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1941 Crown and pinion change


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After driving my '41 Roadmaster (McLaughlin) Buick for a few months, I want to change the rear axle ratio to a higher one.

My car has the stock 70 series ratio of 3.9, and I have a complete axle assembly out of a '55 Century with a 3.4 ratio from which I'd like to take the crown and pinion and put it in my '41.

Though I have done many extensive repairs on my vintage Buicks, I've never got into an axle, except to change the oil seals on the shafts. So, I've lots of mechanical experience and tools, but not with this type of project.

I have heard that the entire "pumpkin" can be swapped from one axle to another.

This will effect the speedometer reading as well, and I have been told that on some pickup trucks GM has had a little gadgets that will go onto the speedometer output on the transmission which the cable then fits onto. These change the speed of the speedo cable to match to the axle ratio used, and they will fit on these old trannies too. Though, I'm not so concerned with the speedo error as with getting the gears changed in the rear axle.

Who out there has done this? What is the procedure, and the possible pitfalls?



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I had this swap done on my 1948 Special that had a 4.45 to 1 originally. I wrote an article about this some years ago that was published in the Bugle.

The approach we used was to replace the entire pumpkin. You would use your original drive shaft and torque tube. It's best done by removing the entire rear axle assembly from the car. There is a pin that secures the drive shaft to the pinion shaft that needs to be removed from your original setup.

My personal view is that the swap from 3.9 to 3.4 to 1 gives about a 13% change and I do not think the work involved is worth this relatively small improvement. But go ahead with it if you want more road speed.

Joe. BCA 33493

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Thanks for the info. I realize that the 3.9 is not too bad, compared to 4.4 or so, but I do want to make the change. I'd go a bit taller if I could, but as far as I can tell, 3.4 is the tallest that was used in the post war Buicks. Do you remember what year your article was published in the Bugle? I don't remember it, but I have been a BCA member for many years, and have most of my back issues.

I know that there are some options out there for overdrives that mount onto the torque tube, but these get too costly for me.


Edited by Buicknutty (see edit history)
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  • 1 month later...

www.1937and1938Buicks.com has good information

More here on this popular & frequent topic http://forums.aaca.org/f165/swapping-rear-ends-354186.html

This may be useful for your roadmster http://www.teambuick.com/forums/showthread.php?20920-rear-gear-swap

Edited by 1939_buick (see edit history)
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