How to read axle housing for ratio...1953, 1954

mgaugustine

New Member
I would like to swap the third member from a dynaflo car into an earlier (1940 Roadmaster) to drop a few RPM off at highway speeds. I have heard that the later style third member should have a number stamped on the bottom of the banjo (housing center) that should read something like 3/2, 3/4, 3/6 referring to 3.20, 3.40 and 3.60 ratios. These are NOT the numbers I find stamped there. I have available to me (for a price) these three to choose from, and cannot turn them to determine the ratio.... but know that they are stamped at the center of the housing with these numbers...788-40, 912-B, 913-70. No sign of anything like the numbers with slashes. Does anybody know what these numbers mean? I assume the "Dash 40" and the "Dash 70" refer to the series, respectively special and roadmaster... but what the heck is the "Dash B"? Could this be the impossible to find optional 3.20 ratio that I have heard about?

In any case, if anybody knows what these numbers mean, I would sure like to know about it. Thanks.
 

1939_buick

Active Member
Most (all?) of the (easy) swaps are into series 40 Special & series 60 Century cars using diff's up to 1954.
I do not have my parts books with me, but from memory series 80(70?) Roadmaster & series 90 Limited diff's are very different. Brakes are a lot bigger. So I suspect not the simple swap like for S40 & S60's. Others may know more (or better). Post WW2 are different
There are articles on diff swaps in the old 1937-38 Buick Club torque tube magazines, now held on line by the http://www.3638buickclub.org/

Also look here (post 5 section 12), but not all links are up to date (links lost in software change)
 
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mgaugustine

New Member
Well, after much cleaning I was finally able to find and read the correct numbers to determine axle ratios. Turns out, they are NOT written in fraction form as shown in the shop manual, but rather simply intended to be read from top to bottom. For instance a 4 stamped over a 1 would be a 4.10 ratio. All are located just off center of the middle banjo portion, near the bottom. They are also faint, and often poorly stamped due to trying to stamp on a rounded surface. If one chooses to do what I did, be sure to bring a wire brush to clear the grease and some sandpaper, these numbers are NOT stamped deeply and impossible to read through any amount of rust without first taking a few swipes at it with sandpaper. Heck, most I could not even LOCATE without sanding the surface first.

Incidentally, I never did find the optional 3.2 ratio. Just the standard ratios.
 
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Well, after much cleaning I was finally able to find and read the correct numbers to determine axle ratios. Turns out, they are NOT written in fraction form as shown in the shop manual, but rather simply intended to be read from top to bottom. For instance a 4 stamped over a 1 would be a 4.10 ratio. All are located just off center of the middle banjo portion, near the bottom. They are also faint, and often poorly stamped due to trying to stamp on a rounded surface. If one chooses to do what I did, be sure to bring a wire brush to clear the grease and some sandpaper, these numbers are NOT stamped deeply and impossible to read through any amount of rust without first taking a few swipes at it with sandpaper. Heck, most I could not even LOCATE without sanding the surface first.

Incidentally, I never did find the optional 3.2 ratio. Just the standard ratios.
Thanks for posting your findings!
 
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