Pigs Fly Garage

Howdy all,

1968 Oldsmobile 442 p/o installed a 68 455 and a th375 long tail shaft. I am looking to switch to a th400 regular tail shaft.
There is a th400 available locally for $150 out of a 75 caddy. It has a long tailshaft. It appears to be a "heavy duty" case(extra webbing and thicker pan mounting surface).
I imagine rebuilding the transmission and changing to a 4" tailshaft housing, and regular length output shaft.

Any thoughts? I am having to replace my driveline as it is now, and so no big deal there. The rear is a 1970 Oldsmobile 12 bolt.

Was 75 a good year for the th400?
Am I to have any unique modifications due to the swap? Most of my research shows a plug and play with the th375...minus the long housing and shaft.
Thanks,
pigs

joe_padavano

Yes, you can change the tailhousing. That also means changing the output shaft, which requires you to completely disassemble the trans. The output shaft is the last thing that comes out of the case and the first part that goes back in during a rebuild. Of course the TH400 output shaft has a larger diameter than the one on the TH375, so you'll need a new front yoke as part of the new driveshaft. Otherwise, that Caddy trans is a fine core. Just be careful that buying the correct output shaft and tailhousing may cost almost as much as buying a complete short-tail trans core.

Pigs Fly Garage

"Otherwise, that Caddy trans is a fine core. Just be careful that buying the correct output shaft and tailhousing may cost almost as much as buying a complete short-tail trans core."
Thank you. I think you're right on the price. I was thinking of just rebuilding the 375 to be a 400, but of course I need the output shaft, housing, and yoke...at least.
The th375 output shaft seal is leaking, probably also needs a rebuild, and I need a different drive shaft. All of these things pointed me to grabbing a "desireable" core, and just spending the time and money on that instead of remaking the 375. Of course if I tripped over a ready to go 200-4R...maybe, too many decisions, I just want to drive the damn thing.

Thanks again,
Pigs

matt69olds

The 375 will most likely have the thick stamped steel pistons, and only 4 frictions per drum. The “good” cores will have drums that will accept 5 plates. the Cadillac will probably still have aluminum pistons that can be machined. If the Cadillac case is heavy duty, it will have 8 pump bolts (GM started getting cheap in the mid to late 70s). You can drill the pump for the additional bolts if desired.

Unless this is something you want to build now and swap at your leisure, you would probably be better off just buying the short extension housing and shaft.


In any case, if either transmission has a plastic accumulator piston, replace it with an early aluminum one. Bad things can happen when that piston breaks.

Pigs Fly Garage

Excellent information, thank you.

"Unless this is something you want to build now and swap at your leisure, you would probably be better off just buying the short extension housing and shaft." - this is where I netted out as well. I like the idea of an "hd" case for chesp, and since I needed to do some work to the trans and driveline anyway, I thought why not? I appreciate the perspective.

Pigs Fly Garage

[QUOTE=matt69olds;1507536]The 375 will most likely have the thick stamped steel pistons, and only 4 frictions per drum. The “good” cores will have drums that will accept 5 plates."

Can I clarify something I don't have a great understanding, please?
I keep reading the th375 is a th400 minus a few discs. It's my understanding this is accurate to the point of being able to take the internals, output shaft, and tail housing from a 400, and then "simply" putting those freshly pulled parts directly into the empty 375 case with no modification to the 375 case.
Is this true no matter what year th400 parts are going into a 375 case?

matt69olds

There are some minor design changes over the long production run of the 400. There are a few different design pump cover and housing, not all compatible. The information is easily found online.

Model year 1969 and earlier used a thick center support that sat directly on the aluminum case. It was determined that the center support was hammering into the case. GM solution was to machine .040 off the bottom of the center support, and place a .040 steel snap ring between the center support and the case.

Try and put a thick center support on top of the snap ring makes it almost impossible to put the top snap ring and bolt in place. Put the thin support with no snap ring will result in a chattering 2nd gear and endplay problems.

The best book I have found it “How to work with and modify the 400”. Last I looked it was out of print.

There are also changes to bearings, thrust washers, etc, but for the most part other than what I pointed out complete assemblies will interchange.

joe_padavano

It is but .

Pigs Fly Garage

On the way. Appreciate ya, both.

matt69olds

That book is worth probably double what you paid. It has casting numbers, design changes, what parts are interchangeable, and some good tech tips.

Pay attention to the art discussing the updated bearings and oil restrictor in the intermediate shaft.

matt69olds

Never mind, since the current transmission is a’375, and the transmission you are looking at is a 75 model, they both should have the updated bearings with no restriction.

Pigs Fly Garage

I'm excited to geek out with this book. I appreciate your time in response.

Thanks,
Pigs

matt69olds

His TH350 book is just as good. Something to keep in mind whenever you want to tackle one of those.