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Projects '49 Chevy 3100 - Rear end swap

Discussion in 'The Hokey Ass Message Board' started by Johnny49, Feb 29, 2016.

  1. Johnny49
    Joined: May 18, 2013
    Posts: 32

    Johnny49
    Member

    FULL DISCLOSURE: Total amateur here, but I do have adult supervision... lol

    Looking forward to our second season back on the road after a 30 year nap, planning to swap out the original rear end for something a little more highway friendly, from a 69 Firebird. Of course, this means we'll also have to swap out the enclosed drive shaft, and maybe the original tranny as well. Just wondering how others have handled this... Is there a particular open drive shaft that will work or will we have to have one made? And will swapping in the Firebird rear end also require a new tranny or will the original 3 on the tree still work? Any advice/suggestions y'all might be able to offer would be greatly appreciated...
     
  2. my truck used a 60s Saginaw 3 spd full sync unit
    hooked up to the original linkage easily
    the driveshaft was some measuring and junk yard shopping
     
  3. you may consider installing an S10 5 spd
    lots of info out there on how to do that
    there is also a Saginaw 3sp overdrive
     
  4. Jalopy Joker
    Joined: Sep 3, 2006
    Posts: 31,179

    Jalopy Joker
    Member


  5. HOLLYWOOD GRAHAM
    Joined: Apr 11, 2007
    Posts: 1,437

    HOLLYWOOD GRAHAM
    Member
    from Ojai,Ca

    Better measure the width of the FB rear to make sure it is not too wide. I used a 70's Chevelle wagon rear which fit just right.
     
  6. The 3 on the tree linkage?
     
  7. factory shifter on steering column.
     
    leadfootloon likes this.
  8. I want to do something with the trans in 47 chevy Fleetline. Stock trans works but could be better.
     
  9. If you are going to go with a open drive rear end, you will need to swap out the stock transmission with a open driveline unit.
    Dave
     
  10. On closed drive models, the leaf spring bolt that also aligns the rear end spring pads is either 1" or 2" (cannot remember) off center to the rear wheel openings. You'll need to drill an off center hole in each spring pad on the new rear axle to compensate. If not, your rear wheels will set forward in the wheel opening.
     
  11. yes
    the stock 3 on the tree for the 60s Saginaw box

    also look at 55-63 gm car rears
    a friend used a 55 in his truck
     
    leadfootloon likes this.
  12. Johnny49
    Joined: May 18, 2013
    Posts: 32

    Johnny49
    Member

    Thanks for all the info, guys! Much appreciated. Looking forward to takin' 'er down the road with the new fixes...
     
  13. gimpyshotrods
    Joined: May 20, 2009
    Posts: 23,264

    gimpyshotrods
    ALLIANCE MEMBER

    What ratio does the Firebird rear end have?
     
  14. 56sedandelivery
    Joined: Nov 21, 2006
    Posts: 6,695

    56sedandelivery
    Member Emeritus

    I believe it's Langdon's that offers 3.55 NEW gears for your year truck now. Unless you really want to tear everything apart and reassemble it with new, used parts. Most rearends, except some trucks, will have 5 bolt hubs as opposed to your 6 bolt hubs still on the front of your truck. By only swapping gears, it'll save you time, effort, and money.
    I am Butch/56sedandelivery.
     
  15. Johnny49
    Joined: May 18, 2013
    Posts: 32

    Johnny49
    Member

    My friend and main mechanical man says 3.55...
     
  16. Johnny49
    Joined: May 18, 2013
    Posts: 32

    Johnny49
    Member

    Thanks Butch! We actually already have 5 bolts up front with a disc brake kit, and with the same in the rear I'll be able to snag the other two free rally wheels that another friend has waiting for me... :)
     
  17. studebaker46
    Joined: Nov 14, 2007
    Posts: 715

    studebaker46
    Member

    the fire bird rear should be the same as the Camaro and nova which work well with an ad truck. if your trans is good and somebody has a junk later model the tailshaft housings will interchange. butch has probably the best and most economical idea in the long run JMO Tom
     
  18. gimpyshotrods
    Joined: May 20, 2009
    Posts: 23,264

    gimpyshotrods
    ALLIANCE MEMBER

    I am almost always a more gears equals more better kind of guy.

    With a 3.55:1 gear, and a tire about the stock height, you might be just over the line for a manual transmission with an overdrive, unless it is the low-ratio S10 T5 (4.03:1 first, 0.89:1 fifth). That might actually work pretty well, if you don't mind a floor shifter. You can get an ultra-short throw base, so the long stick does not feel like rowing a boat. CoreShifters is who I like: http://core-shifters.com/collection...-gm-t5-5-speed-trans-swap?variant=10551810179

    You would need to make a tail-mount, but that is a low hurdle.

    The S10 input shaft is a little longer that the common length for other GM transmissions. It would need to be trimmed, or you'd need a spacer.

    The spacer, for the easy road: http://www.hotrodworks.com/product/t5-muncie-pattern-transmission-adapter-5-18-bore/

    Oh, and my Mom lives in West Haven. I grew up in CT. Still have family there. My brother is in the CSRA. http://www.csra.org/ And runs the 100%Kulture shows.
     
    Last edited: Mar 1, 2016
  19. gimpyshotrods
    Joined: May 20, 2009
    Posts: 23,264

    gimpyshotrods
    ALLIANCE MEMBER

    As has been mentioned, one extremely important thing to note when swapping the rear axle on one of these is the position of the spring pin.

    On a modern vehicle, the bolt that holds the leaf pack together also locates the axle on the leaf spring. It traditionally as a round head, that fits into a round hole in the leaf spring perch on the axle. On a modern vehicle, that hole is dead-center in that perch. Like so:

    [​IMG]
    If you examine how your original rear axle is attached, you will note that it is not affixed by u-bolts, but has a pivoting mechanism. There is a leaf spring pack bolt in there, but it locates the pivoting mechanism, not the axle, per-se. If you put a conventional leaf spring perch on top of that bolt, the whole axle will be about 1-1/2" forward of where it should be.

    The way around this is to either re-drill a new hole in a fresh perch, or to buy perches designed to correct this:
    [​IMG]
    Note the forward hole (in addition to a centered one).

    There are handy kits for this, in-case you don't want to fab, or in-case you need inspiration:
    https://www.performanceonline.com/1947-55-chevy-gmc-3100-rear-end-conversion-kit
     
    wraymen likes this.
  20. Johnny49
    Joined: May 18, 2013
    Posts: 32

    Johnny49
    Member

    Definitely with ya there. Can't tell you how many times I went looking for the nonexistent 4th gear while driving this thing last summer...

    Small world, huh? I grew up here as well, in Bethel. Thanks for all the info, links and photos. Great stuff!
     
  21. Johnny49
    Joined: May 18, 2013
    Posts: 32

    Johnny49
    Member

    Measured up the Firebird rear end yesterday. Looks like it'll be a perfect fit!
     
  22. Forty9
    Joined: Mar 10, 2016
    Posts: 3

    Forty9

    I have a '49 3100 as well. It's an older restoration that I can see had been drag raced and they attempted to patch it up for a quick sale with a new rear end and transmission. I've sorted through a bunch of problems so far and the biggest one was the rear end. They had put a "gm" 12 bolt rear/3:73 in with whatever drive shaft was there already without cutting it, which caused a bad fit. The rear axle was installed at an angle causing the truck to appear lower at the left side when viewed from the rear and the right hand tire was rubbing and actually burning through the side of the truck bed. We took it all apart and redid it and it's good now. Moral of the story, measure 3x install once and good luck with the truck, great year. :)
     
  23. Johnny49
    Joined: May 18, 2013
    Posts: 32

    Johnny49
    Member

    Thanks and welcome to the HAMB! Always nice to run into another '49er! Holy cow, that truck must have looked pretty crazy goin' down the road before you set it straight. We will absolutely make sure everything is right before final install. In fact, I might just drive my mechanic crazy with the measuring... ;-)
     
  24. Forty9
    Joined: Mar 10, 2016
    Posts: 3

    Forty9

    Somebody did a really nice job with it. I'm in love with it, the next one I build though and I'm going to take my sweet time.

    I see you're in CT, I'm on the NJ shore. Bookmarked your blog :)
     
    K A likes this.
  25. Johnny49
    Joined: May 18, 2013
    Posts: 32

    Johnny49
    Member

    Thank you! Hope you like it...
     
  26. Mr48chev
    Joined: Dec 28, 2007
    Posts: 33,861

    Mr48chev
    ALLIANCE MEMBER

    I put a 69 Camaro axle under my 48 in the mid 70's and it fit well. Gears were wrong for the M21 Muncie four speed it had at the time though. Truck would cruise at 80 at 3000 rpm with N50 14 tires on it.
    The only problem you might have is that with some wheel and tire combos the tires can get too close to the bed sides.
    I think I have a 74 Nova rear axle under the truck now and while my regular rear tires clear by a half inch I can't put a standard offset 15 inch rim on the back even with the 75 series tire on it without rubbing.
    I run a full syncro Saginaw 3 speed now and have for years and they work well.
    As far as drive shaft goes it's either do a lot of junk yard hunting or have one made.
     

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