What is the average bit size for a thoroughbred?
You want to avoid a bit that's too narrow that will pinch the corners of the mouth. The correct width will leave about ¼” of metal extending on each side of the horse's lips.
The average bit size for a horse size is between 5 and 6 inches, but will vary by breed, build, and genetic makeup. Horses and ponies with more refined noses such as Arabs and Welsh Cobs often need bits less than 5 inches.
The two most common bits worn by racehorses are a D-bit and a ring bit. Both bits are snaffles, meaning the mouthpiece is made up of two jointed segments of metal. The D-bit is easiest on a horse's mouth and the simplest. Its name describes the D-shaped rings that attach the ends of the bit to the bridle.
How To Measure Your Horse for Bit Size - YouTube
If your horse's bit is too big, you will find that the bit will move back and forth in your horse's mouth which may hit your horse's teeth. If this occurs, your rein aids will be unclear and will not be transmitted effectively to your horse.
A mullen mouth is a plain mouthpiece with a slight curve over the horse's tongue. This makes it more comfortable for the horse to carry than a straight-bar mouthpiece. It's also considered more gentle than a jointed mouthpiece, as there is no pinching effect when the reins are pulled. Continue to 2 of 15 below.
Use a bit 1/64” smaller than the target hole size for softwoods. Use a bit exactly the same size as the hole when working on other materials. If you're not sure which to select, choose a drill bit 1/64” larger than the hole you wish to create. This will account for variables such a wood density and screw type.
The Waterford is the most well known bit for this type of evasion, and can help to prevent leaning but should be used sympathetically. Myler combination bits often work well, the 30 04 being popular or the 30 42 if the horse puts his head down whilst pulling.
HOW TO GET YOUR HORSE ON THE BIT - (Thoroughbred Horses) OTTB ...
Happy mouth bits, which come in a range of different styles, give your horse a lot more room for his tongue, so this could solve a lot of issues in the case of a horse with a smaller mouth. I've found some ex-racehorses go well in a Dutch or two-ring gag as the different action seems to suit them well.
How do you know if a bit is too small?
A bit that's too small can pinch the corners of the mouth, while a bit that's too big can move around too much and clunk against his teeth. Rubbed patches or thickened skin at the corners of the mouth are signs of bad bit fit, but it's better not to wait to see physical evidence that a bit doesn't fit well.
1. A snaffle http://bit.ly/2cpgfAI should be snug against the corners of the horse's mouth. It shouldn't be so tight that it causes wrinkles or so loose that it hangs below the corners of the mouth where it can bump the teeth.

Eggbutt Snaffle Uses
One of the most commonly used English snaffle bits is the eggbutt snaffle. It is useful in training a young horse, general riding, and the beginning stages of dressage. Some horses are ridden their whole lives in this type of bit.
In my experience, however, horses that chomp on the bit are usually nervous or anxious about something. Considerations: If you suspect your horse is anxious, try him in a bit with a roller in the mouthpiece. Nervous horses tend to “play” with the roller with their tongues, which often helps alleviate anxiety.
Loose ring bits help the horse to position the bit where they like it unlike fixed bits such as the eggbutt or hanging cheek snaffle. This helps the horse be more comfortable in the mouth and since the mouthpiece is moveable on the cheek it also helps with horses that are heavy or take hold of the bit.
What causes rubbing of the bit? When the bit hangs too low and the rider takes up the reins, the bit will slide up the mouth. Even with a loose and friendly contact, the bit will still go up and down in the mouth and rub the corners. If the horse is sensitive, this will damage the mouth.
Find the perfect size bit for your Draft or Warmblood horse. DraftTack has one of the largest selections of bits sizes 5.5" to 7" especially for Warmblood and Draft horses.
Generally speaking, Maheu notes that warmbloods are most comfortable with bits that are 5 ¼" or larger whereas stock breeds such as Quarter Horses, Paints and Appaloosas take a 5” mouthpiece. Arabians have smaller mouths and use 4 ¾" while small pony mouths prefer 4 ½" or smaller.