What tool can be used on a Yankee Hill free float tube jam nut without bunging it up?
Strap wrench is not the best IMO.  Is there a spanner type wrench?

Did you call YHM and see if he has the spanner wrench for sale yet. HE was suppose to get some but that was some time ago. This is what I do. I take a brass punch. I put some tape and a couple of patches between the punch and the ring. Then I tap the ring while making sure the tube does not turn. It has worked without marring or having the tube come lose.
If you want you can also use some green loctite on threads.

Quoted: Did you call YHM and see if he has the spanner wrench for sale yet. HE was suppose to get some but that was some time ago. This is what I do. I take a brass punch. I put some tape and a couple of patches between the punch and the ring. Then I tap the ring while making sure the tube does not turn. It has worked without marring or having the tube come lose. If you want you can also use some green loctite on threads.

Hope you never want to take it off with the green stuff on it......

One other idea, is that after you get the tube and receiver lined up, and you are not sure about really torquing the thing to death, is to put a riser rail or even an optic to bridge the tube and the Receiver rail.  I have an ACOG type mounted this way, and you cannot budge the tube ANY at all!  Two 10 inch rail risers from Model 1 would span the entire top of a rifle length YHM FF and the flat top.  Gives it a cool look.  Or, like I said, just a solid piece optic mount with two bolts-one fastening on the rail, the other on the receiver.  Most of the optic is positioned on the receiver though.  Probably won't work for magnifiers very well due to eye relief.  But it will for red dot type optics.  The real solution, and best by far, is to go ahead and torque the crap out of the thing and hope that you do not strip anything.  I have a torque wrench and receiver blocks and barrel blocks.  But the faux ACOG red dot is the lazy man's way.

I used the buddy system.  Put your upper in a vice.  Have someone "hold" the jamnut with a screwdriver and rag (as to no mar).  Then you (and maybe another hand) torque the forearm with your hands and a strap wrench as so it's JUST short of being lined up (I just used a straight edge on the picatinney rail edges).  Now, if you need to go just a little further.  Then at the very back.  Use a good heavy screwdriver on the very back of the forearm (closest to the receiver) and a piece of leather to keep it form marring.  And hit the actual picatinny rail in the correct orientation to get it home.  Mine is very tight.  I did put some blue loctite on, but did not prep the surfaces correctly.  And remember, when loctite gets hot, it loosens up.  Kinda counterproductive.  So I want to just make mine tight.

Gundraw

Get the right tool,its alot easier and you will be glad that you did. i Just completed my build on friday. I used this for the jam nut and a big strap wrench on the forearm it worked great. i got the wrench at eagle firearms.

That looks like the tool I need.  BTW here is how I lined mine up with a flat top upper...

I used a nice piece of flat 13/16 birch plywood cut narrow the length of my upper, lay it on my workbench near the front edge near me.

With my flatop up and FF - 4 rail tube handtight and aligned, lay upside down on the strip of plywood.

Use giant C-clamps or quick-grip clamps along with small wood bocks on the upside-down upper receiver and 4-rail tube to hold the whole thing down with the square surface of the upper and 4 -rail in line upside down on the flat surface, then adjust the jam nut