8 3/4 489 rebuild info

USatC

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Hey guys,


So i got my 8 3/4 suregrip with 3.55 gears, made sure it turned both wheels in the same direction, swapped out my 8 1/4, and tried to do a burnout just to realize it would only spin one tire - under a load, i guess the clutches are trashed. Not too worried about it, as it didn’t cost me much. It’s a 74 489 case. I just wanted to know what it would take to rebuild it. Am i better off than if i had a open diff and wanted to add suregrip (paid a small premium because this one turned both wheels in the same direction). What have you guys done?
 

Daves69

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A cone is typically found in a 489 case from factory..........
upload_2021-1-31_15-34-13.png


A clutch SG will also fit a 489 but typically are found in 742's.......

upload_2021-1-31_15-42-45.png
 

khryslerkid

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Maybe @Cranky would know.

OP needs to know what he has. Any pictures before you installed it or did you install the whole complete housing, drum to drum?
 

Cranky

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Cone type unit. And do not keep trying to make it lock up because it will not. Need to pull it back out and open it up and check it out the parts inside. Chances are if it hasn't been hammered to death it can be machined to work again. And you do not need to take off .100" from the faces to make it work again. It's a little bit of work to open it up but that's not that hard to do and I can walk you through it.
 

Cranky

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Any idea how much is machined off the cones? I assume the shims are the same thickness as the amount machined? Just curious.
The side gears clearances to the spiders will dictate how much shims to use if needed. Generally I've machined them the first time without adding shims unless there's too much clearance in the spiders....
 

71Beeper

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USatC, before you go removing the center section, a couple of things to verify.
One...did you add posi additive when putting the diff oil in?
Two...before trying out the sure-grip did you do a bunch of figure 8's in a mall parking lot to get the additive in between the clutches?
I had a similar situation with a 489 case 3:23 set-up. Did the aforementioned listed above and lo and behold....2 black lines!!
 

khryslerkid

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USatC, before you go removing the center section, a couple of things to verify.
One...did you add posi additive when putting the diff oil in?
Two...before trying out the sure-grip did you do a bunch of figure 8's in a mall parking lot to get the additive in between the clutches?
I had a similar situation with a 489 case 3:23 set-up. Did the aforementioned listed above and lo and behold....2 black lines!!

OP has a CONE. No clutches. :rolleyes:
 

Cranky

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USatC, before you go removing the center section, a couple of things to verify.
One...did you add posi additive when putting the diff oil in?
Two...before trying out the sure-grip did you do a bunch of figure 8's in a mall parking lot to get the additive in between the clutches?
I had a similar situation with a 489 case 3:23 set-up. Did the aforementioned listed above and lo and behold....2 black lines!!
Shouldn't matter. If the unit is in good shape, it will lay down two black lines with or without the additive. The main reason for the additive is to reduce chatter but the clutch units seem to be more suseptible to chatter than the cone units.....at least in my experience they have been. If you have a clutch type SG that's slipping, the additive can help but it's not a cure all.
 

Durandal25

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......I bought a Detroit Locker from our own Cranky. Can't wait to give that sucker a try.....
 

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