If you use our preferred windshield repair bridge (injector), you'll likely need to restock on some of these o-rings at some point. The second picture gives you an idea of which piece this is on the repair bridge.
The bridge uses 2 on the piston for an extra tight seal. They are durable and will last about 40 repairs when you replace both o-rings. This pack of 8 will last about 160 repairs.