Let’s talk about fit
The first place to start, always, is to consider what thickness and size of bit will suit your horse best.
Thickness
The thickness of the mouthpiece needs to be suitable for your horse’s mouth conformation. An equine dental technician (EDT) can help to assess your horse’s mouth if you’re not sure, but you can also check the amount of space between the tongue and the roof of the mouth yourself by gently putting a finger in through the bars and feeling how much clearance there is. If you insert your index and middle finger together, pressure on both fingers would indicate that a thinner mouthpiece is suitable (12-14mm), whereas little or no pressure would suggest something thicker would be better (16-18mm).
The traditional thinking that a thicker mouthpiece is milder isn’t true. If too thick, it can cause bruising, tension, opening of the mouth, breathing issues or grinding on the bit.
Sizing
To find the correct width, a general guide is to look for 1.5 lip wrinkles at the corner of your horse’s mouth when the bit is in place, although this does depends on how thick his lips are and how short his mouth is. If he has a ‘short smile’ there may be a little more wrinkle showing, and this is fine.
The amount of clearance needed on either side of the mouth also depends on the style of the bit. For example, a snugger fit will be required for a fixed ring bit like a full-cheek, eggbutt or D-ring. These are designed to provide stability and support, so they don’t slide across your horse’s mouth. A fixed ring bit would be fitted closely, but not so close that it’s squashing his cheek.
A loose ring bit will need a little more space so that the lip doesn’t get nipped between mouthpiece and ring. If you check the fit of the bit when your horse is at rest, the lip shouldn’t be touching the hole in the bit — then, when a contact is taken, the bit moves back away from the lip and you’ll see a little more clearance. A general guide would be 5mm clearance between your horse’s lip and the ring of the bit when a contact is taken. A horse with a large tongue — a common issue — will be in great discomfort if the bit isn’t fitting correctly. Gently part his lips and you’ll be able to see whether his tongue is bulging out through his teeth — a sure sign of a large tongue. Choose his bit carefully.