Sometimes called a "Western snaffle" because of its broken mouthpiece and medium-length shanks, the Tom Thumb bit is a hybrid between leverage and direct pressure bits. The Tom Thumb can exert extreme pressure on a horse's sensitive mouth. This leverage bit should be used only by riders with skilled, polite hands and upon horses who are neck-reining with ease.
Selecting the Bit
Before the bit can be purchased, horse owners should decide what size they'll need. The length and the width of the bit mouthpiece will need to be considered if the bit is to fit the horse properly. A bit that is too short may pinch the horse's tender mouth, whereas a bit that is too long may slip out of place and cause soreness, according to the Dover Saddlery in Littleton, Massachusetts. A rubber bit sizer can be used to measure the actual width of the horse's mouth. Adding one-fourth of an inch to the size indicated by the bit sizer will ensure that the bit will fit comfortably. The width of the bit is determined by the mouth shape of the horse. A veterinary professional or equine dentist can assist horse owners in determining their ideal bit width.
Bit and Bridle
After the bit is purchased, it is time to try it on the horse, attached to the rider's preferred bridle. The Tom Thumb has ports on either side where the headstall will be attached, either with strips of rawhide or Chicago screws. A small port behind the headstall ring is for the curb chain, which is a crucial component for every leverage bit. Once the pieces are in place, the bit can be placed in the horse's mouth. A properly fitting bit will create one small wrinkle at the edges of the horse's mouth, according to the Weber Training Stables website. Should the bit create more wrinkles, it may be too tight. The bit should not hang low enough to touch the horse's canine teeth.
How it Works
The Tom Thumb bit is a leverage bit, which means that for every pound of pressure the rider puts on the reins, the horse will feel that pressure times three. This is determined by the length of the shank below the mouthpiece, according to the Certified Horsemanship Association. When the pressure is applied, the horse experiences pressure from the headstall behind his poll and a nutcracker action on the bars of his mouth when the shanks beneath his chin come together. The jointed mouthpiece hits the roof of his mouth when coupled with the pinching action of the curb chain beneath his chin.
Using the Tom Thumb
Though the Tom Thumb is a severe bit that can cause horses discomfort, it may have its uses, particularly when a young or small rider is managing a stout, insensitive horse. Though originally designed as a finishing bit for young horses, Colorado-based trainer and clinician Mark Rashid recommends a curb or grazing bit for well-trained, responsive horses. Some trainers use the Tom Thumb as a transition bit between the snaffle and the curb bit, but the Horse Channel cautions that if the transition is made before the horse is ready, he may gape his mouth in response to the bit or toss his head.