Bonas Wrote:Joe Carlos with the AMU settled on retaining compound, specifically Loctite 620, not threadlocker. The max service temperature for Loctite 242 is 300 degrees. This is a link to an abstract of some of Carlos's work:
https://www.thefreelibrary.com/The+relat...0321900231
I've read Carlos's papers. Most people credit the idea of bedding barrel extensions with "Loctite" to Carlos's work, from what I've seen. He has migrated from bedding and shimming to using oversized barrel extensions. He discusses "red" versus "green" Loctite in the abstract linked to above. I think the idea that thread locker makes a useful bedding compound might arise from some people thinking of "Loctite" as just thread locker instead of what it is; a brand name for a full line of adhesives made by Henkel.
I recommend anyone who wants to bed an extension with Loctite to use one of Henkel's products that the manufacturer recommends for high temperatures and is designed to fill gaps. That's 620 or one of the other retaining compounds in the 600 series. Threadlocker isn't designed to fill voids.
But, like I said before, I don't bed at all anymore. I just shim to get a press fit. My last good barrel appeared to come with an oversized extension, which was great.
No disrespect to anyone, but the point of barrel bedding is to fill gaps to remove play..When tightening the barrel nut, the receiver bore thread area of the upper will compress around the barrel extension, similar to how a muzzle nut/devise will compress the end of the of the barrel bore, the higher the torque is applied to the nut/device..
Using a standard upper with no barrel installed and a greased mil spec nut, I have measured .001" compression at 35 ft lbs torque,
.002" at 40 ft lbs torque, .003" at 50 ft lbs torque..
After around 50-60 lbs, the compression seemed to stop..
Mil spec TDP upper receiver barrel bore is 1" +.002", but most modern manufactured uppers will run a little under..
Mil spec barrel extension O.D. is .9987" -.0008"
Upper receiver bores I have measured using + pin gauges were:
Colt .9994"
Wilson .9980"
Aero .9980"
Geissele .9975"
SOLGW .9960"
BCM .9960"
Considering barrel extensions can vary, the need for shimming varies from upper to barrel extension being used..
When Cerakoting the I.D. bore of the upper, the inside bore will also decrease.. I have had very tight barrel to upper fit after using Cerakote..
Considering some uppers are thermal fit to begin with, most modern uppers being under mil spec, the need for for a special bedding compound could vary from a (possible) maybe to mostly pointless IMO..
Sloppy machined uppers is a different story..