Where can I get a replacement for a cracked stitch selection dial where it goes onto the shaft?

You need to select A but also set the Stitch Width dial to zero. The Stitch Width dial should be the one on top of the machine next to the handle.

https://www.manualslib.com/manual/776764/White-Sewing-2380.html#manual (page 25: Select Stitch Pattern A, Stitch Width to zero, Stitch Length to zero to 4)

Perhaps the manual will help:

Manuals PX 100 200 300 Others Brother

The "dial" in the back is for decorative stitches which require a "programmer" to be inserted with the stitch formation you want to form. For a straight stitch no cam is required in the back of the machine. The three dials on the front:

Upper Left: Stitch selector
Upper Right: Stitch lenght
Lower: Stitch width

Using the upper left dial select the straight stitch picture
Using the upper right, choose your stitch lenght
Using the lower, choose your needle position, left/center/right

You can buy a manual for the Simplicity Fashion Pro SW2145 from this link
http://www.sewingandcraftclub.com/index.cfm?fuseaction=product.display&Product_ID=3112&category_id=365&picksku=IBSW2145#top

I would guess from looking at an image of the machine, that you just turn the large dial on the front around to the number for the stitch you want, based on the selection displayed across the front of the machine, blue and red stitches. You may need to switch between blue and red on the selector to get the 2nd range of stitches, probably stretch type where the machine goes forwards and backwards ones, I'd guess.

And vary the stitch width using the dial beside the reversing lever on the lower front.

Hello

look at the cam gear. This is the unit that the cam connects to. It could have a crack or be split completely. If there is a crack the cam stack will have to be replaced. The parts are hard to get and a good technician is needed.

As to your question about leather, the 6460 has a special stitch for sewing leather. Select Colormatic cam C and install in the cam hole at the back of the machine. To remove/add a cam turn the upper left hand dial on the front of the machine to the small dot. This disengages the cam. Gently remove the cam by pulling straight out from the back of the machine. Install cam C straight in, ever so slightly twisting it back and forth into the hole to find the sweet spot. You'll hear a click when it's all the way in.

To use Viking's leather stitch, turn the upper left dial to the color yellow. (Note: The slash line on the upper left dial is a stop. You can't turn past that point. Go in the other direction to get to the icon or color you want.) Turn the upper right and bottom right dial to yellow also. The upper left dial is what selects the particular stitch on the cam, the leather stitch in this case. The upper right dial selects stitch length, and the lower right dial selects stitch width. You can move the right dials within the color range. Experiment on a scrap of cloth to get the stitch size the way you want it. The leather stitch looks like a straight stitch with a block style edging to the right of the straight stitch. This is supposed to give a stronger stitch to the leather rather than a straight stitch by itself.
If you don't have the manual for your machine: Select which of the eight cams you want to use. All the cams are removed/installed by turning the upper left dial on machine front to the small dot. At the rear of the machine, pull/insert cam straight in or out and twist slightly as you push in or remove to find the sweet spot. Move the dial away from the small dot to lock the cam in.
Turn upper left dial to stitch type you want and match the other dials to that same color. Upper right controls stitch length, lower right controls width. Bobbin tension is controlled by the dial at front left of the machine, above the needle.
Cams A-C are the workhorse cams and the rest are embroidery stitches.Straight stitch, button and stitched zig-zag will work on any cam. These stitches do not rely on matching a color code. To use those three: To select straight stitch, on the upper left dial, choose the icon that looks like a buttonhole with a zig that trails off to the right. Select stitch length on upper right dial and turn the lower right dial to zero, going by number rather than color.
To select stitched zig zag, turn upper left dial to the broken zig zag icon (to the right of the straight stitch icon). Select stitch length and width.
To move the needle to the side for buttonhole stitching or other times when you want the needle off center, turn the upper left dial to the button icon. If stitching a button hole, set stitch length to zero and lower right dial to desired stitch width. Push in the small white button that's beneath the lower right dial in order to prevent the feed dogs from moving the fabric forward. Pushing that button in is also handy for putting cloth under the foot before sewing as it prevents snagging and is used for tacking.

I have the same problem. I found the little metal piece down inside the machine. Shake the machine and see if you hear a little rattle. It might be that little metal piece. Bad design on that selector knob, for sure.

It's just in the front panel, where the drawing of all the built in stitches are, it's the drawing number eleven from the left, you'll see a zig zag (red) and a straight stitch (black) Select A group of stitches (knob with the arrow) and then select the eleventh stitch (same knob but use the dial, look the manual) and then select zero on your stitch width dial. Hope that helps :)

Remove the top cover (grasp and lift straight up), spray with "Tri-Flow, liberally inside. Pay attention to the right 1/2 and behind the front dials.

In Front, each dial has a small square opening on the side, use the tube included with the "Tri-flow" to squirt lube inside them. DO NOT use WD-40.

Run the machine daily for 5-10 minutes (set the dial "C" [top front that goes up to 8] to 0 also set the dial on top of the case to 0. Move the Zig-Zag lever (upper right) from 0 to 4 and back again. Respray with Tri-flow. After a couple of days, if it is still stuck place a hair-dryer or light nearby to warm it up to skin temp.

Between the temp and the oil, it should begin working.

Break a Leg!

To select stitches 11-21 you position the stitch length dial on the "SS" setting , and the pattern selecting dial on the desired stitch number.