How do you headspace and what does it mean....Havent had to yet...Thanks

Headspacing refers to the space left between the bolt face and the breech face and it's relation to a round in the chamber.  To perform headspacing you need caliber specific guages.  Basically there is a go and a no-go gauge.

Most AK's will not requiring headspacing assuming they were headspaced when they were me-milled and you have a matching front trunnion and barrel since headspace is set on the AK by the hole and pin.

He is correct about the room for the cartritge.  To much room will allow to much expansion of the case and cause a detonation effect which the chamber may not hold.  To little space and the bolt will not full close on a case.  Those 2 things are bad for the operator and can lead to the rifle blowing up in one's face.

The head space inbetween the 2 (go and no go) allows just enough room for the case to expand and seal in the chamber without ill effects.

Here is one
www.rifleshootermag.com/gunsmithing/headspace_0612/

How does headspacing on a milled AK differ ?  

Head space on a stamped receiver is set by how far the barrel is pressed in to the trunion.  On a milled, the barrel must be threaded into the receiver.  So headspace is set by how far the barrel is screwed in to the receiver.

From what I understand (I have only built FALs and ARs), taking an AK parts kit that includes the bolt, barrel and trunion, the head space has already been set.  There is a pin called the barrel pin that retains the relation between the barrel, bolt,  and trunion.

When you put together a stamped kit, the barrel pin is pressed out, the barrel is pressed out of the trunion, the trunion is mounted and fasted (usually riveted) into the new stamped receiver.  Once the trunion is mounted to the reciever, the barrel is re-pressed into the trunion to the point where the barrel pin hole aligns.  The barrel pin is then re-pressed in.  This re-locks the barrel and trunion and the previous head space should have been maintained.

On a milled receiver, the barrel must be threaded into the receiver to the point where the bolt will close on a 'go' gauge, but not close on a 'no-go' gauge.  If the barrel is screwed to the proper alighnment (front sight straight) and there is not proper head space, the bolt can be manipulated (or swaped for another) until proper head space is attained.

From what others have stated regarding AKs, just because the bolt will not close on a 'go' gauge, that does not mean that the is not enough head space.

This leads me to think that the comercial 'go' and 'no-go' gauges are of different dimensions than that of the the real requirements of an AKs chamber.

For instance, there is a difference between .308 SAMMI and 7.62 NATO 'Go' and 'No-Go' gauges.

Note:  If I am completely off my rocker regarding assembly procedures, please feel free to correct my.

I believe that the proper procedure for modifying headspace on a threaded barrel involves either machining the end of the barrel or reaming the chamber. I think that work is best left to professionals. I wouldn't try it if I had the tools.

Quoted: Head space on a stamped receiver is set by how far the barrel is pressed in to the trunion.  On a milled, the barrel must be threaded into the receiver.  So headspace is set by how far the barrel is screwed in to the receiver.

Anthropy, not all milled AKs have screwed in barrels.  The milled Arsenal AKs have pinned in barrels and Firing Line makes both milled receivers for screw in and pinned.

Thanks for the clarity.  Learn something new everyday.