I had play in my steering wheel after installing a MM solid steering shaft. I had a feeling it was the firewall steering shaft grommet because when I took the stock steering shaft out a piece of the stock grommet came with it. There was 1/2" of play from side to side when the steering wheel was dead center. This mod fixed it and now it's precise, smooth, and the steering feels absolutely amazing and dead center. If I had to relate it to anything, it's like driving a go kart versus a boat. See below. I started this talk in Akurma's thread but expanded it here. One note: on the two bolts that hold the part in, do not fully tighten them until the steering shaft is bolted fully and installed on both the steering wheel side and rack side. The two bolts act as a clamp and once tightened do not allow the part to articulate. I started the bolts by hand to hold the part in place. Once I got the steering shaft fully seated and installed where I wanted it, I then tightened the bolts holding down this part.
This new part can be installed while the steering shaft is on the car. Take the bolt out from steering shaft to steering wheel connection. Retract the MM steering shaft down. Undo the two bolts for the old part. Slide off of steering shaft. Slide new part in and set in place. I used a bit of WD40 on the roller bearing to aid in this.
Below is the part I purchased online from a local Ford dealership. It was the best price I found online at $40 plus $5 shipped and got it within two days. It's made for a 2008 Mustang GT but of course you could punch in any of the model years of that gen and it'll come up.
Old part on right New part on left.
Below you can see what came off when removing the stock steering shaft.
Here you can see the inside of the stock part where it's starting to break. The rollers inside were also locked up and wouldn't spin (the new parts the bearings inside easily spun by my fingers). The rubber was also EXTREMELY soft. The new part has firm feeling rubber.
Here is what I absolutely love about the new part. Check the old part out from the side. It's flat (you'll see what I mean in a minute).
This one is grooved on the side. It allows the inner part to articulate with the steering shaft so you can get it exactly how you want it. Talk about amazing. Made positioning the solid steering shaft extremely easy without damaging anything.
New part installed under the driver side against the firewall.
New part installed from under the car (hard to see. Not the best picture taker while under the car.) The MM solid steering shaft also has amazing clearance with BBK long tubes.
Word of advice: on the MM Solid steering shaft, MAKE SURE you torque it per instructions at 24 ft/lb on both the steering wheel side and rack side. I did 25ft/lb plus a tad more. I also applied blue loctite for good measure. On the MM Solid shaft on the rack side it uses an allen bit. I used the following set up to be able to ensure I could get a torque wrench on and tighten it to spec. 1/4" allen bit from a cheap set I used a while ago to assemble Ikea furniture and a 1/4" socket.