Later transmission transplant into 56?

[Caddy Wizard]

A club member here in GA has a 56 that has had lots of trouble with the 56 Jetaway transmission.   Well, it is broken (again) and won't shift out of first gear.


Before he submits to having it rebuilt yet again, does anyone know from experience whether a later transmission will drop right in?  For example, could we replace it with a 58 Cadillac transmission?  If so, would we need a different rear cross-member and/or rear rear transmission mount?  Would we need a different driveshaft?


Finally, assuming that this sort of swap is feasible, does anyone have a good used 57-and-up transmission that will work for him?  PM me if you do...

[Lexi]

Hi Art. One of my '56 Cads had what I believe was a 1957 Olds Hydramatic casing, with what I presume was a '56 Cadillac bell housing and tailstock. Never had it out or apart, but judging by all of the transmission problems in 1956, that is how this one was finely "tamed". I mention this for what its worth. Might be a starting point for the answers to the local CLC member, as it looked as if this just "dropped" in as it still had the same front and back parts. Only the mid section, i.e. the "brains" (casing had been changed), based on what I could read on the spec plate. Did this necessitate other changes (for example linkage)? Not sure as I did not do the conversion, but it looked to be the same. It also shifted really smooth. Hope this helps if even a bit. Clay/Lexi

[gene harl]

I have a 700r4 in my 59  .. it seems to work good,,  went right in ,, shift linkes and speedo was the only problem... the 700r/4 is a GM ovrdrive trans ... I am stilll working on it ,,,,if you want to talk my # 775-423-8568                          Gene


















































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[fishnjim]

One call to Fatsco or Northwest may give that answer.   

[Roger Zimmermann]

From all 1956-64 Hydramatic transmissions, the 1956 model is the sole with a long rear extension. By using a 1957 to 1964 Hydramatic, the original drive shaft will be too short.

[Lexi]

Roger is correct, in that the '56 transmission tailstock is hugely long. That is why whoever did the conversion I noted above, retained the original tailstock and bell housing, so all would nicely fit back into the car. I am assuming the '57 casing, (the part changed), must have had the same physical dimensions but the parts inside were an upgrade from the problematic '56s. Clay/Lexi

[TJ Hopland]

If he wants to consider going newer here is a place that makes an adapter kit to go to a newer Chev auto.   You sill have to deal with linkage and drive shaft but at least the kit takes care of the rest.

https://transmissionadapters.com/collections/cadillac/products/1954-and-up-cadillac-365-to-chevy-automatic

[Roger Zimmermann]

The modification can be done, if you know what you are doing! The following '56 parts must be retained: the extension, the output drive shaft, the reverse drive flange and the rear sun gear with the main shaft. The selective thrust washer between the reverse drive flange and the output shaft assembly must be choose to have the proper play.
The figure 12-116 from the shop manual, page 12-85 is showing those parts.

[TJ Hopland]

Roger, can you swap those parts without getting into the transmission?  Or does it have to be done as part of the rebuild?   I know with the THM stuff anything related to the output shaft pretty much requires a full tear down since the output shaft is pretty much the first thing installed in the case.

[Roger Zimmermann]

Honestly, I'm not sure. I never tried that because when a transmission is coming to my shop, a complete disassembly is the sole way to check for bad parts. By looking at the drawings/pictures, it could be possible to exchange the output shaft and the associated parts when the rear unit parts are removed. Easy? Maybe not.
For a specific issue on a '56 transmission, I'm those days trying to determine the proper selective spacer by having the rear unit drum on his "head". That way, with the drive shaft upwards, I will be able to feel if I have the proper play when the selective spacers I ordered will arrive. This specific job could probably be done on a partially assembled transmission.

[Roger Zimmermann]

I went to my shop this afternoon. To exchange the drive shaft, the whole internals must go out because the reverse drive flange diameter is larger that the rear case diameter...

[Caddy Wizard]

From all 1956-64 Hydramatic transmissions, the 1956 model is the sole with a long rear extension. By using a 1957 to 1964 Hydramatic, the original drive shaft will be too short.

Roger,

I am not worried about using a longer driveshaft -- that is not a big deal.  But will the cross-member with the transmission rubber mount line up if I use a 57-58 transmission?

[Roger Zimmermann]

From the pictures I have, the rear support is at the same place on all transmissions. To be sure, I should measure...