If you think about it, fly-fishing requires more connections between lines of differing strength, diameter and composition than any other type of fishing. We join the fly line to the backing and a leader system to the terminal end of the fly line. If you use a shooting head or a line that incorporates different tip sections, even more connections are required. Unquestionably, a series of interlocking loops is the most efficient means to make these connections.
Increasingly, manufacturers are offering fly lines with factory loops installed, but there will be times when you have to fashion your own. Many fly-fishers are familiar with the practice of connecting the fly line to the backing via a loop-to-loop connection, but I also use a loop in the front end of the line to make changing leaders and butt sections easier. Contrary to some beliefs, this front-end loop does not spook fish, nor does it adversely affect the way the line turns over on the cast. There are several different methods of fashioning loops in fly lines, and over the years I have used them all.
If the diameter of the fly line is not too large, the easiest thing to do is to fold the line over itself to make the desired-size loop and bind it together by means of several speed or regular nail knots. With the exception of the loops I use for the running lines of my shooting heads, which are large enough to pass a reel or a coiled head through to facilitate quick line changes, my loops are generally about 1¼2 inch long. To tie the nail knots, use a 12-inch section of 8- to 12-pound-test monofilament and bind the fly-line sections together with two seven- to eight-turn knots, leaving a space of less than 1¼8 inch between the knots. If the fly line is limp or has a very small diameter, you can facilitate tying the knots by laying a needle or a length of heavy wire along the fly line to serve as a stiff base. This system has never failed me.
You can make a similar type of loop by whip-finishing the line sections together with a bobbin and fly-tying thread. However, even though many anglers swear by this connection, it is not nearly as secure as the monofilament nail knots, and I have seen a number of these loops pull free. I do not recommend this system for large saltwater species.
With a larger-diameter fly line, like floating and intermediate lines, folding the line over itself produces a bulge that might have difficulty passing through the rod guides. The most popular method for putting loops in these lines involves using either monofilament or Dacron hollow-core braids. I prefer the latter because it doesn't break down as readily as monofilament. Both types come in different breaking strengths and diameters. A 50-pound-test hollow-core monofilament braid will accommodate most large-diameter fly lines; for Dacron you'll have to look in the 80- to 100-plus-pound range.
In either case, begin the process with a 12-inch section of braid (if you want a larger loop, start with a longer length of braid). Take a 12-inch length of approximately 27-pound-test single-strand wire and bend it in half to form a narrow threading loop. Insert the wire into the middle of the braid approximately 6 inches from the tag end, and push it about 11¼2 inches up inside the braid. At this point, push the wire back out of the braid. Insert the free end of the braid into the wire loop, and then grasp its tag ends and pull it along with the end of the braid back inside the braided line that you ran the wire through. Pull the wire loop and the tag end out, leaving about a 1¼2 inch loop in the braid. It's common practice to insert the tag end into the braid a second time for added security, but it's not necessary. As long as the loop is pulled evenly (which is what happens with interlocking loops), the tag end will not pull free from the braid. To make sure this never happens, tie a nail knot over the section of braid just below the juncture of the loop. Now, regardless of how you pull on the loop, it won't give way.
Once you have formed the loop in the braid, you must work the fly line into the remaining straight section of braid. (With a 1¼2-inch loop in one end, about 5 inches of single-strand braid should remain below the juncture of the loop.) To make the job easier, cut the tag end of the fly line on a bias and work it inside the braid "inchworm style" until you reach the tag end of the loop. Lastly, whip-finish or nail-knot the tag end of the braid over the fly line.
I have used braids for many years, but for the past year or so I have reverted to a method I tried a long time ago when I had to fashion an emergency fly-line loop during a trip off Baja. I was tuna fishing, and a yellowfin that I couldn't control ran under the boat, severing the fly line as it sawed across the keel. The only material I had was plain monofilament, and it worked perfectly. In fact, today I prefer using monofilament loops instead of braid. I find they tend to last as long as the fly line, and they have a stiffer character than braids, which assures a virtually hinge-free connection.
Generally speaking, any good-quality leader material in the 50-pound-test range will make very effective fly-line loops. All you need to do is form the desired size loop in the 50-pound monofilament and bind it to the tag end of the fly line with a few nail knots - I use four or five. To make the transition from the fly line to the loop as smooth as possible, first flatten the tag ends of the monofilament with pliers to eliminate any bulges. When fashioning a mono loop to a clear intermediate line, use 20-pound-test monofilament to tie the nail knots. Intermediate lines do not have coatings like other fly lines, so you need the heavier monofilament for the nail knots to make the connection secure. Again, when properly executed, these loops should never pull free.
One final note: I never use binding-type glue with any of the methods discussed here. It contributes nothing to the strength and simply is not necessary. In fact, it can be a disadvantage because it can make the connection less flexible, and in the case of monofilament, the heat that some cyanoacrylate glues generate will actually weaken the line. The only adhesive I sometimes use is rubber cement and only for smoothing the connection.
With that said, this is one case where getting looped is definitely a good thing.