Another question on checking headspace - how tight do you rotate the bolt?

HKSteve said:

How tight are you supposed to rotate the bolt when checking the field gage? Thx

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Very lightly to touch with no op rod in the action. Just enough bolt rotation with gentle finger movement " feel" the end of travel on the bolt. Absolutely no pressure involved. These are precision calibrated gauges and can be easily damaged by improper use. Whatta Hobby!

What Art said… very light, no force. I would add the bolt should be stripped.

Correct,the bolt should be stripped and the bolt face needs to be cleaned to remove any brass and primer residue,then do the same for the bolt lugs and where they contac the receiver. After that move to the barrel and make sure the throat and chamber is properly cleaned.
Thats the only way to get a proper reading.

Something that you hardly ever here anyone talk about is the right bolt shelf and how it comes into play.Many of commercial receivers this shelf is cut too deep. That's what started the roller impact.
Quick way to tell is in full lock up check the radial shuck of the bolt .This movement bolt resting on shelf with the op rod hold lock seat forcing bolt down .

USGI receivers have very little shuck .005- .010"

Some commercial are .030 "+ too deep allowing bolt to rotate forward further .

Condition of the op rod does affect checking the shelf as a worn tab will let rod cant outward .

In an in spec receiver the bolt is closed when it's resting on the right shelf . That is how the M1 design was carried over to the M14.