How much headspace to leave when filling corny

Good Day Y'all.
New to kegging and just picked up 2 corny's. I'm not sure how full to make them before hitting them with CO2.
Also they have held the pressure they came with for 3 weeks now. Is it necessary to change the seals before using them? (waiting for some to come in to local supplier)
Thanks for any helpful replies

My new cornies held pressure, but I visually inspected and replaced all but the lid oring.
One of the orings had a large crack in it, and split as soon as i removed the seat post.
2 post orings and 2 diptube orings cost a total of less than $1.... I see no reason not to replace them, so you know how old they are.

I fill my kegs to the point that i can get the lid back in without it touching the beer, which is about 1.5-2 inches from the top. If i have beer left, I bottle it, and add a coopers carbonation drop.

Arkador said:

My new cornies held pressure, but I visually inspected and replaced all but the lid oring.
One of the orings had a large crack in it, and split as soon as i removed the seat post.
2 post orings and 2 diptube orings cost a total of less than $1.... I see no reason not to replace them, so you know how old they are.

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Seals are cheap enough to replace just for peace of mind. I'm looking to replace a couple poppet valves, those buggers aren't cheap for all they are.

IrregularPulse said:

I'm looking to replace a couple poppet valves, those buggers aren't cheap for all they are.

Click to expand...

$4.50 for a popet valve, $9 for a pressure relief valve.... $2.50 to replace every oring..... Add that up, and that is what i paid for each of my kegs... ($20 each)

If you need to fill one before the seals come in, go ahead. But, make sure and replace them when the keg is empty. If you have time before filling, replace them first.

I fill the keg to the weld line. That gives you enough headspace.

I just make sure the gas tube isn't submerged...because I have yet to install check valves on my gas lines.

i'm lazy.

I rebuilt each of my corny kegs before first use. $2.50 per o-ring set is highway robbery.

Why not order O-rings from McMaster? $20 worth of O-rings should just about last your entire brewing career. Common sizes are conveniently listed on the wiki: O-ring - Home Brewing Wiki

As for the OP's question, I fill my kegs right up to the bottom of the gas in tube.