View Full Version : head space gauges (moved to BUILD IT YOURSELF)

April 06, 2002, 11:32
I'm in the market to buy a set of head space gauges. can you guys please give me your experience, coments and wich is good brand name to buy. also is there a need to buy the field gauge or should I just buy the "go" and the "no go" gauge ? thanks for your help.

[ April 06, 2002: Message edited by: EMDII ]

April 06, 2002, 13:55
here is some info that might help

Headspace is the distance from the breech face to some datum point on or in the chamber or barrel. For a complete discussion of what headspace is, and how to measure it, see the article entitled "A Headspace Primer," in the October, 1999 issue of CRUFFLER.COM. With reference to the FAL, the headspace dimension is that distance from the bolt face to the portion of the chamber where the internal diameter is exactly 0.400". As with many mechanical assemblies, this distance is expressed as a tolerance, rather than an absolute. According to FN, the headspace tolerance for an FAL is:

FN Factory Headspace Dimensions Minimum 1.6315"
Maximum 1.640

Unfortunately, it gets a little more complex. In addition to the absolute maximum, FN also specified a maximum for a new or overhauled rifle of 1.638". That is to say, there are two maximum headspace dimensions, one for a factory new or overhauled rifle and one for a rifle that has been in service. This is shown more clearly below:

FN Factory Headspace Dimensions, New and In-Service Rifles Minimum 1.6315"
Maximum, New Rifle 1.638"
Maximum, In-Service Rifle 1.640"

The equation becomes more difficult when one takes the SAAMI headspace specifications for the .308 Winchester into account. While the 7.62mm NATO and the .308 Winchester are not the same cartridges, and should by no means be considered interchangeable, they are often used synonymously in the United States, and because of their dimensional similarities, only gauges and reamers for the .308 Winchester exist. The SAAMI specification for .308 Winchester headspace dimensions are noted below:

SAAMI Headspace Dimensions, .308 Winchester Minimum (GO) 1.630"
Maximum (FIELD) 1.640

You'll note that there is no "NO-GO" specification. SAAMI doesn't specify one. The NO-GO specifcation, measurement, and gauge are products of the gauge makers, and can be whatever the manufacturer decides it should be (within the bounds of the minimum and maximum dimensions). This is why the Forster .308 Winchester No-Go Gauge measures out at 1.634", while the Clymer .308 Winchester No-Go Gauge measures out at 1.636". However Forster's Field gauge measures out at 1.638", inidicating that Forster builds a .002" margin into its gauges that is not the same as the SAAMI specified .308 Winchester maximum headspace dimension. For this reason, we opted to go with the Clymer gauges. Taking all this into account, the consolidated headspace dimensions look like this:

Consolidated FN Factory and SAAMI Headspace Dimensions, New and In-Service Rifles Minimum, SAAMI 1.630"
Minimum, FN 1.6315"
Maximum, FN New Rifle 1.638"
Maximum, FN In-Service Rifle and SAAMI 1.640"

Optimal assembled FAL headspace range then, is: 1.6315" < Rifle Headspace < 1.638".
:p :p

April 07, 2002, 01:22
thanks chunster, that was a real nice help.
so if I understand correctly, you like the clymer gauges better than forster??? let me know. thanks.

April 07, 2002, 11:33
can some one tell me wich one is better quality: clymer or forster gauges. thanks for your help.