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8.75 - Open to Sure Grip #2714823
11/11/19 02:53 PM
11/11/19 02:53 PM
Joined: Jan 2003
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Johnstown
69dart Offline OP
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I'm swapping out the open differential to a Posi Unit (https://www.jegs.com/i/JEGS/555/62820/10002/-1) for the the 8.75 in my 68 dart.

Does anyone have any tips?

Its a 3.91 gear if that matters and the front pinion seal needs replaced as well.

I checked the tech archives but didnt see a walk thru.

Thanks Guys!


33 Plymouth Roadster - 383 - 5.90 1/8th 9.58 1/4
68 Dart - 340
66 Belvedere - 400

Windy Hollow Garage - https://www.youtube.com/@windyhollowgarage
Re: 8.75 - Open to Sure Grip [Re: 69dart] #2714832
11/11/19 03:08 PM
11/11/19 03:08 PM
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Florida STAYcation
dIc dOc Deity ! Offline
The village idiot's idiot
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For the kind of coin Jugs wants for that ... I would pick up a 3.91 posi pig already set up

Re: 8.75 - Open to Sure Grip [Re: 69dart] #2714836
11/11/19 03:14 PM
11/11/19 03:14 PM
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Kirkland, Washington
Pacnorthcuda Offline
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There are lots of write ups on the net, and the FSM details a lot of info. Unless someone links to a detailed write up here, anything that is posted simply leaves out MANY details that are important.

You ARE looking at a good power-lok clutch unit though, and you might want to contact Doctor Diff (Cass) cause he often sells them at very good prices (and has volumes of information). I remember him talking about a new power lock on the market that had a potential case hardening / cross-pin fit issue that you might want to avoid, but I don’t know if the Jegs unit falls in that category.

Re: 8.75 - Open to Sure Grip [Re: Pacnorthcuda] #2714867
11/11/19 05:08 PM
11/11/19 05:08 PM
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Posts: 1,412
Johnstown
69dart Offline OP
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Well - Thats another story but it was supposed to be a 3.55 sure grip.

O - Well - I'd rather have the piece of mind of a new unit at this point.

So I'm done buying swap meet units.


33 Plymouth Roadster - 383 - 5.90 1/8th 9.58 1/4
68 Dart - 340
66 Belvedere - 400

Windy Hollow Garage - https://www.youtube.com/@windyhollowgarage
Re: 8.75 - Open to Sure Grip [Re: 69dart] #2714877
11/11/19 05:29 PM
11/11/19 05:29 PM
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Posts: 30,251
Florida STAYcation
dIc dOc Deity ! Offline
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Bad luck with swap. Meet units ? You CAN test them on the bench with good results ...

And Doc Diff is a good source too

Re: 8.75 - Open to Sure Grip [Re: dIc dOc Deity !] #2714886
11/11/19 06:06 PM
11/11/19 06:06 PM
Joined: Feb 2004
Posts: 855
ontario canada kingston
aspenrt360 Offline
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100% call DR Diff first he will set you up better than they will. i have bought a few things from the guy and am happy to say he is solid to deal with. and products are good too.

Last edited by aspenrt360; 11/11/19 06:07 PM.

2013 Chrysler 300 touring 1974 Dodge Charger SE. 360-727-8.75-3.23 sure grip 1972 Dodge Dart Swinger 1995 Dodge Ram 1500 4x4
Re: 8.75 - Open to Sure Grip [Re: 69dart] #2714933
11/11/19 08:32 PM
11/11/19 08:32 PM
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Bend,OR USA
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Cab_Burge Offline
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Which case are you using, 741,742 or 489?(last three digits on the casting number under the driver side of the pinion gear scope)
The case, carrier, determines which set of instructions you need when replacing the front seal wrench grin
If it is a 741 or 742 no big deal on replacing the front pinion seal, if it is the 489 case you need to follow the FSM instructions to the ninth degree, dang Mopar for going to a crush sleeve anyways rant whiney
1969 was the change over year going from the 742 to the 489 cases for H.D. applications shruggy I'm sure them dang bean counter had a lot to do with that decision and change down

Last edited by Cab_Burge; 11/11/19 08:33 PM.

Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
Re: 8.75 - Open to Sure Grip [Re: Cab_Burge] #2714960
11/11/19 09:00 PM
11/11/19 09:00 PM
Joined: Jan 2003
Posts: 1,412
Johnstown
69dart Offline OP
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I have two 741 open units that I would like to learn how to convert to posi.

I could certainly pay someone to do it but I'd like to learn and at least give it a shot.

I'm salty about the 3.91 - it was supposed to be a complete 3.55 sure grip but the brake drums were froze so I couldn't test it. Once I broke it apart it turned to be a 3.91 open unit. First time I've really been burned buying parts from a 'buddy'.


33 Plymouth Roadster - 383 - 5.90 1/8th 9.58 1/4
68 Dart - 340
66 Belvedere - 400

Windy Hollow Garage - https://www.youtube.com/@windyhollowgarage
Re: 8.75 - Open to Sure Grip [Re: 69dart] #2714983
11/11/19 10:03 PM
11/11/19 10:03 PM
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Bend,OR USA
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Cab_Burge Offline
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What tools do you have now?
You will need a good inch lb. torque wrench as well as a good foot Lb. torque wrench, a adjustable spanner wrench,a good dial indicator with a good magnetic base and a ton of patience grin
AKA, buy a 1962 to 1969 Mopar factory Dodge or Plymouth service manual and read every part on how to work on their rear ends, the 7 1/4 inch ring gear rear ends all the way up to working on the Dana 60 rear ends as well as the 741 and 742 8 3/4 rear ends scope
The 1969 FSM for Dodge and Plymouth may have a section on how to set up the 489 cases also, depending on when the manual was printed, the 489 cases came out in mid year 1969.
On your deal with the 741 case you need to check the ring gear back lash in 4 different places, with a good dial indicator, 90 degrees apart, and write it down. Hopefully the four places are within .002 of each other on the backlash scope Once that is done you can remove the ring gear form the old center section, the opened rear end. Caution, all Mopar 8 3/4 rear ends have left handed threads on the ring gear bolts, all of them wrench Don't strip or break one off loosening them up tsk You will need to turn them clockwise facing them to loosen and counter clock wise to tighten them up wrench grin Mopar recommends not reusing their stock ring gear bolts but I do. They come with a locking compound on the threads that was designed for one time use, I use blue Locktite on mine when I reuse them up they need to be torque to 55 Ft lbs. wrench You will need to mark both caps before removing them so they go back on the correct side that they where machine for hammer
The FSM calls for no more than .0030 run out difference on the carrier and ring gear backlash, I've learned that I can heat up the ring gear to around 140 F in a oven or on my wood heating stove in the shop in the winter and then the ring gear will drop right on the carrier and sit flatter than trying to pull the ring gear down with the ring gear bolts with the ring gear at room temperature, below 70 F boogie up scope
I need to go eat dinner now so get the tools and FSM and let me know later when your ready to try this, their are several other torque specs you need to know about as well as some other things you don't need to worry about, such as pinion gear depth and pinion bearing preload on your deal now thumbs


Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
Re: 8.75 - Open to Sure Grip [Re: Cab_Burge] #2714990
11/11/19 11:09 PM
11/11/19 11:09 PM
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Posts: 8,128
New Mexico
UCUDANT Offline
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Are you pretty mechanically inclined?
If you mark the pinion and nut and count the exposed threads you can reset the pinion back as it was (if it was ok)
Setting the pinion depth is the first step and it needs to be correct


You'll need:
Spanners
Dial indicator and stand etc
Torque wrenches
[Linked Image]
[Linked Image]
[Linked Image]
I have a FSM but found the Mopar Action August 2016 issue to be very informative.
If you use the ARP bolts TOSS the ARP washers they will not fit and the bolt heads will rub the case


PM me if you need to


Last edited by UCUDANT; 11/11/19 11:18 PM.
Re: 8.75 - Open to Sure Grip [Re: UCUDANT] #2715013
11/12/19 04:37 AM
11/12/19 04:37 AM
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Bend,OR USA
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Cab_Burge Offline
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You don't need to reset the pinion depth when changing the front seal on the 741 or the 742 third members tsk
Remove the pinion nut and replace the seal , retorque the nut back to the factory torque setting and then move the ring gear from the open carrier onto the posi unit and set the back lash and run out correctly on it and reinstall it back into the car and then have fun spinning both tires instead of one hammer twocents
Been there done that a bunch of times over the years wrench
He is not replacing the gear set, he is going from a open carrier to a posi carrier reusing the same gears in the same rear end carrier work shruggy
BTW, I'm not mechanically incline, I'm a drag racer that does all my own engine, tranny and rear end work shruggy
No painting or upholstery work though, I trade my labor for that work

Last edited by Cab_Burge; 11/12/19 04:41 AM.

Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
Re: 8.75 - Open to Sure Grip [Re: Cab_Burge] #2715214
11/12/19 07:27 PM
11/12/19 07:27 PM
Joined: Jan 2003
Posts: 1,412
Johnstown
69dart Offline OP
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Thanks Guys! I dig that bucket idea. I wondered how to secure the pig.

I'll have to dig thru my mags - I might have that one down in a tote.

I've built my engines, transmissions, paint, etc... so I'm fairly handy but I've never setup a rearend on my own.

I finally have a shop with room for projects and my kids are grown so I want to learn some new tricks.

I picked up the Jegs unit today at lunch - Any excuse to buy some new shop tools is good too!!! up


33 Plymouth Roadster - 383 - 5.90 1/8th 9.58 1/4
68 Dart - 340
66 Belvedere - 400

Windy Hollow Garage - https://www.youtube.com/@windyhollowgarage
Re: 8.75 - Open to Sure Grip [Re: 69dart] #2715313
11/12/19 11:55 PM
11/12/19 11:55 PM
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Bend,OR USA
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Cab_Burge Offline
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Did you buy or do you have a good dial indictor that measures in .00010?
Either way you need to use one to determine the run out and backlash on the ring gear on the new Posi center section, make sure the run out and backlash doesn't exceed .0030 scope wrench
If it does exceed that reposition it and make sure and get it warm enough, NOT HOT above 160F, to drop onto the posi unit and sits flat against its mounting surface scope
I use some thick leather glove to handle it out of the oven, it should spin freely around the center section mounting surface for a few minutes so you need to get set up it so you can thread 4 bolts 90 degrees apart in it all the way(don't torque them yet, tighten them up a little, say 5 to 20 ft. lbs.) to hold it down square while it cools scope wrench
Once it is cool to the touch, at room temps. then proceed to mount it back into the carrier and set the carrier bearing preload and finish installing and torqueing all the rest of the ring gear bolts to 55 Ft/Lbs.(left handed threads) and then set the back lash back to what is was before you took it apart up
Let us know your results and what you think of doing these again work grin
Setting the pinion depth requires a pinion depth tool or a lot of patiences (SP?) using the contact pattern on the ring gear to set the depth and a lot more work removing the pinion gear and removing pinion bearing or race to change the shims, not my way of setting the depth twocents


Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
Re: 8.75 - Open to Sure Grip [Re: Cab_Burge] #2715534
11/13/19 09:14 PM
11/13/19 09:14 PM
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Posts: 1,412
Johnstown
69dart Offline OP
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Thanks for the tips!!!

I have a dial indicator so I'll be sure to check the runout.

I'll use the wood stove to warm her up a bit if its too snug.

Momma loves the smell of gear lube. shake_head


33 Plymouth Roadster - 383 - 5.90 1/8th 9.58 1/4
68 Dart - 340
66 Belvedere - 400

Windy Hollow Garage - https://www.youtube.com/@windyhollowgarage
Re: 8.75 - Open to Sure Grip [Re: 69dart] #2715544
11/13/19 09:35 PM
11/13/19 09:35 PM
Joined: Jul 2008
Posts: 30,251
Florida STAYcation
dIc dOc Deity ! Offline
The village idiot's idiot
dIc dOc Deity !  Offline
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Get some original Mopar SKUNK OIL ... she will love you LONG TIME BIG TIME ! penguin

Re: 8.75 - Open to Sure Grip [Re: dIc dOc Deity !] #2715615
11/14/19 02:57 AM
11/14/19 02:57 AM
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Bend,OR USA
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Cab_Burge Offline
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Originally Posted by Doc Fiberglass
Get some original Mopar SKUNK OIL ... she will love you LONG TIME BIG TIME ! penguin

Ford is the only car dealers that I know of that still offer that posi additive scope up
Do NOT use synthetic gear oil with any clutch type posi unit tsk twocents
I had a customer, a real good lawyer, that I help set up a 8 3/4 3rd member with a new clutch type posi and 3.23 OEM Mopar gears that he had a local shop install in the housing and they use synthetic gear oil with no additive, which ended up making the posi unit try to lock up going around a corner which almost made him crash scope puke That shop ended up swapping it out for a different style posi that uses gears instead of clutches and charge him double what I had sold that other unit to him for shruggy

Last edited by Cab_Burge; 11/14/19 03:00 AM.

Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
Re: 8.75 - Open to Sure Grip [Re: Cab_Burge] #2717149
11/18/19 12:51 PM
11/18/19 12:51 PM
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Connecticut
FurryStump Offline
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741’s are underestimated, but they do have one issue. Not a large number of choices for new gears. 741 and 742 both have shims controlling pinion preload. Assuming all bearings are good and it was set correctly to begin with, pop off the pinion nut and change the seal. A set up pinion bearing is the way to go, so you don’t have to press off your new bearing to change your shim stack.


best of 11.39 at 117 mph 1.60 60’. 68 340 S Barracuda Fastback F.A.S.T [IMG]http://i67.tinypic.com/2mnnnnt.jpg[/IMG]
Re: 8.75 - Open to Sure Grip [Re: FurryStump] #2717280
11/18/19 06:52 PM
11/18/19 06:52 PM
Joined: Jan 2003
Posts: 1,412
Johnstown
69dart Offline OP
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My Milwaukee impact made short work of the pinion nut. I've got everything cleaned up.

Currently waiting on the new seals and tools from doctordiff to reassemble.

I've actually got two 741 pigs to reassemble - one with 3.91's and one with 2.94's - party gears and cruise gears.


20191117_130117.jpg

33 Plymouth Roadster - 383 - 5.90 1/8th 9.58 1/4
68 Dart - 340
66 Belvedere - 400

Windy Hollow Garage - https://www.youtube.com/@windyhollowgarage
Re: 8.75 - Open to Sure Grip [Re: 69dart] #2720514
11/30/19 08:46 PM
11/30/19 08:46 PM
Joined: Jan 2003
Posts: 1,412
Johnstown
69dart Offline OP
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Ok - its all back together.

Had .001 of runout and set backlash at .006-.007.

Pattern looks good so hopefully it will live.

Pressing the bearings on the new sure grip was the trickiest part. Next time I'll buy the tool.

20191130_192430.jpg20191130_184415.jpg

33 Plymouth Roadster - 383 - 5.90 1/8th 9.58 1/4
68 Dart - 340
66 Belvedere - 400

Windy Hollow Garage - https://www.youtube.com/@windyhollowgarage
Re: 8.75 - Open to Sure Grip [Re: 69dart] #2720516
11/30/19 09:08 PM
11/30/19 09:08 PM
Joined: Nov 2007
Posts: 21,808
Kirkland, Washington
Pacnorthcuda Offline
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