A crate engine is a complete engine bought assembled from the manufacturer and raced as delivered. Circle-track versions like GM's 602 and 604 use special bolts and serialized tags to prevent modification, and the seals must stay intact.
The names are part numbers. Speedway Motors: the 602 comes from “the last three digits of the GM crate engine part number, which is 19434602,” and the 604 from 19434604. They are different engines — the 602 (CT350) runs cast-iron Vortec heads at “350 hp @ 5,400 rpm,” the 604 (CT400) aluminum Fast Burn heads at “404 hp @ 5,600 rpm.” Sealed isn’t untouchable: rebuilds “must be sent to a registered rebuild facility with the ability to reseal the engines.” Sanctions then add their own layer. IMCA’s 2026 Modified rules require an “unaltered sealed GM 604 crate engine with additional IMCA Cable-Lok system,” and exempt it from claiming: “Any driver using crate or spec engine cannot claim engine or have engine claimed.”
Break a seal outside an approved facility — even for an honest repair — and the engine is illegal wherever it gets teched, no matter how stock the parts inside it are.