There are a lot of different types of thru axles on the market. Here is a quick guide to explain the specification differences.
Forks:
Rock Shox : Rock Shox uses a 15mm axle on most of their forks but downhill forks use a larger 20mm thru axle. They use a standard length with a 1.5 pitch.
Fox : Fox uses a 15mm axle on most of their forks but, the treads are 14mm in diameter. They use a standard length with a 1.5 pitch.
Road : In general, road bikes that are equipped with disc brakes use 12x100mm spacing. This doesn't mean you'll need a 100mm long thru axle since the width of the fork varies between each bike. And there's very little consistency between brands so you need to check all the dimensions carefully. You'll probably need a thru axle that is 118mm or longer.
Frames:
Mountain bike: Most cross country, trail and most enduro bikes come with 12x148 aka Boost spacing. Some bikes use the 12x157mm standard. Since frame width affects the length of the thru axle and widely adopted standard thread, you'll need to check the dimensions carefully. You'll probably need a thru axle that's 170mm or longer.
Road: Road frames, in general use 12x142mm spacing, but this doesn't mean you'll need a 142mm long thru axle since the width of the frame makes a difference for the length of the thru axle. You'll probably need a thru axle that’s 160mm or longer.
The coarseness of threads is called pitch and to find the pitch, you need to count the number of threads over a 10mm section.
There are 3 primary specs for M12 axles – Fine, Medium and Coarse
Fine thread (M12x1.0) : This fine thread is generally paired with a conical (angled) washer under the head. Most thru axles use flat washers. This standard is often referred to as the Syntace standard. To find the pitch, you need to count the number of threads over a 10mm section. 10 threads is P1.0.
Medium thread (M12x1.5) : This thread is the most common on frames and forks, but it's not universally used. To find the pitch, you need to count the number of threads over a 10mm section. Approximately 7 threads is P1.5mm, and just under 6 is P1.75.
Coarse thread (M12x1.75) : Some frame brands use a coarse thread in order to decrease the time it takes to unthread a thru axle. To find the pitch, you need to count the number of threads over a 10mm section. Just under 6 threads is P1.75.
Double lead thread: Some thru axles, most notably Speed Release, use fine thread with a “double lead.” This means that there are twice as many threads which makes it faster to start threading the axle into the frame or fork. All Speed Release axles are found on frames and forks with one open dropout. In addition, the thru axle is notched ensure it doesn't fall out accidentally.
