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Purpose in second set of holes in frame from 89 2wd x member?

Discussion in 'Automatic Transmission' started by Vbb199, Sep 24, 2020.

  1. Vbb199

    Vbb199 B-rate Hillbilly Customs Supporting Member

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    So yesterday, I got my new badass 700r4 in my 89 suburban, bolted up, TC cable, cooler lines, torque converter, lockup harness, all of it.

    So I put a jack under the trans with a block of wood and lifted the tail up so I could put my crossmember in..

    I discover it will hit the deep pan on the transmission!!

    First thought, being who I am, I say to myself, no problem, I'll just notch. The cross member, so I get a marker, and begin marking where I'll cut....


    Then I notice... The frame has a second set of holes roughly 8" away from where I pulled the x crossmember, also, somehow it slipped my attention, so does the transmission (tailshaft housing),

    By moving it down that 8" the x member will clear the pan.
    Not a problem. But then the weight is being applied to the tailshaft housing rather than previously on the body of the transmission right after the pan.

    My questions are =

    1. Why is there a second set of holes in the frame? It's a 2wd, I'd understand if it was 4x4 because the crossmember is then attached to the t case adapter , kinda makes sense? Not really, because a 2wd frame is diff than a 4x4.so what gives?

    2. Will this be safe to do? I realize from an engineering standpoint, the weight of the trans is supported at the motor, and then the four 3/8 bolts on the tail housing would have to sheer off, but it just looks.... "off" to me.

    Tell me your thoughts.
    Thanks guys.

    I'll send pics of the holes in the frame later if someone wants to see.

    20200923_174307.jpg
     
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  2. Keith Seymore

    Keith Seymore Full Access Member

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    Could be for a center support bearing, or for a different crossmember location.

    Keep in mind that frame has to cover as many possible combinations as they can. There are very likely going to be a bunch of unused holes for your specific options.

    K
     
  3. Vbb199

    Vbb199 B-rate Hillbilly Customs Supporting Member

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    Since we're on this topic, and you're the man to talk to, I also wanted to ask, why does my 89 have a 2 piece driveshaft?
    But my 1990 suburban didn't. It was 1 piece.
     
  4. legopnuematic

    legopnuematic Supporting Member Supporting Member

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    From what I have gathered 700R4's got 2 piece and 350/400 trans got 1 piece shafts.
     
  5. Vbb199

    Vbb199 B-rate Hillbilly Customs Supporting Member

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    My old 90 burb was 700r4, my 89 is a 700r4.

    The only differences is, my 90 was a absolute bare base model, the 89 is full options as well as HD radiator, ps cooler, oil cooler, and towing kit,(which is what I assume maybe makes the difference).
     
  6. Keith Seymore

    Keith Seymore Full Access Member

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    It has to do with the driveshaft
    "critical speed": the point at which it begins to swing like a jump rope. It is affected by length, outside diameter, thickness, material, mass and rpm. Highest shaft rpm is affected by vehicle top speed, rear axle ratio and tire diameter. Length is affected by wheelbase and driveline configuration.

    The driveline is also affected by any unusual or unpredicted system resonances, which can destroy a driveline even at a speed lower than the critical rpm.

    From an OE perspective the driveshaft choice (one piece vs two piece) can be manipulated by controlling any one or more of those variables.

    So you could have two otherwise identical trucks but one has a smaller diameter tire driving a two-piece shaft; the truck with the larger diameter tire could have a one piece.

    Or - you could have two trucks and one has an aluminum composite one piece shaft and the other has a steel two-piece shaft.

    Or - you could have a truck that requires a two-piece shaft but is speed limited to 85 mph so that a one piece shaft could be released for that configuration.

    Hence there's a little more going on than picking a length and slapping a couple yokes on it (from a production standpoint).

    There is a chart that the engineer keeps (like the brake system release charts I have posted in the past) that shows all of the driveline configurations and shaft lengths for every GM light duty truck. I have asked the engineer for it on a couple of occasions but he has not come through with it.

    K
     
    Last edited: Sep 24, 2020
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  7. bucket

    bucket Super Moderator Staff Member

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    Wasn't your '90 a 4x4? My 4x4 Burbs have all had a 1-piece shaft.
     
  8. Vbb199

    Vbb199 B-rate Hillbilly Customs Supporting Member

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    2wd
     
  9. jake wells

    jake wells Fast chevys and hot women.

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    My 75 has a two piece drive shaft with a carrier bearing but my trans is also a short tailshaft th350.
     
  10. Vbb199

    Vbb199 B-rate Hillbilly Customs Supporting Member

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    The 89 and the 90 both were/are 31" long 700r4 trucks, throttle body 350, 2wd, 10 bolt rear ends.
    Everything was identical except packages, options, and gear ratios.

    Like @Keith Seymore said, it may be related to gear ratio.
    The 90 had a 3.08, the 89 has a 3.42
     
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  11. Galane

    Galane Full Access Member

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    Different holes in the frame for different transmissions. 3 and 4 speed auto, manual, 4x4 or 2wd. The mounting pad on the tailshaft is where the rear transmission mount should go. That's THICK aluminum and the bolts holding the tail housing to the case are strong, and threaded plenty deep into the transmission case.

    If the mounting ears have broken off the tail housing, or the tail housing bolts have sheared, I'd assume something very bad has happened to the rest of the truck, like getting hit by a train.
     

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