my friend just bought a firewire board yesterday that has a hole on the top about 1cm wide right in the middle on the top that fits a hex key, lot larger than what fcs uses. he doesnt speak a whole lot of english and is new to surfing so he didnt ask the guy what it was for. anyone know? he thought it might be to adjust the rocker. it has no stringer so im guessing its eps?
johnny's right on... don't ever stick anything down in there...for any reason. There's a gortex membrane in there that allows the foam core to breathe, but won't suck in water. If you rupture that membrane, the board will suck water like... well... you get the point.
If the there was no vent, the air in the foam would expand when the board gets hot, and blow the skin right off the core... delamination. Expanded polystyrene foams less than 2lb density are vented for this reason. Higher densities do not require vents... and in fact, really shouldn't be vented.
a more technical explanation is that, b/c of the type of foam firewire uses for their cores (about 1lb eps), it's super light & porous...lots o' air in there...board heats up, the expanded air needs to go somewhere, otherwise you get bubbles & delams on the deck. the vent is a 1-way gore-tex valve. keep it free of wax, debris, etc...& under no circumstances should you ever attempt to adjust or fiddle w/ it.