When it comes to fly rods for pike fishing there are a few key factors such as:
1) Size of the Fishing you're chasing
2) The Size of the Flies you'll be casting
3) The amount of Casting you'll be doing
"Fly fishing for pike" means many things to different people. To some it means constant action catching little "ditch pickles" virtually every cast. To others its casting and searching all day for one giant fish, and still to others its sight fishing shallow bays where you'd never throw a cast until you see the specific fish you want to cast to. Each of these scenarios would be best suited to a different fly rod setup. If you're casting all day then a high modules graphite rod that will be lighter in your hand and more responsive is probably a better bet. If you're sight fishing and doing very little casting then a heavier more durable rod may suffice. Here's some suggestions on rod weights for fly fishing for pike.
Rod Weights for Pike Rods:
-If you're fishing for smaller pike or with smaller flies (3"-5") then a 6wt you may already have for trout could be just fine. One with a fighting butt would definitely be preferred, however with flies that small and pike under 30" it isn't a necessity either.
-If you're fishing for pike up to 36" and flies between 4" & 6" then a 7wt or 8wt would be fine. A #7 or #8 is more than enough to cast flies that size and certainly more than enough to handle mid sized pike
-If you're targeting trophy pike (40" and up) then an 8wt-10wt is what we'd recommend. If you're dealing with plentiful but skinnier fish up north then an 8wt or 9wt is probably lots. If you're fishing some more southern locations with much longer growing seasons and far heavier fish that are thicker and have much bigger girths than a 9wt-10wt would be recommended. Whether you're looking at a 10wt for pike vs a 9wt for pike fishing is not as much about fish size, even an 8wt will handle most fish over 40", but if you're going to be throwing larger flies (10"-16") then a #10 will make that much easier to do. For the largest pike and pike flies I commonly fish an 11wt. Again it's not needed for the size of the fish, a 9wt will handle mid and high 40" pike without issues, but #8-10 rods will not handle most flies that are in the 12"-18" range.
Rod Recommendations: (click pics to view the rods)
Good Pike Rods ($200-$350)
Echo Ion XL Echo Boost Saltwater Orvis Clearwater Big Game & SW Rods
Better Pike Rods ($400-$750)
Orvis Recon Thomas & Thomas Zone Echo 3S (Saltwater)
The Best Pike Rods ($800 and up)
Orvis H3D Thomas & Thomas Exocett Thomas & Thomas Exocett SS