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Help with rearend 742 and 985


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6:48 PM
Apr 18, 2012
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I have two center sections a 985 which has a locker and 4.56 gears, and a 742 open with 3.23 gears. Will the guts interchange, pinion, gearset, shims, ect?
I know the 742 and 489 are different, but isn't the only different the 985 and 742 is the material the cases were made of?
This set up will go in a 69 b body axle housing. I read a 985 with a suregrip or locker will fit this axle housing, but also heard the 985 with a open member will not work because the axles would be to short. But if I put the open from the 742 (If it would fit) would it still be to short?
Any help would be appreciated.

I want to put the open from the 742 into the 985 case
" 1-3/4" large stem pinion (aka. '742')
Carrier casting numbers: 1634985 (1957-1964), 2070742 (~1961-1969). "

The SG and open rotating assemblies are interchangable in the housings (all 8.75 housings) with the correct bearings. The difference in the bearings are the races pressd on the rotating assembly, the cups outside diameters are the same regardles so fit all housings... hope that makes sense.

so... instead of changing over the pinions from one to the other which would just be a waste of time as these cases are the same. (and yes both fit the axle, same opening and bolt patern regardless of year and third member wheather it be 741, 742, 489 or the 985 you have too)

I would just swap the rotating assemblies, swap the ring gears and then all you have to adjust is the backlash as you havent tampered with the pinion... voala!

Hope this helps
Thank you, that is what I needed to know :hello2:
Isn't the center thrust pins shorter in the older rears that used the tapered axles? I know the 41, 42 and 89's are all the same as far as that goes....
Isn't the center thrust pins shorter in the older rears that used the tapered axles? I know the 41, 42 and 89's are all the same as far as that goes....

That I'm not sure as the last one I had, had the clutch type suregrip in it with this type


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Oh now I see what you mean... the SG thust pin vs the thrust pin in an open center...

I dont have an open thrust button out of a case at the moment so I cant effectively measure one and the one I have from a SG I really cant measure because its out! lololol


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Even tho the thrust pins are different looking between the SG and open units, they are pretty much the same length in the 'modern day' rears....but I'm told the ones in the older axles using the tapered axles with the nutted on drums are shorter.
Hey guys I found this on line

There was a difference in thrust blocks prior to 1964 that make center section interchange, as well as axle interchange problematic. The thrust block, or “axle shaft thrust spacer”, it thr block that both left and right axles butt up against inside the center of the differential. Prior to 1964, all open differentials used a thrust block was approximately 1/8″ to 1/4″ thicker than units made after 1964. The Sure Grip thrust block prior to 1964, however, was indentical to all 1964 and later Sure Grips and open differentials. In 1964, the thrust block width was changed to match the Sure Grip thrust block width. This difference in thrust blocks between Sure Grip differentials and open differentials required that two different axles be produced for each 8-3/4″ housing manufactured. This is true of all 1959-1963 cars with 8-3/4″s.
A 1964 and later differential, or any Sure Grip differential, cannot be used with 1959-1963 rearends and axles originally equipped with an open differential. The original axles must be machined or original Sure Grip length axles must be used.

So If I read this right I should be able to put a 742 open (after 1964) in a 985 case (before 1964) and should bolt in since I am using a 69 housing and axles, right?
Thanks for any Help