F NOCK is not a typical pin nock.
In this years we found that bowstrings materials and specially the servings has evolved to different thickness than when  groove diameter sizes was created.
We find in our test  that if  we want the best performance bowstrings we need to lighten the centre.
Now exist smaller diameter servings, more durable and solid.
Actual nock sizes are standards till more than 20 years and don´t change in this time. .98 and .88 are standards, and in most cases don´t fit good enough , some time loose and some times tight. 
Most of archers solve this question using larger servings on the bowstring centre serving. 
Nothing is more harmful to the performance of your bowstring that add weight to the centre of the string, but no other solution on the market.

Now , yes you can.
Archers have now other two options to fit correctly with not loose on the bow performance. 
Oh .. on the bow performance? 
Yes, the bowstring in one of the bow arts : riser, limbs and string.
If you look for the speediest limbs, the more precise riser and you want that both work well, you need the best string, with the minimum weight on the centre and the correct nock.

F NOCKS  are designed for use on most of the pins available today and have two different grooves size : F1 a lit smaller than .88 and F2, a lit smaller than .98.
F NOCKS  shape is aerodynamically designed and offer the minimum surface to deflect arrows on tight groups.
F NOCKS are manufactured 100% in Europe with a high quality polycarbonate.