Information about o-ring replacements
We permanently discontinued our wallets in 2022 so that we can focus more time on our musical instruments. While our wallets may well last a lifetime, the o-rings do have a tendency to eventually fail with use. While we no longer have o-rings in stock, if you're looking for replacements, here's what you should know.
O-rings are sized in 'dash numbers' that indicate their thickness and diameter. The A3 and A4 both use a pair of –227 o-rings. The A4 also uses a smaller –120. If you tend to carry more items in your wallet, you might consider stepping up a size, say –228 and –121.
Rubber hardness is referred to as 'durometer'. We recommend 50A o-rings. Most o-rings you'll find at the local hardware store are probably 70A, and this is a little too stiff for our wallets.
Again, most o-rings you'll find at the local hardware supplier are made from buna-nitrile rubber, which you may also see listed as NBR. We don't recommend this. The o-rings we shipped with our A4 wallet were made from urethane which is a much longer lasting rubber under the conditions that a wallet is typically subjected to. Silicone is another good alternative to NBR. Unfortunately, both urethane and silicone rings are much less common than NBR.
What to do
McMaster-Carr sells 50A silicone rings, both in –227 and –120, but they're not available in black. Instead they're a beautiful orange-red. Unless you're planning on manufacturing custom o-rings (which is what we did), these are probably your best bet. If you really want black rings, McMaster also sells NBR 50A rings.