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Discussion Starter #1
I'll start this off by saying I know nothing about rear ends. I do know that I have a 12bolt, but I am not sure of the gears. I wanted to get in there and check it out...how do I identify the gears and what/how much oil will I need. I know that there are different brands and weights out there so any input would be great. Thanks guys.
 

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Count the bolts holding the rear cover there will be 10 or 12, rotate one wheel a full revolution and count the rotations of the drive shaft (must be posi or one wheel on the ground and one off the ground) this will give you the ratio. There is a plug on the pass side of the diff itself, remove it and you should be able to put a finger in and just touch the oil when the car is level, as for type of oil will depend on the rear, posi or not.

Hope that helps.
 

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rear info

The bolts on the cover will give away the 10 bolt 12 bolt rear as stated.

Ratio- The axle tube will have numbers /letters stamped into the passenger side facing the front of the car parallel with the ground.

If you get those number we can run them to tell you the ratio. BUT this will only be good as long as the rear is totally original. The one wheel spin to the number of drive shaft turns will work on a posi BUT on a open NON POSI rear it is much less likely you will end up getting a good return in info as spider gears will through off the turns to the wheel even with the one wheel on the ground deal. YOU can get lucky and jack the car up and if you just turn the drive shaft and do not touch the wheel they will spin one revolution as the posi will BUT you can not have any resistance on the axles like the drums rubbing etc. or your again doomed for a correct out come.

THE BEST way is to remove the cover and either count the teeth or look at the outer ring gear diameter (10 bolt only) it will have the ring and pinion tooth count stamped in - On 12 bolts you will have to look to the pinion gear machined top of the gear and a flash light. you will have to keep turning the pinion till you get a clear shot of the top of the pinion...the tooth count will also be stamped on that. Good luck.

Any good brand name oil will do for the rear diff. If you have posi (limited slip ) you will need the limited slip additive to keep the posi unit from chattering. I would advise NO synthetic fluid as the clutch and cone type units are friction base units and need the minerals to make the clutches efficiently stay locked. synthetic is very slippery and will work against what the posi is trying to do...STAY LOCKED... :)

If the diff is open or you have a true trac or torsen -gear to gear helical worm gear type posi synthetic is fine. Jim
 

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Discussion Starter #4
The bolts on the cover will give away the 10 bolt 12 bolt rear as stated.

Ratio- The axle tube will have numbers /letters stamped into the passenger side facing the front of the car parallel with the ground.

If you get those number we can run them to tell you the ratio. BUT this will only be good as long as the rear is totally original. The one wheel spin to the number of drive shaft turns will work on a posi BUT on a open NON POSI rear it is much less likely you will end up getting a good return in info as spider gears will through off the turns to the wheel even with the one wheel on the ground deal. YOU can get lucky and jack the car up and if you just turn the drive shaft and do not touch the wheel they will spin one revolution as the posi will BUT you can not have any resistance on the axles like the drums rubbing etc. or your again doomed for a correct out come.

THE BEST way is to remove the cover and either count the teeth or look at the outer ring gear diameter (10 bolt only) it will have the ring and pinion tooth count stamped in - On 12 bolts you will have to look to the pinion gear machined top of the gear and a flash light. you will have to keep turning the pinion till you get a clear shot of the top of the pinion...the tooth count will also be stamped on that. Good luck.

Any good brand name oil will do for the rear diff. If you have posi (limited slip ) you will need the limited slip additive to keep the posi unit from chattering. I would advise NO synthetic fluid as the clutch and cone type units are friction base units and need the minerals to make the clutches efficiently stay locked. synthetic is very slippery and will work against what the posi is trying to do...STAY LOCKED... :)

If the diff is open or you have a true trac or torsen -gear to gear helical worm gear type posi synthetic is fine. Jim
Well, I know that I do have a 12bolt and it's non-posi, as for the rest, I guess I'll be pulling the cover in the next couple days and figure out what's goin' on there and then. But I'm getting lost in terms like, open differential and all of the tru trac, torsen helical worm stuff. Haha. :D I know my rear is non posi-traction....and are you saying that the synthetic is good for posi and not for open diffs, or the other way around?
 

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im running lucas performance posi gear oil in my 12 bolt, it has the additive in it. i think they only had the one kind at pepboys but dont remember the weight, the sales person helped me, he had a built mustang and said it was good stuff and he was right, check it out and hope this helped in some way.
 

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Discussion Starter #6
Turns out that the rear is a 10bolt after all. I'm assuming then that it is completely stock, I would think (will check soon enough to be sure). If it is stock...how much is that rear/axle combo rated safely up to for horsepower/torque? Or does it depend considering the gears also?
 

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How do I find out the size? Do I need the number stamped on the axle tube? If I get the number is there a good decode website to find out?
 

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if its the 8.5 youll be fine.
I was just wondering about how well it would hold up if I upgrade though. I plan to eventually either build a stroker motor or get a nice top end kit for my current motor and do a minor rebuild. I was just wondering what kind of stress it can take considering I'd like to do somewhere in the low 400hp range....though it won't be for a while since this happens I'd like to have everything in my head ready in my head for the complete set-up. So I'm just asking how much this rearend can take. I'll always run automatic, so that plays in my favor I think, but I would like to have posi eventually also.

Sorry for the novel of a reply.
 

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dont see a problem i believe the 10 bolt can handle a 10 second car, my friend has a 400+hp bbc with a th350 and 8.5 10 bolt in his 73' no issues.
 

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I've seen 8.5" 10 bolts running high 8's through the 1/4 in a 3000# car and hold up with no problem. High 1.2x 60' times leaving off a trans-break.:yes:

Recipe is 31 spline axles with c-clip eliminators, Full spool (a GOOD posi will satisfy most---I like the Tru-Trac), Strange gears (Most will probably do), T/A cover and a billet yoke (Typically Mark Williams).

Case preload will make or break you!! I'm sure there's a lot of different ways to get it but I usually (if I remember correctly) shoot for .060" and use the thickest (least amount of) shims possible. Setting up a rear is NOT hard or involve any magical mysteries like some people are afraid of. It does however require a couple tools.

As for gear lube, I usually just stick with plain Valvoline 75-90 and a bottle of the Ford (yes Ford) Friction Modifier if it's a Posi.
 

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why is ford posi lube better then gm whale sperm posi lube?? just curious...
 

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Discussion Starter #14
Thanks for all the help/input guys! I was just wondering considering all of the hype recently with larger rearends. Is there any truth to Posi, Lockers, etc. not being road friendly? I had heard that some make lots of noise and other things that take away from smooth cruising etc.
 

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why is ford posi lube better then gm whale sperm posi lube?? just curious...
Don't know if it's better or worse, I just have a "Sourse" where I got a bunch of it for free and it's worked flawlessly.:D

Thanks for all the help/input guys! I was just wondering considering all of the hype recently with larger rearends. Is there any truth to Posi, Lockers, etc. not being road friendly? I had heard that some make lots of noise and other things that take away from smooth cruising etc.
Lockers are noisy and clunky on the street. I've never really cared for them personally. More common Posi's (or limited slip differentials) don't have those problems and are as smooth as silk. No problems what so ever on the street.:no:
 
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