It probably is a 5/8"
So a 5/8" flare cap from Home Depot is under $5.00
IIRC the thread pitch is 1.5
You plan on wrapping that header?
133whp-126ftlbsTQ Eclipse CD8443,JBL Px300.4, JBL GTO7001, PPI 356cs, JBL p1024(Hooray!) SubThump enclosure, RAAmat BXT, Damplifier Pro
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