• is this 1955 "improvement" ...discussed on a prior thread
• on the 56 Hydramatic?
• ie, I've got 2 in front of me : a 54 and a 56
• which one should i have rebuilt ?

 

For what year engine and car? The Jetaway is the '56, they added Park, it's a different generation of the Hydramatic from the early fifties. A secondary fluid coupling was also added. The '54-'55 were tilted for clearance (called the slant pan). So, it depends what car you are putting it in and how original you want to keep it.

 

The '56 is a Hydramatic. The '54 and '55 are also Hydramatics. The '56 was a revised Hydramatic and they also called it the Jetaway.

 

yes, you win!

 

The 54 and 56 are basically the same transmission. One difference is the rear tailshaft seal. My 54 had a 55 engine and transmission in it. I found that out when I had the transmission rebuilt. On the 55 the seal is actually up inside the tailshaft instead of being in the rear of the housing. If I had the pick between a 54 and 55, I would take the 54 in a minute.

 

Just a few things to note. Oldsmobile used the "1955" version of the original Hydramatic in most 1956 Oldsmobile 88s. These transmissions have a 1955 serial number on their tag. The 1956 Oldsmobile 98s and Super 88s should all have the Jetaway Hydramatic transmission.
Part of this was due to a fire at the Hydramatic transmission plant. The other part of this decision was to reduce the harshness of gear shifts. The Jetaway was designed to produce a softer gear shift.
A softer gear shift is produced by allowing more slip in the gear shift. This increased slip results in increased wear of the clutches and bands. The "luxury car market" desired a smoother/softer gear shift of the automatic transmission, thus the change to the Jetaway transmission.

 

Actually the hydramatic plant burned in 1953. Some 53 Olds and Cadillacs had Buick dynaflow transmissions and some Pontiacs had powerglides.

 

Thanks for correcting me, I am always willing to admit when I make a mistake. After doing some more research, the fire was one of the reasons for the redesign of the Hydra-Matic. The main reason was an effort to smooth the gear shifts.
Apparently I read something that said the fire was later and the main reason they changed production to the Jetaway.
Wally

 

Here's a couple of pics of a '53 98 Holiday with a "Dynaflop" at OCA nats in Alburquerque in 2017.







Sounded like a motorboat when it took off from there.