In the workshop manual, it say's that the steering idler arm is designed to operate with
1/8 inch vertical free play ~ hope this may be of some help for you..

Although my 55 Tbird seemed to track well, there was considerable play at the steering wheel. While rebuilding suspension and steering, I discovered that my steering-gear box had a lot of play, and it was bone dry. I doubt my dad, from whom I inherited the car, which has been in the family for 50 years, ever checked it. I shipped the steering box to Redhead Steering Gears in Washington to be rebuilt. The box came back/w no play, and Redhead had filled it with 00 grease, not gear oil, to minimize leaks. GM did the same warranty repair on leaking steering-gear boxes on early Corvettes. Redhead has a good reputation, and I wouldn't hesitate to use them again. As far as the Tbird's steering-gear box is concerned, it won't be necessary as long as I'm still above ground.

 dbird thanks for your info about their people and their facility. My car is complete and drivable, but has wider tires. I had it on an alignment machine years ago and was checking for any "play" in the steering. The play I found was because of up and down movement of the idler arm. I have recently replaced both the arm and bushings and no improvement because of play in the steering. Back to the play, when moving the the front tires back and forth the idler would move up and down a small amount before actually changing direction of the tires, PLAY in the steering, which transmits movement of the car, (wandering) on grooved or ruff paved roadways. I want to make the car stop doing this,or the best I can. Maybe this is a flaw of 50's driveablity of the car, and the wider tires make it worse, but I remember this happening many years ago with narrower tires and the same thing on my 56 Vicky. And yes the rest of the suspension is rebuilt or in perfect condition and no dropped height.    1st Cav Man

Are you missing an idler arm? I don't see how the arm, itself, can wear out. Replacement bushings are available, but I can't attest to their quality.

Rare Parts in Stockton, CA?   While I haven't actually used any of their parts, but did have an opportunity to tour their facility a couple of years ago.  Nice people who seem to have a lot of pride in what they're doing.  From what I remember, everything is made on site to original spec.  Plus the owner had a 57 'Bird at that time, an E code I believe. 

Don W
55'Bird,  56 F100

I am looking for some feed back on Rare Parts brand idler arm. The parts other venders are selling are a waste of money. This is for 56 Tbird, manual steering. Thanks.   1/8 Cav Man