Why is it nessasary to disassemble the bolt to check for head space?  
Thanks.

Just guessing but I figured it was so the ejector does not rotate against the bottom of the gauge and the extractor does not snap over the edge of the gauge. The paper that came with mine says  it might damage the gauge. Also takes the ejector and extractor out of the mix and possibility of getting a false reading if the ejector does not compress all the way or the bottom edge of the extractor case edge cut out was higher than the bolt face.

You have to disassemble the ejector and extractor from the bolt so that the ejector and extractor and their respective springs do not push the gauges into the chamber. This can give false readings with the gauges and result in misinterpretation of the data derived from the gauging process. Charles the Gunsmith.  

You absolutely need to pull the ejector, but I don't think the extractor really matters unless it's grossly out of spec.
The extractor will hold the gauge into the bolt while you insert and remove it from the barrel.
The ejector will put tremendous pressure on the gauge, not to mention it would be tough to even chamber the gauge with the ejector in place.

I make a lot of barrels and I headspace every one to a bolt.
I always pull the ejector, but only the extractor if the gauge does not sit right in the bolt.
This is not an issue with the forester headspace gauges I have, but the manson's have rims that are too thick.

Randall Rausch
www.ar15barrels.com

the field gauge i got from DrJekl does not require removal of anything



it has a machined rim which does not come in contact with the ejector/extractor.