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They can be flush but do not have to be. At least for smaller sizes, flush is typical, because it ensures control of tolerances of the interference and compression (squeeze %). The groove has excess width that provides empty volume into which the o-ring material displaced by the interference can go. See Parker O-ring Handbook (free pdf) for a pretty good reference text, and standard groove sizing tables.

They can be flush, but do not have to be*. At least for smaller sizes, flush is typical, because it ensures control of tolerances of the interference and compression (squeeze %). The groove has excess width that provides empty volume into which the o-ring material displaced by the interference can go.

[*] note: In case of high pressures, extrusion of the o-ring into the gap must be prevented with a back-up ring.

See Parker O-ring Handbook (free pdf) for a good reference text, and standard groove sizing tables.

They can be flush but do not have to be. At least for smaller sizes, flush is typically assumed because, because it ensures control of tolerances of the interference and compression (squeeze %). The groove has excess width that provides empty volume into which the o-ring material displaced by the interference can go. See Parker O-ring Handbook (free pdf) for a pretty good reference text, and standard groove sizing tables.

They can be flush but do not have to be. At least for smaller sizes, flush is typical, because it ensures control of tolerances of the interference and compression (squeeze %). The groove has excess width that provides empty volume into which the o-ring material displaced by the interference can go. See Parker O-ring Handbook (free pdf) for a pretty good reference text, and standard groove sizing tables.

They can be flush. In fact this is typically assumed because of tolerances of groove depth vs o-ring size, which produces interference that must be within the allowable range of squeeze % for the o-ring material and application. The groove has excess width that provides empty volume into which the o-ring material displaced by the interference can go. See Parker O-ring Handbook (free pdf) for a pretty good reference text, and standard groove sizing tables.

They can be flush but do not have to be. At least for smaller sizes, flush is typically assumed because, because it ensures control of tolerances of the interference and compression (squeeze %). The groove has excess width that provides empty volume into which the o-ring material displaced by the interference can go. See Parker O-ring Handbook (free pdf) for a pretty good reference text, and standard groove sizing tables.