The center hole suspends the weight of the motor and yoke from the carriage on the saw arm. The four smaller holes you see in the photo receive the indexing pin to lock the yoke in the crosscut, inrip, outrip and accessories positions.
After years of use the indexing holes become a little oblong or egg-shaped. Some of the holes wear more than others because they get more use. You can align the saw to make an accurate crosscut, but the blade will heel in the ripping positions. I thought for a long time about how I could restore the accuracy of all of the holes. The key was to make each of the three holes used for precision settings (crosscut, inrip, outrip) capable of independent alignment. That can be accomplished with two pieces of strap iron, four 10-32 screws, and four nuts.
For further explanation, imagine you are looking at a clockface in the photo. 12 o'clock is at the top of the photo. The accessory indexing hole is at 1 o'clock. The inrip indexing hole is at the 4 o'clock position. The crosscut indexing hole is at the 7 o'clock position. The outrip indexing hole is at the 10 o'clock position. When you are ready to do the alignment, you will begin with the outrip hole, no matter how much wear it may have endured and how egg-shaped it may be.
Then you will align the strap iron plate you will make that lays above the crosscut indexing hole. Finally, you will align the strap iron plate for the inrip hole.
Step 1: Remove the Motor Yoke From the Saw Arm
Step 2: Make Two Strap Iron Plates
Grind a concave in one side of each piece of strap iron to make space for the machined ring on the bottom of the motor carriage. You can see a wear pattern from it in the first photo.
Step 3: Drill Mounting Holes in the Yoke
Put the chuck end of a 3/8 inch twist drill through the hole in one of the strap iron pieces. Insert the same end of the bit into the crosscut indexing hole on the yoke. Press the strap iron down on the yoke surface and turn it so the flat side of the strap iron is parallel to the side of the yoke. Carefully mark the centers of the 1/4 inch holes for drilling holes in the yoke.
Drill holes straight down through the yoke. The holes should be correctly sized for a 10-32 thread tap. Tap threads in the holes. (The drawing shows the holes you will drill. Note the black dots on either side of the indexing holes for the crosscut and inrip positions.)
Place a lock washer onto a roundhead 10-32 screw and thread it up from the underside of the yoke. Tighten it. Do the same with another screw in the other hole. Put a 10-32 nut onto each screw, but do not tighten the strap iron plate down too firmly, yet. Grind or otherwise cut the length of the 10-32 screws so their ends are flush with the top of the nut.
Repeat the process in the previous three paragraphs for the inrip hole and its strap iron plate.
Step 4: Alignment
As noted in the first step, turn the yoke to the outrip position and drop the indexing pin into its hole. Align the saw so the blade does not heel and kick up splinters at the back end of the blade on either one side or the other.
Move the saw to the crosscut position. You will need to lift the indexing pin a bit higher than before to clear the extra thickness of the strap iron plate. That is a small price to pay for having a very accurately aligned saw again. Align the yoke. You will not use the bolt heads you see in the photo on either side of the indexing pin (center of the photo), even though that was the procedure before your modification. Now you will make your adjustments for the crosscut and inrip positions by loosening the 10-32 nuts on the strap iron plate (as shown in the photo). 10-32 screws are a bit smaller in diameter than the 1/4 inch holes you drilled, so you can move the plates enough to restore accurate alignment. When finished make a test cut and check for blade heeling. It takes some time, but work with the adjustment process until the saw does not heel.
Repeat the process for the inrip position.
You now have a saw that allows you to align independently each of the three indexing holes used for precision setups and cuts.