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What type of libe do I use in my 12 bolt posi?

Discussion in '1969-1972 K5 Blazer | Truck | Suburban' started by **DONOTDELETE**, Mar 18, 2001.

    I have a recnetly rebuilt 12 bolt post with 3:73's (rebuilt 18 months ago). I just checked the gear oil in the rear dif and it is black! I was wondering what this might mean and what I have to do to drain the diff oil? Also, what lube should I put in my diff with a posi?

    Thanks,

    Rod
     
  1. BluThndr71JMMY

    BluThndr71JMMY 1/2 ton status

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    Buy a 12-bolt gasket or get some oil-proof RTV (blue. I think) Use GL-5 sae 90 weight differential lube. Also get the special Positraction additive from your local GM dealer --it's about a 4 oz plastic bottle.
    Remove differential cover and drain old oil into pan, save and recycle.
    Shoot out pumpkin interior with WD 40 and check for irregularities; metal chips, galling of the gears, C-clips OK etc.
    Good time to take note on what markings are stamped on pinion ring as these can give you your final drive ratio, if you don't know it already.
    Clean off differential plate and mating surfaces.
    Apply new gasket or bead w/RTV silicone.
    Attach diff cover plate.
    Remove squarehead input plug from upper left and pump fresh GL-5 plus GM additive into pumpkin case until it trickles back out of the plug hole.
    It takes about 2 quarts, more or less.
    Secure unit, test by driving around the block and check for leaks.
    The Manual says check level every 6,000 miles and replace lube every 24,000.
    Good luck and don't forget the Lava.


    <P ID="edit"><FONT SIZE=-1>Edited by BluThndr71JMMY on 03/19/01 06:48 PM (server time).</FONT></P>
     
  2. When Im rolling along at slow speeds I can feel my Posi kick in with a clunk(not to bad)when I accelerate and kick out when let off. I just bought my 72 Jimmy two days ago, I havent opened it up yet. Is the "clunk" normal?
     
  3. Steve_Chin

    Steve_Chin 1/2 ton status

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    That oil should not be black unless the person who set the differential up used some sort of black assembly lube (which is bad because moly is bad for the clutch plates). When I bought my Blazer, the differential oil was greyish-black. It hadn't been changed in forever, and there was a lot of wear on the gear teeth (not shiny, kind of greyish with a funky texture). The ring gear bolts were also loose (re-torque them since you're in there). Since I changed the oil and cover, the oil is still nice and light honey-colored (going on two years).

    When I went to buy the limited-slip additive from a parts store, the sales guy advised me against buying the brand that they had on the shelf. He said to go down the street and buy the real GM stuff because it was better. I took his advice, since I don't think that he'd be turning down a sale unless there was a legitimate reason to.

    Oh, I run an SAE 85W-90 EP gear oil with the GM additive.
     
  4. Steve_Chin

    Steve_Chin 1/2 ton status

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    Ptangler, there should be no "klunk" from a clutch-type limited slip differential. If you have a clutch-type diff, you have some sort of problem with the clutches. If it is a locker, it is quite possible for it to make klunking and/or rattling noises as it engages and disengages.
     
  5. BluThndr71JMMY

    BluThndr71JMMY 1/2 ton status

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    When I had the 12-bolt rearend rebuilt for my '67 K/10 Sub, [axles were shot] I had them add an Auburn Posi since we were in there anyway. The diff shop guy put in F*rd limited slip lube because he said it was better for an Auburn(?!). [Or was it better for break-in?] It was blueish.
    I didn't argue, but the next time I had to flush pumpkin fluids, I went all GM.
     

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