Buick Special 1948 rear differential ratio

[Guest WalLie]

Hi,

I just purchased a BUICK SPECIAL 1948 sedanet and i'm looking for a different rear axle ratio that fits. The engine is a straight 8 248ci with the manual 3speed transmission.

Now i have the stock one who is 4,45 but would like to have something more highway friendly.

Somebody knows what would eventually fit??

Edited December 24, 2015 by WalLie (see edit history)

[1939_Buick]

A diff from an early to mid 50's Buick with dynaflow will (sort of) bolt in.  Many look for 3.56

Look at part section 12 in post 5 here '39 Buick Team Membership

http://forums.aaca.org/topic/3287-40s-rearend-gears/

http://www.teambuick.com/forums/showthread.php?20920-rear-gear-swap

Edited December 24, 2015 by 1939_Buick (see edit history)

[Guest]

Greetings 1940 thru early 1955 are a direct bolt in. It's correct that at first glance the lowest rpms available would be best for less noise and less wear for miles driven, etc. The 440s in my 1940 Super coupe are annoying when I have to hit second halfway thru an intersection! I plan on changing them out. Just don't forget about oil pump pressure and the 30 amp charging system and the condition of your engine in terms of available power. The 55 Century I have has a 3.4 to one ratio, but a 236hp nailhead pulls it around. In 55 my Spec parts car with standard trans its a 3.9 ratio, the 55 Spec dynaflow car is 3.6 to one(if its factory), I haven't checked yet. And Centuries with dynaflow are standard with 3.4. There is some info in the service bulletin books where Buick is explaining changes about different ribs on the differential casting on the later 55s vs earlier, 54 and earlier back to 40 bolt in, The Limited models prolley wont come up and are geared numerially in about the 4.5s as well, its just some late 55s you have to be careful about. The Hollander interchange manuals tell which driveshafts interchange and are the same length, but unfortunately, don't state the actual length in feet and inches, so measuring is prolley good cheap insurance! You can change out two different ways. The differential comes out of the front of the housing after removing "C" clips holding axles in. Diff can be removed with driveshaft attached and if you find a replacement with the same length driveshaft you can swap that way if the torque ball matches. The shaft is splined to the front of the diff and then a pin is driven thru both to hold together. Separation can be challenging and reassembly requires jigging the whole shaft and diff for a check to make sure the shaft and diff spline don't have "runnout" causing "wobbling" instead of smooth vibration free operation. Who flipped the switch on this long winded guy anyway! Hope some of this helps. Just remember, advice is usually worth what ya pay for it!

[Guest WalLie]

Thanks for the reply.

Do you think a ratio of 3.4 could be a proplem in terms of pulling it around? I just found one from a friend who has parts from a 55 century with the dynaflow. I would opt to change it by moving it out from the front of the housing... Wouldn't that be the easiest way? instead of with the driveshaft attached??

[Guest WalLie]

A diff from an early to mid 50's Buick with dynaflow will (sort of) bolt in.  Many look for 3.56

Look at part section 12 in post 5 here '39 Buick Team Membership

http://forums.aaca.org/topic/3287-40s-rearend-gears/

http://www.teambuick.com/forums/showthread.php?20920-rear-gear-swap

 

Thank you for all the links. Very usefull info.

[Guest WalLie]

Greetings 1940 thru early 1955 are a direct bolt in. It's correct that at first glance the lowest rpms available would be best for less noise and less wear for miles driven, etc. The 440s in my 1940 Super coupe are annoying when I have to hit second halfway thru an intersection! I plan on changing them out. Just don't forget about oil pump pressure and the 30 amp charging system and the condition of your engine in terms of available power. The 55 Century I have has a 3.4 to one ratio, but a 236hp nailhead pulls it around. In 55 my Spec parts car with standard trans its a 3.9 ratio, the 55 Spec dynaflow car is 3.6 to one(if its factory), I haven't checked yet. And Centuries with dynaflow are standard with 3.4. There is some info in the service bulletin books where Buick is explaining changes about different ribs on the differential casting on the later 55s vs earlier, 54 and earlier back to 40 bolt in, The Limited models prolley wont come up and are geared numerially in about the 4.5s as well, its just some late 55s you have to be careful about. The Hollander interchange manuals tell which driveshafts interchange and are the same length, but unfortunately, don't state the actual length in feet and inches, so measuring is prolley good cheap insurance! You can change out two different ways. The differential comes out of the front of the housing after removing "C" clips holding axles in. Diff can be removed with driveshaft attached and if you find a replacement with the same length driveshaft you can swap that way if the torque ball matches. The shaft is splined to the front of the diff and then a pin is driven thru both to hold together. Separation can be challenging and reassembly requires jigging the whole shaft and diff for a check to make sure the shaft and diff spline don't have "runnout" causing "wobbling" instead of smooth vibration free operation. Who flipped the switch on this long winded guy anyway! Hope some of this helps. Just remember, advice is usually worth what ya pay for it!

Thanks for the reply.

Do you think a ratio of 3.4 could be a proplem in terms of pulling it around? I just found one from a friend who has parts from a 55 century with the dynaflow. I would opt to change it by moving it out from the front of the housing... Wouldn't that be the easiest way? instead of with the driveshaft attached??

[Guest]

Thanks for the reply.

Do you think a ratio of 3.4 could be a proplem in terms of pulling it around? I just found one from a friend who has parts from a 55 century with the dynaflow. I would opt to change it by moving it out from the front of the housing... Wouldn't that be the easiest way? instead of with the driveshaft attached??

My friend restored a 48 Super convert and we put 3.6 in. He loves it! Don't think 3.4 is different enuff to make it unusable. Granted his engine is totally brand new, but if you think yours runs well it should work. Rear axle must be pulled back if not removing engine regardless of driveshaft removal or not, but the less difficult way if the shaft length is correct is to remove shaft and diff together to avoid the possibility of fighting with the alignment(concentricity) when reinstalling! Edited December 30, 2015 by Guest (see edit history)

[Ben Bruce aka First Born]

 WalLie, I have a 1950 Special with the 248 and three speed. I changed the rear end to a 3.36/1 from a '55. I am happy with it. I have driven about 10,000 miles since changing. I do have to use second a little sooner than with the 4.10, but really like the 600 to 700 rpm LOWER rpm in third.

 

  Anything through '55 will work.  Will "Old Tank"  Pittman has a  tutorial on his web site for changing. 

 

  Ben

[old-tank]

 WalLie, I have a 1950 Special with the 248 and three speed. I changed the rear end to a 3.36/1 from a '55. I am happy with it. I have driven about 10,000 miles since changing. I do have to use second a little sooner than with the 4.10, but really like the 600 to 700 rpm LOWER rpm in third.

 

  Anything through '55 will work.  Will "Old Tank"  Pittman has a  tutorial on his web site for changing. 

 

  Ben

http://www.buickrestorer.com/driveshaftool.html

BCA members and regulars on the forum can borrow the tool.

 

If using 1955 be advised that if it needs new bearings the rear pinion bearing is a one year only item and sometimes hard to find and expensive.

 

Willie

[Guest WalLie]

 WalLie, I have a 1950 Special with the 248 and three speed. I changed the rear end to a 3.36/1 from a '55. I am happy with it. I have driven about 10,000 miles since changing. I do have to use second a little sooner than with the 4.10, but really like the 600 to 700 rpm LOWER rpm in third.

 

  Anything through '55 will work.  Will "Old Tank"  Pittman has a  tutorial on his web site for changing. 

 

  Ben

 

Hi evrybody!, I read several times that rear differentials from 40 till 55 would work... I found a 55 from a friend and the car is from september 55... In several posts i discovered that a late 55 wouldn't work.. Anybody nows the difference?? Are there more splines like with the 56??

[Rooster]

One difference, the 55 ring gear is bolted to the carrier and earlier years are rivetted. 

[old-tank]

All 55 will as well as any...see post #9.

[Guest WalLie]

Finally received the 55 rear axle. Now checking if this will work. Anybody knows by head how many splines a rear axle of a 48 has? The 55 has 16....

[Leif Holmberg]

Same splines from mid 30s to 1955.1956 has about 2 times more.

Leif in Sweden.

Edited February 1, 2016 by Leif Holmberg (see edit history)

[Guest WalLie]

Same splines from mid 30s to 1955.1956 has about 2 times more.

Leif in Sweden.

Hi Leif,

Thank you for you the reply. Is that on the driveshaft? If not... Do you know if it is also the same on the driveshaft?

[old-tank]

Axle and pinion splines are the same so everything is interchangeable.

Edited February 1, 2016 by old-tank (see edit history)

[Guest WalLie]

Axle and pinion splines are the same so everything is interchangeable.

Thank you.

[Joseph P. Indusi]

Based on my experience with a late 40's Buick Special, a rear ratio of 3.90 or 3.60 would be ideal although a 3.40 (3.36) will work, but you will need to shift into 2nd gear more frequently on local streets.  As stated above, I think all pumpkins from 1940-55 will bolt in.   In my case, we changed out the pumpkin but used my original drive shaft and torque tube.

I always wondered why Buick sold these cars with the 4.45 ratio.   I realize that the speed limits were typically 45-50 MPH here in the northeast in those days, but that cannot be all there was to it.   I have suspected that Buick did this because they did not have an automatic transmission until 1948, and then only on the Roadmaster, a relatively expensive car.   With the 4.45 you could ride around town all day and rarely had to shift.   Any opinions?

Joe, BCA 33493

Edited February 3, 2016 by JPIndusi (see edit history)

[old-tank]

Based on my experience with a late 40's Buick Special, a rear ratio of 3.90 or 3.60 would be ideal although a 3.40 (3.36) will work, but you will need to shift into 2nd gear more frequently on local streets.  As stated above, I think all pumpkins from 1940-55 will bolt in.   In my case, we changed out the pumpkin but used my original drive shaft and torque tube.

I always wondered why Buick sold these cars with the 4.45 ratio.   I realize that the speed limits were typically 45-50 MPH here in the northeast in those days, but that cannot be all there was to it.   I have suspected that Buick did this because they did not have an automatic transmission until 1948, and then only on the Roadmaster, a relatively expensive car.   With the 4.45 you could ride around town all day and rarely had to shift.   Any opinions?

Joe, BCA 33493

Yes, and the same reason some classics used 12 and 16 cylinder engines.  When those Buicks were contemporary, drivers would usually skip 2nd gear...or start in 2nd.

[NTX5467]

Agreed!  The lower-geared rear axle cars would also require less clutch-slipping to get started from a stopped position.  Then, the sign of a perceived "good car" was how it lugged around town in high gear, rather than having to shift to a lower gear.  BUT traffic patterns were much more leisurely back then, too! 

 

What I remember "older drivers" doing when I was growing up was to start in Low and as soon as the car was moving about 15mph, shift to High gear and drive on. Even a few stories from mechanics of getting a 3-speed manual (column shift) car in the shop that would not shift 1-2-3, but only 1-3 as that's how the owner always drove it.

 

What changes to the speedometer gears are needed for those rear axle ratio changes?

 

NTX5467