Intro...
So you want a quick ratio box in your Mustang? Better yet a quick ratio box with feedback that feels like a modern car instead of the "pinky finger" power steering it came with? It can be done, with wrecking yard parts and a moderate level of tools. Best yet it can be done with no change in the appearance of the steering box, and no modifications to your car. The GM A-body guys have it easy, since the Jeep Grand Cherokee box will bolt right up in their cars. However the Mustangs use a different casting, with different mounting points. There were no applications that used the 71-73 Ford mustang style Saginaw 800 casting with the 12.7:1 ball screw and piston. So if you want a quick ratio box you must swap in the 12.7:1 ball screw and piston out of a GM or Jeep vehicle that had one into your casting. The thread below is a general guide to what has to be done.
I started building up a steering box today. My 71' Grande had the super slow fixed ratio box. The variable ratio box came from another member on the forum. Add in a 91' Firebird 12.7:1 fixed ratio box and I have a mountain of mix and match parts. Goal is to use the Mustang casting, with the quick 12.7:1 firechicken ball screw and piston. Also going to use the firechicken input since it has a much larger T-bar (Torsion Bar). We'll see how it turns out, I was thinking of using the Mustang input shaft, but the T-bar (the thin little shaft I'm measuring with a caliper in a photo below) on both of the Mustang boxes was so tiny that I decided to go with the Firebird input. That means I will have to get an adapter rag joint since the splined input shaft is smaller. Hopefully it will be worth the trouble to get more feedback that the larger T-bar will offer.
These are all Saginaw 800 series boxes and the parts interchange minus the input shaft and the casting itself. This is a pretty neat DIY project and the 12.7:1 quick ratio gears are plentiful in many 1980's GM F Body (Camaro & Firebird) along with many of the Jeep Grand Cherokees. If you are go to a pick-n-pull yard you should be able to walk out including tax and core charge for less than $75.00 and have the parts you need minus a $30.00 rebuild kit from Rock Auto. The info on what exact models to harvest from is posted a few pages ahead.
Pictures...
You can see the differences in the internals.
Ball Screw. Quick ratio 12.7:1 ball screw in the bag on the far left, stock variable ratio in the center, slower fixed ratio on the right. Note the two lines between each "thread" on the 12.7:1 screw. This is unique to the 12.7:1 screw and can be used to identify it in a wrecking yard. Just remove the pitman arm, then unscrew the four bolts on the sector shaft cap. Make sure the steering is in the center of it's travel and pop the entire sector shaft and cap out (mallet from the bottom). With a flashlight you should be able to turn the junkyard box to full lock in one direction and see the ball screw. If it has the two lines like the one in the photo you are good to go!
View attachment 33437
Variable ratio sector shaft and piston. Note the deeper center tooth for the variable ratio.
View attachment 33438
Fixed ratio sector shaft and piston. Notice that all the teeth are the same depth. The fixed ratio teeth seemed to be identical to the GM F body ones.
View attachment 33439
.185" diameter torsion bar out of the SPA S fixed ratio box. This is pretty small and is why the stock steering boxes take so little effort to turn with next to zero feedback. Larger torsion bars = more feedback. You should never take apart an input assembly this far if you intend to reuse it. I took it apart for learning.
View attachment 33440
Slightly larger .190" torsion bar out of the variable ratio SPA AF box. Still small but better.
View attachment 33441