've seen additives help an auto trans that sat for quite some time. The additives seem to soften the seals and make them a bit more supple and pliable to where the transmission will work again. I'll also add, if it does work out, don't have high expectation for a long lived transmission. But hopefully, it'll be patch job to get you by for a couple to a few months to when it's a better time to get a rebuild done. You can add 1 bottle and see what happens. If it's needs 1 more, go for it. If 2 bottles don't do it, then give it up, it's not going to work this time.

Hi Everyone!

Finally got settled in enough to reply back: the issues were all my own damn fault.

Turns out that the transmission had a deep pan already, I just added one with a drain plug. I was relying on the factory capacity for a shallow pan, and I was way too low on fluid. Once I really saw the fluid level on the dipstick it's been working just fine. Git it out on the highway finally and it's great, now I can put it to use.

Next issue: determining if the right speedometer gear is installed. The previous owner said he had "replaced" the transmission, I'm wondering if he reinstalled the right gear or not. First thing I need to do is determine the rear gear (78 C20 2WD) and then pull the speedometer gear and see how many teeth I have. But for now I can drive it, and I'm happy with that.

Take Care,
KS in KCK

 

[DoubleDingo]

My wife used to work in the transportation department of the high school district, I had her ask around to her old co-workers in the shop area if they had any recommendations on a solid transmission shop locally. I got a hit from one of the supervisor's. I'll be checking out All Automatic Transmission either this week or next. He also gave me the name of the guy to ask for. We'll see how it goes. If they do me right they'll get more of my business, as my AW4 in my Jeep needs to be gone through as well.

The trans tune additive has helped with the TH400 in the '81, but with everything being so old and it sitting most of its life, those seals have to be brittle inside there, and who knows how much varnish. I did run the modulator in another two more turns, it shifts a little better, but still not that TH400-sit-you-back-in-the-seat 1-2 shift unless I manually shift from 1 to 2, then it slams you back in the seat.

Any suggestions on what to ask the guy, or mention when I go in? Recommended trans kit for a step up from stock? I want to ask if he opens up and inspects the pump, and if he checks the case for true-ness at the valve body.

 

[82sbshortbed]

Maybe put a shift kit in it. They're pretty cheap.

 

[ChuckN]

@DoubleDingo sorry if it’s not exactly applicable, but after much research seems like TransGo is a reputable kit, but that’s in the realm of the TH350. Nick may have a different idea- he has been helping me a lot with my TransGo install.

 

[NickTransmissions]

My wife used to work in the transportation department of the high school district, I had her ask around to her old co-workers in the shop area if they had any recommendations on a solid transmission shop locally. I got a hit from one of the supervisor's. I'll be checking out All Automatic Transmission either this week or next. He also gave me the name of the guy to ask for. We'll see how it goes. If they do me right they'll get more of my business, as my AW4 in my Jeep needs to be gone through as well.

The trans tune additive has helped with the TH400 in the '81, but with everything being so old and it sitting most of its life, those seals have to be brittle inside there, and who knows how much varnish. I did run the modulator in another two more turns, it shifts a little better, but still not that TH400-sit-you-back-in-the-seat 1-2 shift unless I manually shift from 1 to 2, then it slams you back in the seat.

Any suggestions on what to ask the guy, or mention when I go in? Recommended trans kit for a step up from stock? I want to ask if he opens up and inspects the pump, and if he checks the case for true-ness at the valve body.

Install Alto high energy frictions, Sonnax TH400-LB1 boost valve kit, dual feed the direct clutch, drill the spacer plate per application, shift firmness preferences and converter stall. If pushing more than 500 hp, id also install a billet rollerized forward clutch hub. Rollerize the rear of the case as well.

You dont need any shift kits for those units - waste of money...

 

[Bextreme04]

Thank you for your speedy reply.

That's what I thought you would say. Is it okay to drive it like this until I get the parts to rebuild it? If I opt to try the rebuild myself, do I have to clean the entire case, or can I just make sure the internals are cleaned? Granted, I would scrub and clean the outside the best I can before tearing it down. I can always see if Pop is willing to help me, but I don't know if he's wanting to do this stuff anymore.

This being a work truck, and no more power adders will be installed, what kit/parts do you recommend? I need to order them sooner than later.

Having done a few of my own rebuilds now... my personal recommendation will be to take the whole thing apart and inspect for obvious damage, then put all the parts in a plastic tote with the stripped case and head down to your local transmission or machine shop. Give them a few bucks or a case of beer or whatever they want to put all of that through their hot pressure wash tank. Takes about an hour and comes out cleaner than you could ever get it with 10 hours of elbow grease in your own garage... trust me.

Cleanliness is everything with a transmission, they have so many itty bitty clearances and spots for metal to get stuck and hide.

 

[DoubleDingo]

I really want to try the rebuild myself, but I don't have any spot to do it unless I do it in the backyard with all the dust. Not ideal at all. Sucks, because a lot of money could be saved. I'll write down the key points mentioned above and ask good questions when I talk to him. Getting some fast responses today, I can now go talk to him sooner than later. Thanks.

 

[DoubleDingo]

I don't know what the HP numbers are but they're no where near 500. A gen1 sbc with vortec heads and the old school 300hp/327 camshaft that really woke up the engine. With the 4.10 gears this truck is very quick off the line but slow on the highway...lol...

@NickTransmissions This truck is not being used to race on the streets or highway, just want a good reliable build to last another 43 years like the factory setup did.

 

[Bextreme04]

I don't know what the HP numbers are but they're no where near 500. A gen1 sbc with vortec heads and the old school 300hp/327 camshaft that really woke up the engine. With the 4.10 gears this truck is very quick off the line but slow on the highway...lol...

@NickTransmissions This truck is not being used to race on the streets or highway, just want a good reliable build to last another 43 years like the factory setup did.

Lots of the parts are the same between the TH400 and 4L80. I've got a 400 behind my stock high compression 1970 455 that was rated at 370hp/500ft-lb and it has no performance upgrades at all. Runs and shifts great.. no issues ever.

My 1980 K25 still has the original Th400 and I regularly tow and beat on it with no issues also. It has a mild rebuild 350 in it with the compression bumped up to 9:1 and a Summit 1102 cam in it. No issues with the stock TH400 ever either.

I rebuilt a 4L80 to go into the K25 with the mildly build Gen VI 454 and the only upgrades I put in it are a sonnex line to lube PR valve, sonnex no walk bushing kit with the roller rear bushing/shims, HD intermediate snap ring, and manual dual fed the directs. I also did the transgo TCC regulator valve and AFL repair kit during the rebuild because the donor trans had 288k miles mile it behind the big block and I assumed they were probably worn. Other than that it was rebuilt with Borg-Warner HD clutches and new steels, new 2004+ solenoids and wiring harness, and a rebuilt stock Big Block converter.

 

[DoubleDingo]

Lots of the parts are the same between the TH400 and 4L80. I've got a 400 behind my stock high compression 1970 455 that was rated at 370hp/500ft-lb and it has no performance upgrades at all. Runs and shifts great.. no issues ever.

My 1980 K25 still has the original Th400 and I regularly tow and beat on it with no issues also. It has a mild rebuild 350 in it with the compression bumped up to 9:1 and a Summit 1102 cam in it. No issues with the stock TH400 ever either.

I rebuilt a 4L80 to go into the K25 with the mildly build Gen VI 454 and the only upgrades I put in it are a sonnex line to lube PR valve, sonnex no walk bushing kit with the roller rear bushing/shims, HD intermediate snap ring, and manual dual fed the directs. I also did the transgo TCC regulator valve and AFL repair kit during the rebuild because the donor trans had 288k miles mile it behind the big block and I assumed they were probably worn. Other than that it was rebuilt with Borg-Warner HD clutches and new steels, new 2004+ solenoids and wiring harness, and a rebuilt stock Big Block converter.

The 4L80E is basically a th400 with OD, right? Each of them are great transmissions. Your K25 350 is probably close in power as my engine. I was thinking a stock rebuild on this TH400 would suffice. There are so many kits, so many videos, waffle frictions, smooth frictions, dual feed, it is a little overwhelming for a first-timer trying to figure it all out and make a decision on which route to take.

 

[Bextreme04]

The 4L80E is basically a th400 with OD, right? Each of them are great transmissions. Your K25 350 is probably close in power as my engine. I was thinking a stock rebuild on this TH400 would suffice. There are so many kits, so many videos, waffle frictions, smooth frictions, dual feed, it is a little overwhelming for a first-timer trying to figure it all out and make a decision on which route to take.

Yeah, basically. They share quite a few parts. I'm sure Nick could give actual specifics, but I know that some parts are shared.

My K25 TH400 has the original truck VIN stamped on it and I don't even think it has ever been rebuilt. When I did the engine rebuild a few years ago I found the torque converter nose had a chunk missing, so I got a used unit from someone locally for $50 and slapped it in. It had some leaks, so I fixed those while I had easy access. Basically pulled the pump and put a new gasket on it, took it to a local trans shop to see if the guy had the right stuff in stock and the guy cleaned the pump, pressed in a new bushing and seal and assembled the pump in about 15 minutes for $20 including the cost of the seal and bushing. I also pulled the governor and inspected it, replaced the gasket, and put the cover back on. The last thing that was leaking was the shaft seal and I just bought the kit with a new seal and the puller/installer on amazon for ~$15. Haven't had a single issue or leak since and I think I've put ~15k miles on it since then in the last two years. That includes several trips pulling my trailer through the mountains on hunting trips.

 

[NickTransmissions]

I don't know what the HP numbers are but they're no where near 500. A gen1 sbc with vortec heads and the old school 300hp/327 camshaft that really woke up the engine. With the 4.10 gears this truck is very quick off the line but slow on the highway...lol...

@NickTransmissions This truck is not being used to race on the streets or highway, just want a good reliable build to last another 43 years like the factory setup did.

Then a quality build with a rollerized output thrust, Sonnax LB1-TH400 kit, feed orifices enlarged a tad per your preferences along with the usual stuff (clutch module, new seals, sealing rings, o-rings, etc plus detent solenoid) should be sufficient...I'd have him install 4L80E center support one-piece teflon sealing rings on the pump and center support. Your builder can take it from there...

The 4L80E is basically a th400 with OD, right? Each of them are great transmissions. Your K25 350 is probably close in power as my engine. I was thinking a stock rebuild on this TH400 would suffice. There are so many kits, so many videos, waffle frictions, smooth frictions, dual feed, it is a little overwhelming for a first-timer trying to figure it all out and make a decision on which route to take.

The aft section (center support, gear train) plus direct drum are all a direct carry-forward design of the TH400 while the front half of the case, architecture-wise, is based on the 200-4R.