converting an 8 3/4 to the correct sure-grip

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Frank Miller

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So I'm converting my '65 Dart GT 7 1/4 to the 8 3/4 rear-end which I found months ago. After learning late and reading lots I see that "bolts right up" is not true and the correct "sure-grip" conversion set up has many combinations of right or wrong pieces that has to be chosen now if I don't want all the headaches you guys tell of. Example: the 4-12-10 thread by member 71 DUSTAR, titled 'axle lengths for 8 3/4' scared me straight that I need to learn quick about the variables that could create friction and driveline failure and if I was to listen to my local Mopar wrench buddy who says " bolt it in, it'll work" I could get hurt. In the Dodge '65 Dart-Coronet Tech. Serv. Manual on page 3-15 titled 'Rear Axle-General Info, it states 'The Dart model cars are equipped with a conventional type diff., however a Sure Grip diff. similar to those used in the Coronet model cars is available.' 'Refer to the Sure Grip Diff. section of the axle group for the serv. proced.'
This is the last part of my power-train resto. that's left and I want to make it correct as possible before the huge body-interior project ahead. In the recent thread by 71 DUSTAR it mentioned one of 'several Sure Grip packages' to choose from? What the heck? Simply put, is there a specialty shop that I should turn this over to that knows everything about making this rear-end the perfect Sure Grip that's highway geared and ready to bolt on? I don't trust the "this will work" feedback I'm hearing from my buds.
 
It sounds like you are thinking about it a little too much. There are 2 kinds of sure grip units; the clutch type and the cone type. The clutch type is generally considered to be more desireable, but I've never had any problems with a cone type. These 2 sure grip units can be interchanged between cases (741,742,489) as long as the correct ring/pinion combo and bearings are used. If you want highway gears, I would go with a ratio of 2.76, 2.93, or 3.23. There are several companies that sell a complete, set-up third member, but it's pretty pricey. I think Randy's Ring and Pinion does.

Putting a rear end together is not that difficult as long as you have a factory service manual close by. I've put all of my rear ends together in my garage. No special tools, just a factory service manual and a couple hours worth of work.
 
you will be fine if you have a factory "A" 8 3/4 rear. The only thing you will need to do is shorten the driveshaft and have it reballanced. You may need to transfer emergency brake cables and get a new brake flexline. Are you keeping SBP axles? Do you have 10 inch brakes? Either suregrip is much better than an open rear. Bolt it in first then add a suregrip later if you want.
 
Thanks for ithe info. I finally went into a profesional shop (Rearend specialties in San Jose, CA) today and got the straight scoop about my options. He said that the Chrysler 'Sure-Grip' was obsolete and no accompanying parts are available anywhere, though I did have the perfect 8 3/4 for my '65 Dart which has a couple of choices of the Dana or Truetrak modern replacements that would work perfectly. The whole setup rebuilt with green bearings would be about a thou without the driveshaft changeup. (from a diffeent Co.) Does this all sound reasonable?
 
not really good advice. I do not like the Idea of getting rid of the original mopar tapered roller bearings and it IS possible to get a "cone type" sure grip. Green bearings are not as strong. They use ball bearings, not roller bearings. I would not spend a grand for something that is not as good of quality as the original equipment.
The best advice; call Randy's Ring and Pinion. They know the 8 3/4 probably better than anyone. They will give you the best advice. (try googling them).
...(I'll probably get dissagrements on the "bearing" controversy)
 
He is trying to sell you something. You do not have to set the pinion depth to add a suregrip so labor should be relatively cheap to install. Original suregrips are not obsolete, they last forever. Good used is fine and I agree with omahamoparguy about the green bearings. Keep checking. Moser gets my nod for expertice.
 
It sounds like you have two conversions going on here. The first is the 7 1/4 to 8 3/4 swap which has been discussed extensively on many threads lately. The other is to make an existing 8 3/4 a limited slip rear end. Assuming you have the gear ratio you want, it's fairly easy to add a sure grip. If all the bearings and other hard parts are in good shape, it's only a 2-3 hour job for anyone handy with tools and has a good manual. Two important areas are heating the ring gear to install on the new carrier and a dial indicator to set the axle shaft end play. Don't ignore either one! The last time I checked, you could get a new cone type carrier for around $350 or so. Used clutch types are available from time to time on E-bay, Craig's list, Hemmings, etc. The cone type works great when new but when it gets worn out there is not much you can do to it to make it like new again without spending almost as much as another new one. The clutch type can be rebuilt with a new clutch pack kit for around $50 the last time I checked. If you'd rather send it out to have converted, either Randy's or Moser know how to do it right. Don't use the green bearings for a street car. They don't handle side loads very well. $1000 sounds a bit excessive though....even with new bearings and seals. If you're going to put more than 500 horsepower through it, Moser has some nice upgrades to handle it.
Thanks, Mark
 
I like the idea of going at it as several suggest with just putting in my 8 -3/4 as is for now to enjoy hanging up the new complete spring system I got from Springs n things without changing out the diff for now. As easy as that is (along with getting the driveshaft correct) I wonder if there is any reason to open up the box to inspect it's overall condition first? From all appearances the case and axles don't look like they've been beat around but my mopar buddy insists "just hang it and run it around" to listen for noise is what most do. I love the nuts and bolts aspect of this part of my restoration and understand that popping in the posi later is simple once everything is hung. I intend for this to be highway gear perfect show driver.
 
I would agree with your buddy in that you could just hang the entire rearend in the car and hook it up as is, then run it to make sure that it is serviceable. Then if I were you, I would start looking for an extra center case with the desired gear ratio in it to have on the bench. Doing a bench rebuild on an extra center will allow you to get the bugs worked out of your housing install, and if there is nothing wrong with the rear you put in, will still allow you to enjoy the car while you rebuild your extra center. Then, make the extra center what you want, suregrip and desired gear ratio. When it's rebuilt, just swap out the center sections, which should take just a couple hours and you're done. Then you still have an extra case to rebuild to something different should you decide to upgrade from highway gears to something more steep.

IMO, either suregrip unit will work for your application, and if you were to buy a brand new Auburn (cone) style suregrip, it would give you years of service in a highway application. Good luck with your project, Geof
 
"When it's rebuilt, just swap out the center sections, which should take just a couple hours and you're done. Then you still have an extra case to rebuild to something different should you decide to upgrade from highway gears to something more steep. "

That is the beauty of an 8 3/4. I have 4 or 5 centers with different gears. Going on a trip, put in the 2.76's. Rematch with a 327 nova, put in the 4.56's. Normal(?) diving, run the 3.55's
 
Amen to that 66fs, I have at least one of every ratio "just in case", and I like to build the 742 case with clutch style SG. Not that I have anything against the other cases, but if I find a 742, I always pick it up "just in case"!!! Geof
 
"if I find a 742, I always pick it up "just in case"!!! Geof" Well said. My favorite combination too.
 
Hey Frank, You've come to the right place. "The straight scoop"? I dont think so. "Obsolete" ? What rock did this guy crawl out from under? I'll bet he has something he can order for you that's a lot better. Randy's Ring and Pinion is where i shop. There are some after market units out there that work very well but dont be afraid of the "obsolete" units either. Good luck with your project.
Small Block
 
You members have been great on alerting me that the original Mopar sure grips are out there and not be talked into the thou plus local shops modern suggestion. I've just located an original 8 3/4 posi from a 489 case (the one which I have w/o posi) which the seller says is in excellent condition and been in storage over 15 years since he swapped it out for a spool? What does that mean? It's got the tapered bearings and is for 30 spline axles which I believe is what mine should be. Shouldn't I expect if I drop my original 8 3/4 and this posi diff off at this local "reputed" diff shop that they would pop right in (at a low cost?) and benefit with an overall inspection of the whole rearend and these "used parts" to feel "correct and safe" to pop it on my new spring kit. (And of course get the driveshaft modified). Thanks for all your help, Frank.
 
If it is the cone style Auburn suregrip unit, I doubt they will take it apart to inspect the suregrip unit unless you insist. It will, however, be able to be installed into your center section in place of the open unit that is currently in there. To just change the suregrip section will yield you insight into the condition of the pinion gear and ring gear and the carrier bearings, but not the suregrip unit itself.

To check the condition of the cone style suregrip unit, it must be disassembled and then the distance from the bottom of the cone to the bottom of the housing must be checked with a crushable measuring medium. The clearance between the two can be reset to factory specs by milling the bottom of the cone and adding the amount removed in shims on top of the cone. Once the cone bottoms out in the case, the suregrip will no longer function.

Good luck with your install, Geof
 
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