Originally Posted by Kt-88
Maybe they mean 2.5 slides? I'm not clear on the needle jet size there, either.
There are 26 needle jets in the 159 series and they increase in size by some arcane formula I don't understand but can regurgitate. The letters increase by 0.050mm, so O-2 is 0.050 larger inside diameter than N-2. The digits are increments of 0.005mm, so N-3 is 0.005mm larger inside diameter than N-2.
The arcane point I don't know enough about is the lack of uniformity between the letter series: 5ea N, 6ea O, P, Q, 4ea R. So the diameter makes a bigger jump between sizes at the lean and rich ends than it does in the middle ranges. Haven't found an explanation for that yet.