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1940 Buick Super Rear End Swap


Chuck Griffin
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I want to change out the diff gears in my 40 to make it more highway friendly. I am aware that it’s a straight exchange from any 1940 to 1955. My question is that somewhere on this forum is a step by step instructions, but I can’t seem to find them. Anybody know the link to where they are? Or can somebody tell me what the procedure is? What would be the best gears to use?

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30 minutes ago, Chuck Griffin said:

I want to change out the diff gears in my 40 to make it more highway friendly. I am aware that it’s a straight exchange from any 1940 to 1955. My question is that somewhere on this forum is a step by step instructions, but I can’t seem to find them. Anybody know the link to where they are? Or can somebody tell me what the procedure is? What would be the best gears to use?

The biggest problem will be separating the (pinned) drive shaft from the diff input spline. Torque tubes are different length year to year

You have the option of just replacing the crown wheel&pinion assembly (~pumpkin to some) or the complete diff

 

Look here (section 12). Has links to other threads on this common topic

https://forums.aaca.org/topic/207201-39-buick-team-membership/?do=findComment&comment=1057715

 

This thread is useful

https://forums.aaca.org/topic/267259-buick-special-1948-rear-differential-ratio/

 

Edited by 1939_Buick (see edit history)
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2 hours ago, Chuck Griffin said:

I want to change out the diff gears in my 40 to make it more highway friendly. I am aware that it’s a straight exchange from any 1940 to 1955. My question is that somewhere on this forum is a step by step instructions, but I can’t seem to find them. Anybody know the link to where they are? Or can somebody tell me what the procedure is? What would be the best gears to use?

 

Chuck, scroll on down to the Buick forum and look for Old Tank then go to his website. He documents the process. 

 I have done the change but did not document it as well. 

I used a 3.36-1 ratio.  20,000 miles, still happy. You will use 2nd a little more.

 

  Ben

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20190619_093944.jpg.949d9f960342ea936c31b57d11eb2e93.jpg

On 5/27/2021 at 7:47 PM, Chuck Griffin said:

I want to change out the diff gears in my 40 to make it more highway friendly. I am aware that it’s a straight exchange from any 1940 to 1955. My question is that somewhere on this forum is a step by step instructions, but I can’t seem to find them. Anybody know the link to where they are? Or can somebody tell me what the procedure is? What would be the best gears to use?

Have you taken the rear cover off the back of your differential housing? The ratio of your rear gears is stamped in the outer edge of the ring gear. Knowing this will help with deciding which ratio you want. 4.40/1 were standard & 3.90/1 was optional for 1940 Supers. Its very simple to do and change diff fluid at same time. 3.6 was optional for 1940 Roadmasters and 3.4/1 standard on my 55 Century with dyna. Which will also fit in your car. 

Edited by 2carb40 (see edit history)
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