1958 hydramatic...rebuild or swap for TH400??

[D. Mailan]

Hi everyone,
I recently have had some issues with my 58s hydramatic. It started slipping in 4th on the freeway when going about 55. I hardly drive on the freeway. Now its starting to slip around 35-45mph. The engine rpm is super high just to maintain speed.

I replaced the transmission fluid about 6 months ago. It was black. It was shifting good till recently. I am assuming the soft parts in the transmission are worn.

I put some lucas in and it is still malfunctioning.

I am debating whether rebuilding the original transmission or swapping in a th400 in.
The price seems to be about the same.

I preferably have my original transmission rebuilt but I am open to the option of swapping it. I drive the car very frequently around town and it is not a show car. It appears to me rebuilding it would be the easier route.

Has anyone done the swap before? Anything to keep in mind? Any recommendations/ thoughts/ or concerns that I should be aware of with swapping?

Anything to keep in mind when rebuilding a hydramatic? Are they reliable and well built?

Thanks!!
Hope everyone's doing well!

Derick

[dadscad]

My experience with the hydromatic is good. I'd rebuild what you have, it fits.

You don't have to alter the mounts, redo the drive shaft, or buy adapter kits for attaching the transmission to the engine or flexplate or make shift linkage or alter the dash shift quadrant indicator in the dash. Much cleaner to use what you have.

I can attest that the 58 car/transmission is tough and dependable. When I was 14 to 17 years old, (a long time ago) I had use of my family's 58 CDV. Believe me, the 58 cars are tough.

[The Tassie Devil(le)]

The good thing about the original transmission is that the 1st gear is really low, and this is good for getting the car up and going from a stop.   The TH400 1st gear is a bit higher, and therefore puts a bit of strain on the engine.

Plus, installing a TH400, you have to change the shifter quadrant on the steering column as the shift pattern is totally different, plus, the Neutral Start Safety Switch has to be replaced, as the Reversing Light operation is on the opposite end.

Then there is the adaptor, tailshaft alterations, etc, etc.   Sometimes it is cheaper to rebuild what you have than perform the swap.

Bruce.

[bcroe]

Continuing to drive a failing trans will just do
great harm and make repair far more difficult.
A TH400 is a great trans, but there is so much
different that a conversion is a monumental
project as described above.  To be done properly
the rear axle ratio must be changed to a much
lower ratio, to match the advantage of the torque
converter.  I once wanted to do the same, but was
never able to do it.  good luck, Bruce Roe

[TJ Hopland]

Just the adapter kit that lets you bolt it to the engine and still have a starter is $1100. 
https://transmissionadapters.com/collections/cadillac/products/1954-and-up-cadillac-365-to-chevy-automatic

If this still has the single front mount and the side mounts at the bell that is another problem. And you still haven't dealt with the shift linkage or different pattern that was already mentioned.  Then the drive shaft and gearing issues......

There are people that can rebuild those and soft parts are no problem and not that expensive.  IF you keep pushing it which it sounds like you may have here you could have hard part damage and those are a lot harder to find and more expensive but the parts are still out there. 

[Roger Zimmermann]

Another bonus: the Hydramatic is more efficient that the THM400. I'm using way more fuel with my '72 de Ville than I used with my ex '56 de Ville or my '56 Biarritz. Something to consider...

[Clewisiii]

This guy is good he rebuilt my transmission. About $2000 in total. Depends on what your shipping charges will be. He recently moved to North or South Carolina

https://www.classichydramatic.com/

[64\/54Cadillacking]

I would also do a rebuild. Too many modifications to convert to a TH400 and it might cost more in the end.

I plan on rebuilding the Hydramatic that's in my '54 Fleetwood this year as it leaks like it got shot in the chest, and it won't upshift anymore.

Plus these are great transmissions and I am sure once a rebuild is done, it will probably be better shifting than new and will last another 100 years.

[D. Mailan]

I figure that swapping to the th400 would definitely require some number of modifications. That is something that I dont want to go through. I was hoping for more of a drop-in situation for it. Like Bruce said, the neutral safety switch would have to be replaced and some modification for the TV rod. 

I think rebuilding the hydramatic would be more strait forward and less troublesome. Like David said, there is just too many modifications. If this transmission lasted 66 years, I'm pretty sure it can last another 66 after a rebuild.

Roger: I did read about their differences and heard that the original hydramatic reduces engine speed and tends to be more efficient than the th400. My dad also has a 56 Seville, with those dual quads, it never wants to skip a gas station!! Sometimes when idleing at a long light, you can watch the gas gauge drop! I cant imagine a car that gets wore mileage than the eldorados with dual quads!

Since I do drive my 58 frequently, that was also a concern of mine.

C.Asaro: Thankfully mine has never leaked. It seems the rebuild is the way to go.
 

[Jon S]

Rebuild!

[Roger Zimmermann]

Roger: I did read about their differences and heard that the original hydramatic reduces engine speed and tends to be more efficient than the th400. My dad also has a 56 Seville, with those dual quads, it never wants to skip a gas station!! Sometimes when idleing at a long light, you can watch the gas gauge drop! I cant imagine a car that gets wore mileage than the eldorados with dual quads!

When I still had my'57 Brougham with basically the same drivetrain than a '56 Eldo, I never got under 18 liters for 100 km (sorry, I don't know how to translate that in MPG). However, on long distances, my '56 Biarritz is using between 15 and 16 l/100 km. My '72 de Ville is using about 18 to 19 l/100 km.
Cars with dual carbs are extremely inefficient on short distances; I suppose this is one of the reasons why    many were modified with just one carb.

[Daryl Chesterman]

When I still had my'57 Brougham with basically the same drivetrain than a '56 Eldo, I never got under 18 liters for 100 km (sorry, I don't know how to translate that in MPG).

Roger I feel your pain, having to convert from metric to USA standard units!!!  I wish that the USA would just make the change, since most of the rest of the world is metric.  All of the scientific world is metric, and anyone involved in manufacturing that exports to other countries, has converted to metric, so the rest of us in the USA need to make the switch.  It would be difficult for us older people to make the adjustment, but the younger people would have no problem, then the switch would be done.  There would still be those of us who have to deal with the USA standard units on our old cars and equipment, but most of us have the tools already, and we are a small minority of the world's people.  The internet makes it really easy to make the conversions, when necessary!  In your situation, this website makes the conversion of liters per 100 kilometers easy:

     https://www.calculateme.com/gas-mileage/liters-per-100-km-to-us-mpg

In your example of 18 liters per 100 kilometers, that converts to 13.07 miles per gallon, which was decent mileage for a heavy car like your cadillac!

Daryl Chesterman

[Roger Zimmermann]

Daryl, thanks for that tool! Inches, miles are not a problem to "translate" but MPG and °F are more difficult...I have graphs for the conversion but, as I'm not at home, I don't have them with me...
I doubt that the US will ever switch to metric. The first time I came in the US (1979), I remember that the distances on the freeway were given in miles and km. The last time I was in your country, I saw only miles!

[bcroe]

The efficiency claim for the old hydro is based
on the idea it locks up better than the torque
converter input to the TH400.  But that ignores
the fact that the old hydro needs an axle ratio
well into the 3s because of its lack of torque
multiplication.  The TH400 can perform just as
well (much better here) with an axle ratio in
the 2s, so the economy of running the engine at.
much lower rpm completely exceeds whatever the
hydro might claim. 

I have taken that a step farther by using the
switch pitch version of the TH400.  It gives
even more torque multiplication (allowing an
even lower axle without performance loss)
while slipping even less in cruise mode.  I
quite regularly log MPG in the 19 to 30 range
at road speeds between 55 and 75 mph, cars in
the 4000lb range.  Bruce Roe

[The Tassie Devil(le)]

I figure that swapping to the th400 would definitely require some number of modifications. That is something that I dont want to go through. I was hoping for more of a drop-in situation for it. Like Bruce said, the neutral safety switch would have to be replaced and some modification for the TV rod....... .....Since I do drive my 58 frequently, that was also a concern of mine. 

The TH400 does not use a TV rod and this removal would be the least of your worries.

But, don't forget that back in the days, these cars were every-day drivers.

Bruce.