OEM vs aftermarket fasteners: what actually differs
OEM fasteners come from the original equipment supply chain; aftermarket parts are produced to match the specification after the fact. In practice the difference is paperwork, not always quality.
A serious aftermarket mill runs the same cold-heading lines and the same heat treatment ovens as an OEM supplier — what changes is traceability. If your application is regulated (automotive, pressure vessels, lifting gear), insist on full MTC 3.1 certification regardless of channel.
For general construction and DIY, a certified aftermarket fastener at half the price is usually the rational choice.