This motor belt measures 13.75 inches by .22 inches and has 84 teeth.

Singer 223, 237, 239, 240, 241, 242, 247, 247AP, 248, 250, 251, 252, 257, 259, 263, 285K, 288, 293B, 547, 563, 588, 2404, 2430, 3101, 3102, 3103, 3105, 3130, 5705, 5710, 13 3/4 - Alternative number 196388

To replace the belt:
Loosen the large screw that holds the motor in place (loosen – do not remove).
WHEN YOU LOOSEN THIS SCREW - PROTECT THE PAINT ON THE ARM OF THE MACHINE FROM THE SCREW DRIVER.
To locate this screw; look at the motor as if you are sewing – this screw is below and behind the hand wheel.
Loosening it will allow the motor to slide up and down.
Loosen the small screw on the silver stop motion knob on the large hand wheel.
Unscrew the stop motion knob and remove from the machine.
NOTE the stop motion washer - its 'ears' should be pointing out.
Remove the washer.
Pull the hand wheel off of the machine – the belt will come with it.
Remove belt from motor area.
Clean the backside of the stop motion knob, the stop motion washer, the inside of the large hand wheel, backside of the hand wheel, and the shaft the hand wheel goes on (inside and out).
Put a couple of drops of sewing machine oil on the shaft the hand wheel rides on (inside and out).
Install new belt on pulley on motor. (This is usually a two-handed job.)
Make certain it is on the pulley, riding in the groove, not on the outer rim.
Loop over hand wheel (the hand wheel is not on the machine, yet).
Replace hand wheel on machine. Replace washer ('ears' out) and then the knob. Tighten screw on knob (not too tight!).
Check to make certain the stop motion knob will release (partially unscrew).
If it will not, remove the knob and rotate the washer 180 degrees.
Replace the knob and check again.
Your new belt should be just tight enough to pull machine without slipping -- NOT ‘plucking’ tight! Actually, it needs to be pretty loose.
You can adjust the tightness of the belt by sliding the motor up or down.
It is VERY important NOT to run the machine with the belt too tight.
Having the belt too tight can cause the machine to run slowly and can damage or ruin the motor.
When the belt is adjusted correctly, retighten the screw that holds the motor on (remember to protect the paint on the arm of the machine from the screwdriver this time, too!).
You will be most successful adjusting the belt if you hold the motor in place while you are tightening the screw that holds the motor.