CALLING ALL DYNAFLOW GURU's Question About Dynaflow differences

My55buick

New member
I dont know if anyone has ever asked this question, I cant seem to find it. I know bell housings and designs have changed over the years, but do all Dynaflows share the same external dimensions? Im asking the question because Ive never found a 100% answer to my idea. I want to swap another Nailhead into my 55 special, Ive found 364's and 401s reasonably priced, but they use a different bell housing. Then I read that you can swap the bell from a 57 only 364 Dyna flow onto the 55 Dynaflow, then Ive heard they bell is cast into the trans and cant be swapped. Then I started thinking, they improved the Dynaflow over the years maybe a later one from whatever donor car I choose would fit. But if the Trans is longer then I have to get a shorter torque tube and god only knows whats involved in that, or if the trans mount is in a different spot etc... So I was thinking we have a lot of smart guys on this board and to help my self and everyone else maybe we can compile a list of year to year differences. It would be nice if we could just look them up like a TH350 or 400 and know all the various measurements.
 

jnelson

Member
I dont know if anyone has ever asked this question, I cant seem to find it. I know bell housings and designs have changed over the years, but do all Dynaflows share the same external dimensions? Im asking the question because Ive never found a 100% answer to my idea. I want to swap another Nailhead into my 55 special, Ive found 364's and 401s reasonably priced, but they use a different bell housing. Then I read that you can swap the bell from a 57 only 364 Dyna flow onto the 55 Dynaflow, then Ive heard they bell is cast into the trans and cant be swapped. Then I started thinking, they improved the Dynaflow over the years maybe a later one from whatever donor car I choose would fit. But if the Trans is longer then I have to get a shorter torque tube and god only knows whats involved in that, or if the trans mount is in a different spot etc... So I was thinking we have a lot of smart guys on this board and to help my self and everyone else maybe we can compile a list of year to year differences. It would be nice if we could just look them up like a TH350 or 400 and know all the various measurements.
Ok. First off all dynaflows bellhousing unbolt. The Dual path which looks like a dynaflow but smaller dnt. 57-63 bellhousing use the same part number. (1171803). They'res different lengths dynaflows thou. It's the tail housing and output shaft that varies. In 1955 40-60 series used a 9 3/4 tail housing. In the 50-70 series used 10 3/4 tail housing. So if you use a newer dynaflow make sure it's the same size tail house so you can switch rear housing.
 

jnelson

Member
Ok. First off all dynaflows bellhousing unbolt. The Dual path which looks like a dynaflow but smaller dnt. 57-63 bellhousing use the same part number. (1171803). They'res different lengths dynaflows thou. It's the tail housing and output shaft that varies. In 1955 40-60 series used a 9 3/4 tail housing. In the 50-70 series used 10 3/4 tail housing. So if you use a newer dynaflow make sure it's the same size tail house so you can switch rear housing.
I forgot to tell you that from 1948-60 uses the same tail housing gasket. So you'll have to keep it at 1960 or older.
 
And here is a Dealer Service Bulletin sent out in 1965 when the factory was discontinuing the early rebuilt units. The interesting thing is it shows the number of versions the factory stocked to cover all '48-63 automatics.
 

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Andy S

New member
Looks like a fine spline was added on 56 u joints so would have to check that out to swap that into a 55 Special/Century.
 

322bnh

Member
Looks like a fine spline was added on 56 u joints so would have to check that out to swap that into a 55 Special/Century.
It will "work" if you change the U-joint to a 55, but that 56 transmission is about 1" longer overall and your axle will not be centered...the springs will rub in the pockets and the panhard bar will rub the differential cover. When I did mine I put a bunch of grease on the rubbing areas and turned up the radio.:sneaky: The 56 tranny swap works in the 55 Super and Roadmaster which have a longer tranny than the Special/Century.
 
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