Steering...

At the risk of being overly tutorial, let me describe how steering works on '50s vintage cars.  The steering wheel is attached to a long shaft, inside the steering column, which is in turn attached to the input shaft of the steering box.

Exploded view of Thunderbird steering.  The steering wheel would be above and to the left of this diagram.  The front wheels attach to the right and left ends of the spindles.

The steering box is bolted to the frame on the bottom front driver's side of the engine compartment.  It contains gears that translate the motion of the steering wheel into a much smaller motion of the Pittman arm, which projects out below the chassis frame behind the front wheels.  A rod called a drag link is suspended between the Pittman arm and a similar sized idler arm bolted on the opposite side of the engine compartment.  It is set up such that turning the wheel to the left will move the drag link very precisely to the right, in a very controlled way.  Left and right tie rods connect this drag link to the front wheel steering knuckles, which in turn control the direction of the spindles and thus the front wheels.  Turning the steering wheel turns the steering box input shaft, the internal gears then translate this motion to the Pittman arm, which moves the drag link slightly, pushing/pulling both the tie rods, and then the steering knuckles, moving the spindles and turning the front wheels.

Modern steering boxes are actually very remarkable pieces of engineering... they typically use either "recirculating ball" or "rack and pinion" technology, which is very efficient, reliable, and they don't wear out... at least not for hundreds of thousands of miles.  In 1955, however, Ford  used "worm and sector" technology.  These are not so reliable, and they do indeed wear out.  The worm gear is on the input shaft (connected to the steering wheel), and the sector has a roller gear to tightly engage the worm.  Here's a sequence with the top cover removed showing the worm and roller.  The input shaft is on the right side of the picture, the worm gear at the bottom, and the sector arm at the top.

Full right turn.  Note how rough the worm gear is.  It needs to be replaced.

Center

Full left turn

It's pretty clever, and it works well.  The problem with this arrangement is that, over time, the worm gear tends to wear down in the center (because the wheels spend almost all of their time pointed straight ahead or at very small angles), but it doesn't wear much on the ends.  This wear can create a lot of "slop" in the steering... up to several inches of steering wheel travel before the front wheels move.  That's actually pretty dangerous at highway speeds!  The gears can be tightened together  to remove this slop, but then the steering will bind up and jam in a tight turn.  That's even more dangerous.

The steering box on my '55 t-bird has exactly this problem... too much slop.  It needs to be rebuilt.  It is also still on the car, at the body shop.  I needed to leave it on the car... otherwise, the front wheels couldn't be turned together, and moving the car around would become very difficult.  Since I have to rebuild the steering box anyway, I looked into replacing it with something better. 

1957 was the last year Ford used worm-and-sector steering boxes.  1958 Fords used recirculating ball steering boxes, which are much stronger and don't have the "wear in the middle" problem.  Sadly, this newer steering box won't just bolt onto on my t-bird.  The steering column would need to be shorter.  Also, '57 and earlier Fords routed the horn wire from the steering wheel down the hollow center of the steering shaft, all the way through the steering box, and out the end to pick up the horn relay.  Later Fords don't do this, and use a brush and slip ring for the horn signal.  To use the later steering box, I'd need a whole new steering column, and steering wheel, and turn signal switch.  Now we are into some big $$!

My t-bird does not have power steering.  While I can easily bolt on a period-correct power steering setup for $2K, I really don't want to.  Vintage power steering units are basically a hydraulic ram that moves the drag link back and forth, based on the position of the Pittman arm.  They tend to leak, and don't provide much driver feedback.  This is very unlike modern power steering.  I've decided to keep the manual steering, and it's better road feel.  I may have to work a little harder when parallel parking, especially with radial tires.

Note that the roller on the sector arm in the pictures above has three teeth... this is a 1957 t-bird "three tooth" box.  It is a bolt-on replacement for the steering box on my 1955 t-bird, which is a very similar but weaker "two tooth" box.  My plan is to rebuild the three tooth box, since it should last longer and be better able to handle the stress of wider-than-stock radial tires.

I got the '57 steering box on eBay for $.99!  Alas, I spent $40 on shipping.  Even though the gears are trashed, it was a very good deal!  Buying a complete rebuilt three tooth box would cost about $800.  I can have this one rebuilt locally for about $400. 

Overall, I feel quite fortunate.