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"39 Special Rear Gear Swap


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A friend of mine has a '39 Special that maxes out between 45-50 MPH. Can a gear set from a Century be swapped for the std. Special rears or are the carriers different? Some one suggested the swap to increase roadability. Car has a rebuilt engine and runs strong. Any help appreciated.

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Guest Grant Magrath

It's been discussed here a few times, and I don't believe there are any issues giving a Special some longer legs.



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I am not an expert on this but believe others have said that the rear end up to '49 is the same and the newer rears have even better road gears than the '39 Century.

The '39's all have 4.44 but theCentury which has 3.90 final drive.

The service manual indicates there was a 3.61 available for Century as an option.

My '39 Century with stock tires is running 3,000 rpm at 60 mph.

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1939 S40 & S60 rear ends are unique with many unique 1939 only parts. Some internal parts are the same as 1938 and some same 1940 and later. Spring, locating sway bar position & torque tube attachment are 1939 only. Probably other difference in design - dimensions.

So easy to put 1939 S60 gear set into a 1939 S40. :)

Or swap the complete rear so you get the bigger S60 brakes. Not sure is S60 torque tube fits S40

Anything else takes welding & machining :mad:

From 1940 to 1952? on they kept the same design & dimensions, apart from torque tube length. Have read that gear sets up into mid 50's can be fitted to 1940 - 52 dif housing (but no 100% sure).

The old 1937-38 Club magazines have a range of interesting & some details articles on high speed dif for 37-38's. Are good reading

Edited by 1939_buick (see edit history)
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Thanks for the info guys! I'll pass it along to my friend:now to dredge up a good gearset!
Very hard to get a 1939 S60 gear set. Occasionally they came up on ebay. Sell for good $. Have met someone who purchased a a 1939 S60 parts car just for the diff

The 3.9 was an option on the series 40. Marked by red dot on the end of the shaft (this written in some Buick book).

Have also seen photos on diff housing with ratio 3.9 stamped on

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