Still don't have my Yankee Hill 7.62QD can in yet but did get a 556 QD today at a gunshow. I had a phantom FH on and now want to install the QD. I shouldn't have to use a crush washer since the QD has holes all the way around and there is no shoulder before the threads. So do I just install the QD and call it good or shim or is there something I don't know. Right or wrong please give me some input.
Here's a few pics

The old FH


QD half way threaded on.


QD threaded all the way down but not torqued yet

Just torque it down without a washer, or anything.

Yeah just put it on there without any washers or anything. For my AAC M4-2K mount they included some rocksett, so maybe put a drop of that on the threads if you have it.

Ya YHM states in their manual iirc not to use crush/lock washer.

thanks for the info guys.

Check concentricity (you'll have to mount the can), remove the can, then use a locking agent (e.g. Rocksett, high-temp lock-Tite, etc.) on the barrel threads and torque the mount down to manual specifications.

Check thread depth before torquing, as the FH is designed to index on the shoulder, not on the muzzle.  If you're bottoming out (this happened on my barrel, for instance) then it will not be mounted properly.  YHM didn't have a stretch specification for me when I asked, but I left 0.050" of room with a spacer and everything seems to be fine.  You might not need that much at such a low torque rating...