42RH / A-500 in a Duster with pictures.

Transmission and Drivetrain Tech

  1. gdonovan

    gdonovan Well-Known Member

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    This thread is for detailing the install of a 42RH or A-500 in an A-Body. This should also apply to either an A-518 or 46RH as well since they have the same overdrive unit as a 42RH.

    The car in question is a 1974 Duster with a late model 1994 5.9 with Enginequest heads, CompCams .510 roller & Edelbrock RPM intake. The car currently has 318 manifolds which are going in the trash since I had to cut off the front section of the dual exhaust and H-pipe anyways. The manifolds will be replaced with a '73 340 manifold on the drivers side and a early magnum Dakota manifold on the passenger side with 2.5 inch outlet.

    The purpose of the 42RH swap is to improve the launch of the car (2.74 first gear vs 2.45 in the 904 and better converter,) improved mpg (1900 rpm at cruise vs 3500 rpm,) higher top speed and less cabin noise at speed due to the lower operating rpm.

    The 42RH is pretty much just a slightly newer version of the A-500 from a local yard (1994 Dakota 3.9 application in this case) It had a 4/4 clutch setup which will be updated to a 5 clutch setup with either a different drum or thinner steels.

    Pictures and more details will proceed this post! Gary

    PS: Please do not inquire in this thread about weakness or strengths of the 42RH / A-500, transmission upgrades or how it will be shifted into overdrive and lockup. The narrow focus of this thread is how to install one in a car, period.

    Trans upgrades and shifting of the overdrive unit and lockup will be covered in their own threads and I'll request moderators to remove any non thread related posts so things remain concise and to the point for your reading pleasure

    Thanks!
     
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    • gdonovan

      gdonovan Well-Known Member

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      Ok the first picture-

      At this point the 904 has been pulled out, the engine blocked into position (1.5 degree down angle to the rear using the carb as the point of reference) the front part of the exhaust along with the H-pipe has been removed and the torsion bars have been backed off 17 turns per side to unload the tension on the cross-member.

      What you are looking at is the 42RH case bolted into place to see what has to be clearanced. Some minor hammering gave the floor enough room to not hit the case.
       

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      • gdonovan

        gdonovan Well-Known Member

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        Here you can see the area that has to be removed to clear the overdrive section. The letters A indicated where I used a ball peen hammer to move the tunnel outward which gave me enough room for case clearance AND enough room to cut out the cross-member.

        At this point I have ample room to install the transmission, indeed it is in the car as we speak. I'll take a picture this afternoon and update the thread. There is NO cutting of the floorpan required to do this which is a bonus in my book.
         

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      • 66plyValiant

        66plyValiant Well-Known Member

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        interesting thread, I hope that if your using a 3.9 transmission behind a 360 that your going to build it for intended use, the 3.9 tranmission is built with lighter duty clutches and gears and bearly servives life behind a mild 3.9

        also I have heard that the OD can fail from lack of fluid if your intended use is mostly 1/4 mile
         
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        • gdonovan

          gdonovan Well-Known Member

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          Ignoring what I have to do in the guts of the transmission here is a list of stuff to be completed to install the transmission. Shifting OD and lockup will be detailed later.

          1) Find a fairly flat mount to use as a transmission mount that is close to the 42RH bolt pattern. One board member states Energy Suspension 3-1108G works which is a GM application, 1964-2001!

          2) Reenforce the area that has been cut out without hitting the transmission.

          3) Fabricated a new crossmember (with four holes) for the trans mount.

          4) Get the drive shaft modified. (local shop)

          5) Longer speedo cable from another application like a C-body or truck perhaps.

          All the other stuff like the linkage is a bolt up so should be a non-issue and I already have the magnum flex plate.
           

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        • gdonovan

          gdonovan Well-Known Member

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          These subjects are not the focus of this thread, I'm trying to keep it simple as possible. Upgrades to the 42RH are not going to be covered here but in another detailed thread. People run hemi's with 904's, a 42RH with minor mods will have no problem behind a 5.9
           
        • gdonovan

          gdonovan Well-Known Member

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          Ok some quick pics!

          3 shots of the underside showing how the overdrive now fits and one showing the floor from the inside of the car.

          I'm going to grind the truck mount boss off where the parking breaking cable passes, its not used in the this application =)
           

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        • BRONZEBEE

          BRONZEBEE Well-Known Member

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          Here is a few of my 70 dart and the cross member and tunnel.Still need to finish the upper plate that goes in the tunnel on to the floor.The drivers side inner cross member mount hole i moved out 1 1/2 in to make more room for the o/d housing and i shaved the right side mounting bracket on the transmission housing for more clearance.
           

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        • gdonovan

          gdonovan Well-Known Member

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          Ok more work tonight!

          1) I fired up the plasma cutter and gutted a spare transmission crossmember

          2) Here is the trans mount from Autozone, $6 I only oblonged one hole so the mount sits a bit to one side as the overdrive bolt pattern is not centered.

          3) The mount installed and the cross-member notched to clear it.

          This is where I'm at now, I'll chew on what I'm going to do next overnight.

          First cruise in April 4th, I will be ready!
           

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        • C130 Chief

          C130 Chief Mechanical Genius

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          I'm interested to see how you replace the crossmember above the transmission. Plan this swap myself, but still soaking up ideas.
           
        • gdonovan

          gdonovan Well-Known Member

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          What I'm planning to do is take and reinstall the two cross-member bolt tubes in location "B" and take a piece of 1/4 inch thick steel and weld it into place like shown in this picture.

          I'll then follow it up with another piece (maybe thinner like 3/16) on the topside of the floorpan.

          For good measure, it looks like there is enough room under the car (near the B locations) where I can weld in a vertical ridge along the sides where the cross-member is cut out to make it even more rigid.

          I'm going to make a cardboard template of the transmission tunnel tonight and start bending the steel in my 12 ton shop press.

          Gary
           

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        • BRONZEBEE

          BRONZEBEE Well-Known Member

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          In the process of bending a 3/16 plate like in the picture,had to borrow a torch to make the bend.I have the top welded but i have to cover the ends of the cross member.I will get pics soon.:glasses7:
           
        • Clamcake

          Clamcake Well-Known Member

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          This is excellent. I would love to have an O/D trans but fear making the torsion bar support too weak. With your large press, you should take orders for the properly bent steel for the upper part of the cross member (You should sell a kit to people like me). Thanks for posting, this is great.
           
        • gdonovan

          gdonovan Well-Known Member

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          I have the hoop done and test fitted, two new bolt "tubes" for the cross member welded in but need to take a few days off due to a minor accident.
           
        • BRONZEBEE

          BRONZEBEE Well-Known Member

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          • ateam

            ateam A motor can

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            This is first rate information since I plan to use an A500 with a Gen3, in 67' cuda. I suspect, with coil or bag suspension in front, crossmember area will need much less structure. Thanks, ateam.:cheers:
             
          • AnotherA

            AnotherA Well-Known Member

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            Any reason why that cross member would have to stay in the car if i'm going with coil over suspension?

            I picked up that Duralast 2378 mount today. Going to give it a try, want to stay away from the poly stuff for now.

            I installed a poly tranny mount in my 98 Camaro a few years ago. It put a loud drive train vibration and noise into the vehicle. Put the rubber mount back on and it was silent.
             
          • BRONZEBEE

            BRONZEBEE Well-Known Member

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            With coil over suspension i don't think it would make much difference,but connecting the the two cross member together makes me feel better.Still planing on running the bars,and putting money aside for the fuel system.Thinking of setting one up like Mad Dart did,but maybe with two pumps instead of three.Unsure at this point.

            I used urethane engine mounts inserts,so i figured that i would do the transmission also.I was looking into urethane front suspension and after pricing it out i think i will stick to Moog.I like the:blob:
             
          • gdonovan

            gdonovan Well-Known Member

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            Welded in the lower "hoop" today, not sure if I'll do a matching unit in the passenger compartment as it is already pretty stout.

            Tomorrow I'll work on the mount cross-member as the weather is getting nice and I'd really like to drive the car soon.

            Gary
             

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          • gdonovan

            gdonovan Well-Known Member

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            Transmission mount complete! Lots of welding, grinding and slicing to come up with this, I'm rather pleased how it came out. When I was done, slathered it with POR-15. The one in the foreground is the original of course,
             

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          • gdonovan

            gdonovan Well-Known Member

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            I'm really looking forward to reassembling the trans and going for a road test!
             
          • BRONZEBEE

            BRONZEBEE Well-Known Member

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            Looks good,let us know how it drives:cheers:
             
          • gdonovan

            gdonovan Well-Known Member

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            Some more test fitting today- Stuck on the pan and bolted everything up. Was surprised to see the pan is the lowest hanging item! I would have thought it was the trans mount but was proven wrong. Everything looks good, the H-pipe even clears everything. Still have to grind the rear mount off the tail shaft to clear the brake cable, I'll do that when the case is out and I'm reassembling it. Right now I'm heading out to take measurements for the drive shaft. There is a local shop would can do the shortening for small money.
             

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          • inkjunkie

            inkjunkie Well-Known Member

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            If you have subframe connectors there is an easy fix for this concern. Just triangulate the torsion bar crossmember into the subframe connectors.

            View attachment IMG_3184r.jpg
             
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            • gdonovan

              gdonovan Well-Known Member

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              Current speedo cable length is 62 inches, going to order a 80 inch one from Rock Auto for $13 plus shipping. Need a longer unit to reach the speedo gear location at the rear of the overdrive housing of course!

              42.25" center to center on the new driveshaft.
               
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