There are a number of posts on SP over the years that address this exact issue, so you could also check them out. Ultimately windows are just recommendations, as Bob B says above, so don’t sweat it too much. It’s not that kind of exact science. However it helps to at least understand WHAT is being recomended by the manufacturers, and the confusion might have a little to do with the fact that there are really (at least) two kinds of gain windows out there. Yep, it can be confusing when you first start out.
The main kind in terms of numbers - If you see a grain window that has a range of 100 grains plus/minus a bit that is a grain window that is meant to indicate the weight of the HEADS you use, and the range is meant to encompass what weight range heads are most efficiently castable with that rod. The range typically includes the range in the best weights of different types of lines, AND the natural variations individual’s have in their casting preferences. So for example the weight of a skagit head you like on a particular rod wiil typically be 40-60 grains heavier than the weight of a scandi head you like, and different people (and possibly the same person in a different mood) may have a general preference for lighter or heavier lines in general, so add another 30-40 grains. That is where the range comes from, and this type of window really doesn’t have anything to do with tips which are a whole other thing. There are tips that match the lines you choose, but the weight of the tips does NOT come into this type of grain window, which FWIW is what the vast majority of rod makers out there use. But to not understand this, as even many fly shop guys that don’t spey cast themselves sometimes do not, is a common prescription to get a crap, way too light recommendation on which head to use when sink tips are involved. We have seen this many times from stories posted on here.
Then there are the Bob-Meiser-style grain windows - you will know them by the fact that they are FAR larger - 250-350 grains wide - and possibly by the fact that it is a Meiser rod. There Seem to be far fewer companies that use this type. By all accounts Meiser invented the idea of a grain “window” for a (modern) spey rod, but if I had to guess some of the confusion out there come from his pervasive influence rather than the actual prevalence of these big windows. The big Meiser windows are the full-featured Rolls Royce version of what a grain window can be, fully thought out, and perfectly explained at the proper length on the Meiser web site. That is like a users manual for grain weight. If you haven’t read the Meiser grain window page(s), regardless of whether you own one of his rods, you are still a grain window virgin. ‘Nuff said, go do it.
There are a few clarifying caveats we could add here ...
- Everyone that does this long enough will develop their own rules of thumb (that work for them) on how to treat these windows, some examples of which have been given above. Try them or ignor them. There is nothing rigorous about any of these shorthands, but hearing them MAY help - that is, unless you are the kind of person that just gets more confused by the lack of a single standardized set of rules.
- Both Airflo and Rio
have extensive line charts available online listing many, many rods and the suggested weights of their various lines for each rod. Rio
even has an A and B rating - one for those who like lighter lines and one for those who like heavier. I would start there - if you can’t find your exact rod there you will find something close.
- A lot of rods just have one number and leave it up to you to have the knowledge of how to adjust for different style lines and to taste. With one number there can be no confusion about exact instructions - plausible deniability for THEM when YOU muck things up.
The same thing could be said about the regular narrow widows - a bit of plausible deniability. With the Meiser window (plus the “manual”) all the info is there and the rest is up to your personal tastes.
- From many, many stories (there are lots on here) we know that the guy at your local fly shop it’s not to be trusted out of hand when it come to grain weight. So take those guys with a grain of salt the first time you ask a new one for info. Its not necessarily that they are just trying to sell you something no matter what, but sometimes they don’t spey cast themselves or even fully understand the systems. I imagine things are getting better over time, but if you just walk in to say a Cabelas and ask the first guy you see, and not the one guy that is sometimes there and actually know what he is talking about you are likely to get advise that is bad. If you crowdsource the answer to the question on speypages you typically get far more reliable info.