Basic 8 3/4 question

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    Author Topic: Basic 8 3/4 question  (Read 2085 times)

    Offline mjb765

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    Basic 8 3/4 question
    « on: June 25, 2010 - 09:55:02 PM »
    I see people who have 8 3/4 rears describe as a "741" or a "489" center. How can you tell the difference? I know this may seem like a dumb question, but I am just getting back into the hobby after about a 20 year break in Harley Davidson land......now I am splitting my time between my 71 R/T and the bikes.....
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    Offline 72cudamaan

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    Re: Basic 8 3/4 question
    « Reply #1 on: June 25, 2010 - 10:19:23 PM »
    The three digit number is the casting number. It is cast right into the diff housing. Most say either the 489 or 742 casting are the strongest.
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    Offline mjb765

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    Re: Basic 8 3/4 question
    « Reply #2 on: June 25, 2010 - 10:20:20 PM »
    OK--I will look....is there a better one to have or are they both about the same strength?
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    Offline 72cudamaan

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    Re: Basic 8 3/4 question
    « Reply #3 on: June 25, 2010 - 10:27:48 PM »
    For most applications, either will usually work just fine. I just read a thread somewhere here that the 742 is a bit stronger, though the 489 is close and can be made a bit stronger with mods. If you are really concerned then you could always upgrade to a Dana 60.
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    Andy  (phukker whither)

    Offline mjb765

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    Re: Basic 8 3/4 question
    « Reply #4 on: June 25, 2010 - 10:40:07 PM »
    Not that concerned...just curious...

    Thanks for the info
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    Offline djwhog

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    Re: Basic 8 3/4 question
    « Reply #5 on: June 26, 2010 - 08:49:40 AM »
    the 3 diffs all have diff pinion shaft sizes, the 489 is actually the largest and suppose to be stronger, than the taper on the other 2, but they are all good and what he said, you can build a 489 to be better as far as tuffness.

    Also there are 2 posi main types, cones and clutches, clutches again are preferred, cones are strong, but when worn not reallt service but can be modded and serviced.

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    Offline HP2

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    Re: Basic 8 3/4 question
    « Reply #6 on: June 26, 2010 - 11:22:01 AM »
    Most common castings are 657, 741, 742, and 489.

    The 657 and 741 are small 1.38 stem pinions. Which are actually close in size to some of the bigger chev and ford offerings. The 742 pinions are 1.75 in diameter. The desireable thing about all three of these castings is that they use washers to set pinion deth, therefore they can withstand huge amounts of shock loading ie. drag strip launches with high powered engines. 

    The 489 case has an even larger 1.875 diamter pinion, but typically uses a crush sleve to set depth. The only problem with that is the crush sleeve can crush and destroy the gears if subjected to repeated high shock load. There are solid spacers out there that can replace the crush sleeve and eliminate the problem but they ar enot commonly used.
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    Offline Changin Gears

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    Re: Basic 8 3/4 question
    « Reply #7 on: June 26, 2010 - 11:38:55 AM »
    All these are plenty strong for street aplications.  The only strength problems I've ever had with an 8.75 is the spider gears/sure grip which is common to all of these.

    I ran a 741 with a 440/4-speed and slicks for years.
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    Offline 1970 RT Challenger 1970

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    Re: Basic 8 3/4 question
    « Reply #8 on: June 26, 2010 - 01:14:22 PM »
    Quote from: 72cudamaan on June 25, 2010 - 10:19:23 PM
    The three digit number is the casting number. It is cast right into the diff housing. Most say either the 489 or 742 casting are the strongest.

    Is the casting number internal and you would have to take it apart to see it?
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    Offline Katfish

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    Re: Basic 8 3/4 question
    « Reply #9 on: June 26, 2010 - 01:21:25 PM »
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    Offline Chryco Psycho

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    Re: Basic 8 3/4 question
    « Reply #10 on: June 27, 2010 - 02:54:58 AM »
    casting # is external & along the side of the pinion
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    Offline thedodgeboys

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    Re: Basic 8 3/4 question
    « Reply #11 on: June 27, 2010 - 09:41:21 AM »
    Quote from: Chryco Psycho on June 27, 2010 - 02:54:58 AM
    casting # is external & along the side of the pinion

    Drivers side
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    Offline RusTy/SE

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    Re: Basic 8 3/4 question
    « Reply #12 on: June 27, 2010 - 08:28:30 PM »
    There are some pics of my 741 case in the link below. Thanks to Moparal that thread's subject now has a happy ending :)

    http://www.cuda-challenger.com/cc/index.php?topic=67724.0

     :cheers:

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    Russ
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