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german6563 08-15-2015 04:32 AM

Help with my transmission
Hello everyone.
I will have a 1977 Argosy 24 and cracked my transmission where broken? in the bell housing, I never saw anything like it.
Now I'm looking for an equal, by the data I have is a TH 400, but which? , I thought it was a heavy duty, but did not see any HD Discription
Now I need your help, where is the difference between a transmission TH 400 for example to pick up and transmission for Argosy with 6.2 tons?
I bought a 400 TH last week of a Chevrolet G 30 but the output of the transmission is thinner in diameter my transmission.
So before continue spending money and throwing it away, I need your advice
Thank you
greetings from Germany

StevieB 08-15-2015 05:06 AM

I would recommend that you visit a local transmission speciality shop. They might be able to assist you with some minor modifications to make the recently purchased tranny adaptable to your existing set-up.

Kota 08-15-2015 05:03 PM

Help with my transmission
As stated above, it might be as simple as swapping out the tail housing and output shaft from your transmission. That does require disassembly of the transmission. Not a bad idea to get it freshened up.

Not knowing much about your transmission, you tight be able to swap the yoke on the front of the driveshaft or have the driveshaft modified.

german6563 08-16-2015 12:11 AM

thanks for your reply ,
but I saw on the internet, to change the driveshaft, must disassemble the entire transmission, oh the other thing is that the HD Transmissions come with a disc more.
StevieB for your information, I used to ask here in the forum I visit 6 different professionals, and none of the 6 said the same thing, so it may be that one of the 6 this correct or none of the "professionals" know nothing

Keyair 08-16-2015 01:43 AM

The 3 speed transmission fitted to most of our motorhomes is a HD version of the TH400, or 3L80, and is called the TH475, or 3L80HD.
I am told that the main difference between the 400 and 475 are straight cut planetary gears instead of helical cut.
This makes the transmission stronger, and able to take the weight of the motorhome.
There is also a difference with our TH475's as they have the parking brake on the tailshaft.

If the transmission is in good shape, why not have the case welded?

german6563 08-16-2015 12:01 PM

hello keyair
Thanks for the explanation, helped me a lot,
I answer to your question, why not ?, weld the Simple, housing that the tempering of the weld material is lost, so it's back to stiffen the aluminum (I say that working with aluminum) and my experience, time becomes long break, when aluminum does strength, welding is fine but if it breaks soon
Now I have another question,
all the houses of transmission are the same inside? (TH 400)
if so, only I have to change and ready
Thank you

TomKirk 08-16-2015 12:47 PM

Make no mistake, TH 400 is the GM workhorse of all time. Generally speaking, the only differences are bell housing bolt configurations (engines are different across GM car lines) and things like output shaft diameter and extension housing (tranny mount "location, location, location"). All the guts, the important stuff, are pretty much interchangeable.

At a minimum, you're gonna need a case. But I've got a feeling you're gonna have a tranny builder go thru it anyway. And when you do, you're gonna want install a B & M RV kit.....and, you're gonna hafta swap out some of your old parts - like in this case, the tranny extension housing and rear output shaft. You should also talk to the tranny guy about swapping out the torque converter, valve body, and front pump. You may also want a deeper pan (for increased tranny fluid capacity).

Just so you know: The one ton chassis varies across GM truck lines. A P-30 Series is not the same as a G-30 Series. The P-Series is significantly heavier duty all across the board. But that doesn't mean your G-30 TH 400 isn't up to the task. Because it is - no doubt about it.

Before going any further, you should talk to an experienced, old school, after-market tranny builder at an independent tranny shop (where transmissions is the only work they do). You should also do a little after-market research online to bring you up to speed.



Keyair 08-16-2015 02:01 PM

All valid info Tom.

I believe that the lighter duty transmissions have a 27 spline output shaft, but our have the stronger 32 spline.
I would also add a 34 element sprag, Kevlar frictions.
A deep cast pan will add capacity and also stiffness/strength to the case.

Digging around on the web I found this PDF which might be useful...

Look at the well known TH400 builders sites, like Monster Transmission, and these guys on how they build strong TH400/475 units and it will help you get a list of upgrades to put in if you build.
TH400 TH475 Transmission

I DO NOT know if there is a difference between a normal TH400 case and a HD case, but there may be...
Here is a picture of the TH400 that came out of my 1987 Suburban 454 3/4T 2wd.
Note the HD cast in the upper RH side of the bellhousing...

Other side.

martin300662 08-16-2015 02:03 PM

Guys, please note Herman's location. In Europe in general even standard turbo-hydramatic transmissions are about as common as hens teeth, and whilst each town may have a guy capable of repairing a 6 speed sequential Mercedes box the chances of there being an old school tranny guy is practically zero, and no doubt he had to search high and low to find the G-30 box.

My advice Herman and what I would do if I was in the same position is look for a rebuilt/warrantied tranny State side and ship it.

NavyCorpsman 08-16-2015 02:38 PM

I use the Fairbanks kit and a B-3 HD torque converter. Its a real stump puller.

Keyair 08-16-2015 03:21 PM

I did note that Martin, being a displaced European myself.

The TH400 is not actually as rare in Europe as you might think, but its definitely not mainsteam. Rolls and Aston Martin have them, as well as a host of other V8 powered specialty cars.

Your advice is valid, and unless he can find a shop that knows how to build them... even then, dependent on shipping/core charges I might still be the way to go.

martin300662 08-16-2015 04:34 PM

Sorry Keyair I wrote and posted my message almost exactly as you posted yours and I hadn't seen yours before I did.

The future is daunting as even in the US fewer and fewer repair places seem to want to touch our units, and there are less and less old school guys around. For example I have decided to change out the steering/suspension for SuperSteer units and have phoned around every truck repair place in a 20mile radius of my place. I am still waiting on a reply from a garage near streamquest (Charly), but all the others, including a step van specialist basically said 'no chance we don't work on those'. It doesn't matter they are a truck chassis - no one is interested.

Grosspoluter 08-16-2015 10:01 PM

Me to😠
Ok I'm going to pay close attention to this thread as I have the same problem
I was lucky enough to find a uncracked used th475 off of a sob. RV
I've thought about removing it and having it welded but wasn't sure if welding aluminum was strong enough.
Also wondered about making some sort of screw on bracket / brace / bandaid sort of thing?
I'm also worried If I give my transmission shop a th475 to rebuild I'll get a th400 back (I'd never know what the planetary gears inside look like)
Also interested in the aftermarket up grades torque converters etc. other people used or installed
Has anyone here swaped transmissions? Is this something a backyard guy like me can do, and save a little money doing it myself? (Not the rebuild just the pull.and install of the same type trans?)

Keyair 08-16-2015 10:54 PM

Not a problem Martin!

Yes, pulling the trans is not tricky... maybe easier in the MH than most due to space... But I might be wrong.
I have done maybe 4 now and none of them have been as bad as the 4 other trans removals I have done.
The TH400 is a pretty simple beast, and there is LOADS of aftermarket support for it. Any good trans shop should be able to build you a beast that will last longer than the stock unit.

The "Straight cut" vs "helical cut" planetary is this simple...
Helical gears run quieter, and also are slightly weaker as a result of the "end load" they produce under extreme loads. Straight cut gears are stronger but noisier.

The TH475 will last longer, and perform better with a few upgraded parts.
A deep pan and external cooler are very important, and because the transmission does not have a "Lockup" Torque convertor, there is slippage and extra heat developed that needs to be dealt with.

The next step would be to replace it with the next generation transmission... The 4L80E, or its HD counterpart, the 4L85E.
These 4 speed Electronically controlled transmissions will bolt behind a 454 no problem, but are longer and heavier, so will require some X member fabrication, and careful parts selection for the parking brake, and some driveshaft shortening.
The great news is with an 4th gear Overdrive, and a lockup converter, the word is that at least 10% gas mileage improvement. Being electronically controlled, they also need a controller... which runs at least $600, or more if you want to get into pushbutton or paddle shifters!:bb:

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