Dave, few things about a chainsaw are more confusing at first glance than chain selection. Every chain manufacturer has their own special "code" to identify their chain. In truth it's really simple once you understand what it all means and get past the manufactureeeze.
The two main things your concerned with are pitch and gauge.
Pitch is the distance between any three rivets divided by 2
..... The reason pitch is important to you is it tells you weather or not the chain will match your drive sprocket and bar tip sprocket (if you have one). Think of it like this, the chain on a saw is kinda like the chain on a bike, only backwards. The bike has a sprocket with teeth that fit into holes in the chain as it goes around..... a saw chain is just the opposite, it has teeth on the chain that fit into holes on the sprocket to drive the chain around the bar. If the chain is one size and the sprocket is another then it will be impossible for the chain to stay on the sprocket and you will throw chains and cause premature wear on both the chain and sprocket assembly. The reason they measure pitch as the length between any three rivets divided by two is that the spacing between two rivets on the chain isn't the same as the spacing between the next two rivets and so they average it between any three rivets to give you the chains "pitch"..... confused yet??? Don't be, all you need to know is that the pitch of the chain you are buying matches the pitch of the sprockets on your saw.
Gauge is a measurement of the thickness of the driver (that "tooth" that fits into the sprocket) on the chain. The gauge is important because it must match the gauge of your bar groove. The bar groove keeps the chain on the bar and cutting straight. If the gauge of the chain is thicker than the gauge of the bar groove the chain will be to tight and not fit into the bar groove at all... not really a mistake you can make you'll know right away it's wrong. If the chain gauge is thinner than the gauge of the bar groove the chain will be loose in the groove and let the chain rock from side to side causing the saw to "wander" in the cut and cut poorly, or not at all.
There are a few other factors when picking a chain.... cutter shape is probably the next biggest thing to consider, and is where a lot of confusion comes in...... Full chisel, simi-chisel, square chisel, chipper, round ground, square filed, round chisel.......
It's really not that complicated (or at least it doesn't have to be) really there are only two cutter shapes you need to worry about
Full chisel (also called square chisel by some) has a cutter that when looked at straight on (looking down the length of the bar) has a sharp corner at the top of the cutter and will look like the number 7.
Simi-chisel (also called round chisel by some, also sometimes wrongfully called "chipper chain" by some) has a cutter that when looked at straight on will have a more rounded edge and look like something between the number seven 7 and a question mark ?.
Full chisel chain is the most efficient/aggressive profile and is faster cutting than simi-chisel chain but it dulls more easily... once you blunt the "point" on the cutting edge at the corner (also called the working corner) the chain quits cutting well and it's time to sharpen. That "point is pretty easy to dull if you hit anything but clean wood with it.
Simi-chisel chain is not quite as efficient/aggressive and is a little slower cutting than full chisel chain but because of the rounded edge of the cutter there is no "point" to dull. The whole top corner radius is the "working corner" and is much harder to dull and will go longer between sharpenings.
I'm not gonna go into "chipper chain" much because it's really kinda become obsolete and you likely will never see any but it will have a very rounded cutter profile and look more like a question mark ?.
The next thing to consider (or probably not for you) is cutter sequence.... full comp, skip, or simi skip....... here we go again right
......
"Full comp" (as in full compliment of cutters) just means the regular chain you already use and has the usual number of cutters..... cutter-driver-cutter-driver-cutter-driver
"Skip chain" just means that every other cutter has been left out..... cutter-driver-driver-cutter-driver-driver-cutter-driver-driver-cutter.
Skip chain is typically used on longer bars in big wood and leaves more space between the cutters to let the chain carry the chips it makes out of the bigger logs without clogging up the cut (thats the simplified version for those who would correct me). The skip chain also has about a third less cutters and puts less strain on the power head which can help a smaller saw to pull a longer bar when necessary.
Simi-skip is just a compromise between skip and full comp and has every third cutter left out of the sequence..... cutter-driver-cutter-driver-driver-cutter-driver-cutter-driver-driver-cutter-driver-cutter-driver-driver.
And thats all there is to it.... clear as mud right??
Anyway it's really not as complicated as it seems. Like I said you really don't need to worry about skip chain yet, all you have to know is your pitch and gauge and then decide if you want full chisel or simi-chisel chain. Hope I didn't cornfuse you too much.
