I have original 1966 7.25" rear axle with open diff and 3.55 gears. As well I have sure grip unit which I will install (or ask someone to do for me). I found NOS 3.91 gears from Ebay but they are advertised to be for 1960-61 axle. What is the difference and can I convert something to get those 3.91 gears fit to my rearend?

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1966 Dart. 170 cu.in. 200 rwhp.

I found that early pinions have been with 29 splines and late ones with 27. So I ordered 3.90 gears listed for 1966-

Update: 4.10 gears coming.

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1966 Dart. 170 cu.in. 200 rwhp.

That is an interesting detail I wasn't aware of, thanks for posting! Other than
having to use the appropriate 29 spline yoke, I wonder if there are other
changes as well, brg. size etc., those 3.91's are probably hard to come by
for such a limited app!

I have got installed Motive 4.10 gear set and original Mopar cone type sure grip unit in to the 1966 A-body 7.25" axle.

The drive shafts will not go deep enough. Seems to me that splines in the drive shatfs are not long enough. Missing something like 0.08" inch both side.

Drive shafts dont seem to touch to center shaft of the SG.

Do I just cut spline grooves longer about the missing lenght?

Edit: No problem. Transmission specialist has measured something wrong. Shafts were in correct position. And I got a very good discount because he was sad about the problem!

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1966 Dart. 170 cu.in. 200 rwhp.