I am considering swapping my 4spd BB to an automatic. I see lots of posts to convert from auto to 4spd, but not many on the other direction. Anyone with knowledge or experience with this type of conversion in an early model (71) C3?

 

I know it can be done, I have a friend here who had to do his 68 4 speed back to an auto because his left knee was giving him trouble.

It was easy enouh for him as his workshop has a 2 post lift and he has one of those g/box hyd. Lifter units. But even with that he said it' s a struggle to get the box in.

 

GM did not make the center crossbrace removable on auto frames just for fun. It was removeable so the larger auto transmissions could be removed in service. I have no idea if it is possible or not, since I have never done it. Without the body on the car.....I am sure it is easy, but obviously that is a project.

 

Jeff,
You said that a bit contradicting,
To clarify,
The cross member is removable on auto frames not removable on manual frames,

When my auto 69 received the rebuilt chassis from a manual 68 i had a special cross member made up that is bolt in as i wasnt sure if i wanted to go 4 speed auto or old 4 speed manual but knew whatever i did the removable crossmember would make it easier...

On my manual 66 when i had to pull the muncie to get it freshened up i only had that car as a dd,
We pulled engine and trans together and slid the engine and porkslide ( powerglide ) back in, of course it wasnt "that" simple....

 

Silver Sport, SST makes a kit that includes the correct torque converter, driveshaft , trans mount and cross - member. They use a 4 speed auto with overdrive - the best of both worlds if going automatic.

I would call and talk to their tech department.

I spoke to them at Carlisle, they really know what they are doing. My friend was with me, also spoke to them, he wants to do the auto conversion also. He said when you do the math - their price is really mot that bad. SST explained the procedure to him - he was impressed.

I used their 5 speed conversion and it fit perfectly.

 

21.5 years ago I moved to Florida with my muncie '72 vert....trick is that up in the DC region I shifted from 2-4 all the time in traffic 1&3 hardly ever used.....3.36 rear......but with the move south, I found the cruise speeds here are 80 mph witch was ~4k on the SBC engine...a tad much....so I put in a 700 r4 at first, but the 1-2 shift was a bit much with the shift kit, so built my own 200 4r and it's there now.....

The number one thing is to have a good welder, which a buddy is.....he can weld aluminum to concrete and make it stick.....removed the muncie and had the driveshaft shortened as the 700 is too long, the trans mount fit fine on the muncie forward extension off the cross support, the cross support was cut on each side about 4" from the frame rails, ~2" angle welded onto the upper rear of the support and a 1/2 inch bolt going fore/aft on each side the vertical load held by the angle iron....and the bolt of course, a piece of steel not as large on the bottom too, to keep the sections squared up and together, no flex.....the removable section was modified to make removal easy with the exhaust pipes, being lazy and all....cut the bottom of the loops out, and welded up the top sections with some steel to prevent flex, lowering the exhaust pipes maybe an inch or so, still high enough.....
sorry no pix as they gone long ago on another computer.....
The 200 trans takes the muncie drive shaft, no mods, and the trans also set right on the cross member just fine, using the two bolt holes holding the muncie support in place.....
Now for the shifter, the stock shifter mounted to the body plastic, not the cross support, but my version uses the muncie mount off the frame, and I found a HURST chrome shifter bar/shaft/arm at a car show, and to put an 8 ball on it.....just not cool if you don't have an 8 ball on your shifter....there is no buttons/interlocks to the shifter, move the lever and it rolls, steering column lever was tied to the upper position, and later removed with the change to a late shark TT column.....the console plates were removed from the main bezel and new ones cut and etched by a guy making plaques for sports awards....did same for engine specs....
see interior pix below......

OH, forgot, need to cut the floorboard on pass side and pry the tunnel out about 2 inches to clear either trans pan and cooling lines, glass it in......keep same carpets, no problem.....I just cut the clutch pedal off and had done with it, but it could be lowered down and hidden under the carpet....

one of the shifter rods worked fine length wise and used one of the muncie levers on the auto trans,
I don't remember about the output yoke, maybe have to change from muncie to a smaller drive shaft to fit the 200 trans...?? I forget to remember.....

 

You can do it with some "massaging" of the crossmember, etc. The main thing you should consider is the loss of value in the car. An early C3 BB w/manual trans will bring more $$$ than an auto tranny car. But, if you are going to keep the car forever and want it to have an auto tranny, go for it! I got tired of shifting a long time ago, and if they are set up right, an auto trans car will be about as quick as a manual shift car. But on a daily basis, the auto car is a lot easier to drive.

If you do this, I highly recommend that you install an overdrive trans in your car. It would be the right time to do that.