A nock is a few cents worth of plastic, and it’s the only thing holding your arrow to the string while sixty-odd pounds of stored energy turns loose. Size it wrong and you get flyers you can’t explain, a cracked nock at full draw, a chewed-up center serving, or on a crossbow, a dry fire that ends your season.
The trouble is that “nock size” isn’t one measurement. It’s three, and they’re independent.
Shaft fit
This is what most people mean: the inside diameter of the shaft the nock presses into. Easton’s letter codes are the industry shorthand, now published with metric IDs alongside. S is 6.5mm standard (.244″), H is 6mm reduced (.233″), X is 5mm micro (.204″), and G is 4mm ultra-micro (.165″). Gold Tip and Victory VForce shafts run .246″, usually called GT.
That last gap is the one that bites people. Two thousandths of an inch separate S from GT, close enough to look identical on the bench. An S nock will physically go into a Gold Tip shaft, but it sits loose, and loose nocks fail under load.
Throat fit
The throat is the groove that snaps onto the string, and it has nothing to do with your shaft. What determines it is the finished diameter of your center serving, not your draw weight or bow type. Easton sells small-groove and large-groove versions of the same nock for exactly this reason.
Too tight and the nock pinches the string, throwing your release and eventually splitting. Too loose and the arrow drops off at the worst moment. The bench test settles it: nock an arrow, hold the bow horizontal, and let go. It should hang on its own, then fall free when you tap the string an inch or two away. If it takes a real rap to release, go up a groove size. If it falls unprompted, go down.
Crossbow nocks
On a crossbow the critical spec isn’t diameter at all. It’s shape. Flat, half-moon, capture, and TenPoint’s Omni-Nock and Alpha-Nock each engage the string differently, and modern bows have narrowed enough that string angle at the shot is much steeper.
The failure mode is serious. If the string slips over or under the nock, you get a dry fire, and that can happen even on a bow with a dry-fire inhibitor, because the trigger released a bolt that was never attached. Half-moon nocks especially have to be rotated so the groove captures the string, with the cock vane oriented as your manual specifies. Turned the wrong way, the nock edges can slice your serving.
This is why manufacturers stopped treating nocks as generic. TenPoint mandates its own profiles. Ravin uses proprietary clip-on nocks and says plainly that only Ravin-branded arrows and nocks belong in a Ravin crossbow, and even within the line, the R500 and R470 series take a different nock than the rest.
One clarification: the .001 and .003 stamped on crossbow arrows is a straightness tolerance, not a nock spec. Two arrows with different ratings take the same nock.
Finding yours
Check the manual or the shaft maker’s spec sheet first; ID is published for every current shaft. For older arrows or a mixed quiver, calipers settle it in about ten seconds. For crossbows, don’t substitute. Run what the bow maker calls for.
Bring a nock with you when you shop, or a single fletched arrow. Matching one in hand beats guessing from a letter code.