BOP 8.2 guidance

Discussion in 'Got gears?' started by msantimanzano, May 24, 2019.

  1. msantimanzano

    msantimanzano New Member

    Greetings from Colorado. New member here.

    I come to this Buick forum, because I am striking out in locating the information I am looking for anywhere else.

    Though I do NOT have a Buick, I do have a 67 Firebird and my question regards the 8.2 posi unit, which I believe is the same or very similar in the Buick as in the Pontiac. I have found a few 8.2 posts on your forum, and it seems that the knowledge resides with some of the members here.

    Some background, my car is a 67 Firebird with a factory Pontiac 400. Transmission is a TH400. I am in the process of changing to a Muncie 4 speed, and with that, I need to change my rear end gearing as well. I have a new set of gears in 3.73 as well as 3.90 to put into the rear end. Also just acquired a 67 Pontiac Firebird posi unit to replace the current open rear in my replacement diff housing.

    I would like to open up the posi unit to inspect the internals before I go through the trouble of installing it, only to find out that perhaps, it is not usable or that it is in poor shape.

    I cannot locate anywhere online, a good reference, perhaps with pictures, as to how to go about opening up this unit, what to inspect, how the parts inside should look like if in good usable condition etc etc.

    I don't want to just start taking the bolts off the unit, only to discover there should have been some kind of specific process to follow, or to have it blast apart on me when the last bolt is removed, and find myself without a reference to re-assemble it afterwards.

    I have seen one post that did talk about machining some surfaces, but not highly descriptive. I am a gunsmith by trade, do have both a mill and lathe in my garage, but some better descriptive guidance as to what to look for, what to machine etc, would go a long way in ensuring I don't F this up.

    I can, if need be, take pictures of the unit if that will help at all. Hoping someone here can guide me or throw me a lifeline.

    Thank you all very much for even taking the time to entertain my non-Buick post.
  2. RoseBud68

    RoseBud68 Well-Known Member

  3. msantimanzano

    msantimanzano New Member

  4. wovenweb

    wovenweb Platinum Level Contributor

    You may want to try
    455monte likes this.
  5. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

  6. wkillgs

    wkillgs Gold Level Contributor

    The 1967 Pontiac Service manual likely describes the complete process and assembly specifications.
    Since you are not reusing the gears or carrier, the primary things to keep track of during disassembly, would be to mark the carrier bearing caps left/right, and note the thickness of your pinion shim as a starting point for setting up your new gears.
    Jim and Brian are likely to offer guidance when they see your thread.
  7. jmos4

    jmos4 Well-Known Member


    Jim is a good source for technical knowledge and parts as has been mentioned.

    You didn't list your goals, but you may want to consider your end use of the car as 3.73-3.90 gears will leave a bit to desire if you do any interstate travel. I have 3.55's in my 65 Skylark (245 60R14 tires) and doing 70 I am pulling 3500 rpms.

    Also consider which muncie you get as a close ratio M21/M22 have a low first gear and work better for the gears you listed, although I have a M22W Autogear ratios based on the M20 and have heard the wide ratios are better in a daily driver. And know a few people who changed from close ratio to a wide ratio.

    Cool project, good luck with getting it the way you want.

  8. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    Part of the problem is the short tires. At 25.6", they are a full inch shorter than what came on the car. Your 3.55's are actually 3.69's with those short tires.
  9. jmos4

    jmos4 Well-Known Member


    Yeah I talked to a guy who had a new 65 Gran Sport with 3:73's and said his rpms were similar to mine.

    14" tires don't leave many options for height

  10. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    P225/70R-14 (26.4") is the closest to a G70-14 (26.8")
  11. jmos4

    jmos4 Well-Known Member


    Sorry if I somewhat hijacked your thread, I have had 225/70r14 before and really didn't see much of a rpm change, in a perfect world I'd get 265/60r14 but unfortunately BFG's don't come that tall.

  12. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    336/tire height X rear gear X MPH = RPM (no converter slip)

    Add 150-200 RPM for converter slip

    For your P245/60R-14 at 25.6" tall

    336/25.6 X 3.55 X 70 = 3261 RPM

    Add 200 RPM for converter slip and we are at 3461 which jives with your observed 3500 RPM

    For the P225/70R-14 at 26.40" tall

    336/26.4 X 3.55 X 70 =3163 RPM

    Add 200 RPM for converter slip and we are at 3363 RPM

    Yes, only 100 RPM difference.

    P265/60R-14 is only .12" taller, not even worth it. You need 15" tires and rims.
  13. monzaz

    monzaz Jim

    You NEED an overdrive transmission...:D Jim
  14. monzaz

    monzaz Jim

    ThAT is why 75% if not more car were installed with 2.73 - 3.08 gearing in the Muscle car era... Cars that had 3.55 3.73 3.90 were special performance cars...even back then. They were not being build for freeway cruising.
  15. BrianTrick

    BrianTrick Brian Trick

    To add to what Jim said,you also need to remember that back when these cars were built,the highway speed limits were a lot lower. I’m not sure about your area,but in my area,they are 65-70,so you guys that drive your cars on the highway probably want to keep up with traffic. This is why I like to ask each customer exactly what they plan on doing with the car,so I can install a ratio that best suits their needs.
  16. monzaz

    monzaz Jim

    Well Yes and no... lol. States that were LONG and rural Montana Utah, Arizona etc. did not even post speed limits signs in the 60's... lol. (cars were still getting 2.56 2.73 2.78 ratios plentiful then too) V8 Motors with little cams just made alot of torque and really did not have much top end rev. Give people what they want pep and go and try to keep some mileage. :) Caddys had 472 motors and 2.56 or 2.41 gearing they would do 100 mph plus ALL DAY LONG... You also need to figure in the Interstate HIGH way was just getting going with freeways that could even handle those kinds of speeds... I remember my Dad saying they would do 100 MPH on 2 way Black top in rural areas all the time to get to the dates after work.
    Less road Traffic back then too. NOW you go anywhere there is a side street or a light all over. But as with everything ...... it all changes.
  17. monzaz

    monzaz Jim

    1973 OIL EMBARGO did us all in. I remember sitting at the gas stations for long lines... in the Big 1974 Kingswood Impala wagon 400 sbc... GAS HOG...
  18. jmos4

    jmos4 Well-Known Member

    Hi all,

    Yeah a OD would be nice or taller 14's or 15's but early 65's don't allow for really big tires when trying to keep it stock looking.

    I am running a stick so no slippage to calculate.

    Back to the original post, just wanted him to consider end use as everyone reads old magazine write ups and has a old timer swear he ran 4.11's in something that was his daily driver.

    My car came with the 3.55 and it's not the worst, not something to go on a trip from MI to FL but 2-3 hour drive is ok, even on rural highway doing 70 isn't bad. And is a lot of fun from light to light lol.

  19. BrianTrick

    BrianTrick Brian Trick

    I just remember my dad getting a ticket for doing 71 on the highway that had a 55 speed limit. Now we set our cruise above that.
  20. monzaz

    monzaz Jim

    We live in a populous area compared to the Big sky country and the four corner southwest. LOTS of land to cover. :)
    Tickets all depend on how good or bad day the trooper is :D

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