8.5” BOP rebuild

Discussion in 'Got gears?' started by Darron72Skylark, Oct 13, 2019.

  1. Darron72Skylark

    Darron72Skylark Well-Known Member

    I’m looking for a little direction on rebuilding the axle on my 72 Skylark.
    It currently has 28 spline shafts, look to be original, and in good shape; just pulled, inspected and measured them this afternoon. Open differential, 3.42 gears.
    I’m in the process of building up a Buick 350 for the car with the new TA heads, forged pistons and rods, TA 310 camshaft. Projecting power in the 400+ hp range, and torque in the 425 to 450 foot pounds.
    My question is whether this power/torque level needs 30 spline axles. I’m planning to replace the open diff with a TrueTrac unit and keep the 3.42 gears.
    This is a manual transmission car, if that makes any difference. The 3 speed Saginaw is coming out in favor of a Tremec TKO 500 when the new engine goes in.
    Planning to use this car mostly on the street with some occasional road course, autocross and possibly one or two trips down the drag strip.
    Last edited: Oct 14, 2019
  2. DasRottweiler

    DasRottweiler -BuickAddict-

    I'm only running low 13s, 13.09-13.14 or so. 3853 lbs with me in the driver seat on MT drag radials. Launching, 2500 to 2800 rpm, 8.5 rear 3.42 posi, 28 spline stock axles. No issues as of yet. Was told the stock 28 splines are good into the high to mid 11s. JIM
  3. Bens99gtp

    Bens99gtp Well-Known Member

    I wouldn't tell others to do it, but I ran stk axles into the 10s in a 3800 car, I did twist a few axles from time to time.

    But for your bound I wouldn't worry about them as long as your not doing constantly high rpm clutch drops

    Attached Files:

  4. Darron72Skylark

    Darron72Skylark Well-Known Member

    Wow, that’s a twisty axle shaft!
  5. Bens99gtp

    Bens99gtp Well-Known Member

    Yeah, I would say mid 11s is saf . Much lower than that its time to upgrade for safety
  6. agetnt9

    agetnt9 Agetnt9 (Dan)

    I got the truetrac and 33 splines from Monza Just because i have grease on the clutch pedal .. Ha
    Freakazoid likes this.
  7. BrianTrick

    BrianTrick Brian Trick

    You will be fine for what you are doing. However,most of us will add more power as time goes on. If you have ANY future plans to do that,just get a set of 30-spline shafts made,and a matching unit,and do it once. The price is the same for the unit,whether it is 28 or 30.
    Darron72Skylark likes this.
  8. Darron72Skylark

    Darron72Skylark Well-Known Member

    Thanks Brian, sounds like 30 spline shafts are cheap insurance, in the overall scheme of things.
  9. DasRottweiler

    DasRottweiler -BuickAddict-

    Bear in mind , you have to change posi carrier to a 30 spline unit as well.
    2 axles, carrier and bearings etc, not so cheap. Jim
    Just re-read original post, that open carrier has gutta go so , I'd go 30 spline as well. Doh...
  10. BrianTrick

    BrianTrick Brian Trick

    He doesn’t have a posi yet,so it makes more sense to do it now. He currently has a 3:42 non-posi.
  11. Bens99gtp

    Bens99gtp Well-Known Member

    Posi, axles, bearing etc, will be close to 1000, but there really isn't much cheaper way to go and be strong.

    We just went low 6.50s in the 1/8th on our this weekend......I will guess that a low 10.40-upper 10.30 pass. And plenty of ppl have gone faster

    The 8.5 is a good rear for sure
  12. Skyhawk

    Skyhawk Well-Known Member

    got a complete rear for sale if interested. it will be in b.g.
    built 8.5 10 bolt. narrowed slightly. fully braced. moser axles, spool, 3.73 etc etc etc
    make a great deal if interested. probably have 3.42 gears for it also.
  13. Darron72Skylark

    Darron72Skylark Well-Known Member

    So I'm running into a fair bit of confusion as I discuss this project with various suppliers.
    I've had two suppliers insist that what I have is an 8.2" axle. When I finally convince them that what I have is an 8.5" with bolt-in axles, that throws them for a loop.
    One supplier is quite certain that 30 spline axles won't fit, the other is doubtful that there is a Truetrac limited slip carrier that will fit in the housing, since the applications chart only lists Chevy trucks from '88 and up.
    In my naivete, I thought that the only issue with a limited slip carrier was choosing between clutches and helical gears, 28 or 30 spline axle shafts.
    Do all 8.5" ring gears have the same number and spacing of ring gear bolts? Is there any variation from different models of 8.5"? Does the limited slip carrier work with either bolt-in or C-clip axle shafts?
    Here I thought this would be a fairly straightforward upgrade, now it seems really complicated.
    I guess I could just stick with the 28 spline shafts and buy a 28 spline TrueTrac, but it seems like I should make upgrades while I have it all taken apart.
  14. Bens99gtp

    Bens99gtp Well-Known Member

    Ok first off stop talking to some of these other supplier. They will tell you your rear doesn't exsist.

    If you want 30 spline axles get ahold of Brian trick or jim from race and restoration. They can order you axles that will fit from moser........don't call moser they are dumb when it comes to this axle. It's considered a custom axle and if you measure wrong you own it. Both these gentle have the proper sizing to get it right.

    I can't answer if all 8.5 are the same I know they front axle 8.5 as well.

    A posi unit for a c-clip 8.5 rear.....will drop in and your axles will fit..

    8.2 axle are super similar but the splines are slightly different than the 8.5

    30 spline is the largest you can fit in the factory size axle bearing...Going more requires new housing ends......or a hard to find vista cruiser rear axle which has larger axles and in think you get 33s in them.

    I dont know if there is a truetracs unit. But you can still get posi units for 28 spline and 30 spline.

    I have used quick performance b4 for many parts inside our 8.5, but if doing g axles just order through Jim or Brian and save tons of hassle
  15. telriv

    telriv Well-Known Member

    AND, they can both save you some $$$$.
    Lucy Fair and DasRottweiler like this.
  16. Darron72Skylark

    Darron72Skylark Well-Known Member

    Thanks, you are completely right on the “that axle doesn’t exist” response. Gotten it from several suppliers, including the ones you mentioned.
    I’m talking to Brian offline about getting the right set up.
    Hope to have it figured out soon ... and if there is some savings- even better!

  17. Bens99gtp

    Bens99gtp Well-Known Member

    Well rear ends are not cheap. I just upgraded mine to 30s.....I think the axles with studs and bearing shipped was 700-750sh......i.think posi are 300-350ish.......install kit about 100-125ish......gears if needed or wanted 150ish.
  18. BrianTrick

    BrianTrick Brian Trick

    Don’t feel bad Darron. I have been ordering axles for these rears for nearly 25 years,and I STILL get a call right after my order,with someone telling me the specs or wrong,or the axle doesn’t exist. I simply tell them to follow the specs,or look at any of my past order history. I usually get “Wow,you get a lot of these”.,but yet they are still considered a custom axle,which yes,costs more.
  19. Gary Bohannon

    Gary Bohannon Well-Known Member

    If stock 28 spline axles are good to 11.50's, then what performance level do you need to weld the axle tubes?
    60 foot time may be more accurate. So how about 1.50 vs 1.45 for, 30 spline axles and welded tubes?
  20. BrianTrick

    BrianTrick Brian Trick

    I don’t get too concerned with welding the axle tubes. I will do it with some strip/race applications simply because I usually already have it in a jig. One of my own 10-bolts in one of my cars is not welded and it’s been going 10’s for nearly 20 years. I’ve also seen rears with welded tubes bend the tubes and I have also seen them break deeper in the pumpkin. A lot of this is the result of wheel hop and the weakest link in the chain.

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