With all the questions reguards Head Space I found this & thought it might be of use

 Headspace - What's the Scoop?
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        Most modern rifle cartridges are referred to as rimless, but that's not 100 percent correct.  They all have a rim - it just doesn't extend past the wall of the case.   Rimless cases use the distance measured from the bolt face to the middle of the shoulder to determine headspace.  This length or measurement is standard.  It has been predetermined and accepted by the industry as a given length.
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. But what about the 303 British?  It's a rimmed cartridge! .
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.         Looking at the picture, you can see a shaded area just forward of the bolt face.  This is a recessed area that houses the rim when a cartridge is chambered.  The size of this area is measured from the bolt face to the end of the recess, when the bolt is closed.  How large this area is, determines headspace.  If this distance increases, your headspace is moving toward excessive.
        There are two "standards" for measuring headspace - military and SAAMI.

          Military    SAAMI
Go          .064        .064
No Go    .070        .067

                                                   Base Maximum/Field    .074         .070
        You'll notice that the military measurement is more generous.  Remembering that it was designed for war use, its dimensions are more in line with the original purpose.  The military gave no thought to reloading cases, so stretching was not an issue.  Military ball is a "one use" item.
        Most gunsmiths use the tighter SAAMI gauges when accessing headspace.  I said most - not all.  If your rifle is assessed as "serviceable" using military gauges, there is a distinct possibility that you will experience more case stretching.   If this is so, you might want to tighten up the headspace.

A Third Standard??  (Information provided by David Moses - Thanks!)

        There is one more standard for the .303 and that is the CIP norm. CIP states that the max ctg rim thickness R is 1.63 mm; min barrel relief R is also 1.63 mm. The max allowable headspace (in the case of the CIP, this is the air space between the max ctg R and the breech face) is 0.15 mm.  In our terminology, "go" would be 1.63 mm = 0.064"; "no go" would be 1.63 + 0.15 mm = 1.78 mm = 0.070". This is obviously based on military specs. Naturally, there is no "field" gage for CIP proof norms.
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Adjusting Headspace for the No 4 Rifle
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        If you've got a No 4 rifle, check the bolt head.  On the top, there is a number stamped into the metal.  The numbers go from "0" to "3".  Each increment of one adds 3 thousandths of an inch to the length of the bolt head.  If the number stamped there is 0, 1 or 2 you will be able to adjust the headspace by simply removing (unscrewing) the bolt head and installing another with the next highest number.  It will be necessary for you to check using the gauges again.
        If you've got a No 1 rifle, the fix isn't as easy.  Leave it with the gunsmith and let him work his magic.  The bolt heads in these rifles are not interchangeable like the No 4s.
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Back to the Chamber or (This Case Will Grow on Ya)
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       So, the cartridge is chambered and locked into place when you fully close the bolt.  When you fire the cartridge, the sides, shoulder and neck all expand outward to fit the chamber wall.  They do spring back slightly, but what you have, in effect, is a tailor made case that is unique within the world of Lee Enfields.  Most people notice that the shoulder moves forward when they hold a fired case up beside an unfired one.  Remember, shoulder position is not important for headspacing rimmed cases!
        This, my friends, is where many people start causing the trouble of cracks and separations.  They take this marvelously formed case, which, like a snowflake, is one of a kind, and run it through their full length die.... crushing it back to its approximate, original size.
        Subsequent firing and full length resizing overworks the brass and its life is reduced.  It's this complete cycle that prematurely ends its usefulness.  The solution is simple - neck resize!
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    Dave
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A little more info

 

Lee Action - Trigger, Bolt, Safety, Headspacing, and Barrel Threading

The Lee-Enfield trigger assembly
This graphic shows the 2-stage trigger pull.

 

 

The SMLE bolt

 

 

A - Bolt head,
B - Extractor,
C - Bolt head lug which housed the extractor and extractor spring,
D - Bolt guide rib/locking lug
E - Cocking piece,
F - Left locking lug and
G - Gas vent hole in the bolt head

The Safety
This image shows the positioning of the Lee-Enfield safety/locking bolt in the cocked and uncocked positions.

 

 

Lee Enfield Barrel Threading

 

 

 

Headspace

Headspace adjustments can be easily accomplished with the Lee-Enfield, simply by replacing the bolthead. As the Enfield rifle was a military, combat designed rifle there are actually two sets of specs listed for headspace. These are:

 

 

Remember that the .303 is a rimmed cartridge, so headspace is the distance between the bolt face and the cartridge rim. Unlike the rimless cartridge, which draws it's headspace from bolt face and the cartridge shoulder.

For the SMLE/No1 family of Enfield's adjusting headspace can be accomplished by swapping different boltheads until one is found to produce the desired distance. This is necessary because the SMLE was only manufactured with one size of bolthead and the unit armourer stoned the bolt face to correct for headspace.

In designing the No4/No5 family of Enfield's, this minor headache was cured by producing 4 different bolthead sizes. This allowed the unit armourer and now the private owner to simply select the correct bolthead to quickly adjust headspace to specs.

Each No4/No5 bolthead is numbered 0, 1, 2, or 3. Each number specifies an increase in length, nominally .003 of an inch.

 

       Dave

For the new folks, please note that not all bolt heads will fit properly. They should screw on tight, and go just past the vertical. If it won't turn to the vertical position it needs some adjustment, if it goes past the vertical more than 20 degrees, you need to try another bolt-head.
Another fix if the bolt head won't do it, is try a different bolt. If your bolt lugs are set back excessively it will give headspace problems. A less worn bolt may correct it. Again, have it checked by your gunsmith if you're swapping bolts around.
Ed

It was something a Mate e-mailed me ,after we'd been discussing the head space question. I'm with you on the isue of the mk1  as over the years I've had ocassion to have to change them over. sometimes I had to hone the face down to obtain the correct head space . I do not recomend thougth that this is a practice for the backyard gunsmith !

      Dave  

Here is a picture of my bolt head,  my rifle swallowed a field gage, so I have too much headspace.  I have found a bolt head that is .719 I am thinking of buying.  Notice to in the pic the marking  I thought was a number '4'




I wanted you all to notice the offset when the bolt head is tight on the bolt.  Is this normal or something that changes too.

CJROCK, my Duh! I was thinking the No.4 rifle. As noted above, the bolt heads on the No.1 are not marked. The offset is normal and looks OK.
What size field gauge did you use? If it wasn't a .074, I'd try and find one and try it again.
Ed 

 Ed,  My field gage is a .070 size.  not .074 so I probably need to get another one to test with.

Thats only  4 hundredths of an inch bigger!  the Bolt head I found is .719 and my old one is .630 a difference of  .089.

if my Headspace is just bigger that .070 say its .080  and I lengthen my bolt to .089 my head space would go from .080 to .169.  Thats way too big. 

say for instance if my head space is .080 it needs to be .013 smaller to get me to .067 (.080 - .067 = .013)

My bolt head is .630 + .013 = .643 (new Bolt head size)

This is all true if my actual head space is .080 which there is no way I know to actually measure it accurate with out a set of gages field gages.

I never knew that I would be doing so much math just to shoot a rifle!

its very hard to feel with the cocking spring pushing against you.  should I remove the spring?