Truly a thing of beauty.
Dill, headspace is the available room in the chamber measured parallel to the bore with the bolt in battery. It is important for it to be correct for a particular cartridge.
To little headspace will either:
A:Not allow the bolt to go into battery (impossibly short headspace.)
B: Increase pressures to the point of damaging the weapon or shooter (excessively short headspace.) or
C: Cause increased pressure to the point of sticky extraction and/or failure to cycle (short headspace.)
To much headspace will either:
A: Not allow the firing pin to contact the primer (impossibly long headspace.)
B: Cause a split case and allow escaping gas to spray into the shooters face (excessively long headspace) or
C: Cause shoulder deformation which will impair accuracy and decrease reloadable case life (long headspace.)
it is important that headspace is correct and set where you want it in the acceptable tolerance range of a give cartridge. minimal headspace is commonly thought to increase accuracy at the possible expense of reliability. Maximum headspace is commonly thought to decrease accuracy and improve reliability. Everyone has an opinion about how headspace should be set up for a specific application.
To put it in the simplest terms possible, heapspace is "the gap between bolt end and the end of the round." (although there should never really be a gap.)
Hope that enlightens you
We also have to take into consideration the "tilt bolt" locking system of the SKS.
Aside from some of the rimmed cartridges,
headspace is actually more than just the distance of the bolt face to the end of the cartridge.
Length of the case, angles of the shoulder and many other factors are considered.
When fitting a bolt in an SKS, we are already figuring that all these factors are correct and that the
case fits the chamber and the round is fully seated.
So the last thing, is the distance from the end of the bolt, to the end of the fully seated round.
And this is supposed to be the minimum of what it takes to allow the bolt to fully lock into battery on the round.
Too much headspace (and this can be measured in as little as a few thousandths of an inch)
and all the pressure that is supposed to be contained by the chamber,
will end up being held by the exposed part of the soft, thin, cartridge casing.
And this is usually the part right around the base of the shell.
Too little headspace, as is the case with a new bolt, is not normally dangerous because,
in an SKS, if the bolt cannot chamber the round and fall into battery,
there are safety mechanisms built in that prevent you from pulling trigger and dropping the hammer.
BUT! I have seen in the past, Yugo M59/66 models that have Bolt follower disconnectors,
that are a tad long, and it makes me wonder if, this possibly could align the trigger bar and allow a trigger pull,
dropping the hammer, even though the bolt is not fully locked down
The biggest danger of fitting a bolt is that it is very easy to "go too far".
And as previously mentioned, "excessive", can be described as little as a couple of thousandths of an inch
over the maximum allowable headspace of the rifle.
So, by no means is this write up meant to be a tutoral to fitting a bolt.
In fact, if you are not savvy about what I'm saying and are not comfortable with the thought of
working with precision parts and thousandths of an inch,
Please have your bolts done by a qualified Gunsmith.
The cost will be well worth the peace of mind.
But if you do decide to fit your own, remember to take it little at a time and test frequently.
And remember, do not test bolt fitting with a live round in the chamber.
Use a demilled round (not a fire round, as this distorts the casing).