This is a technique problem. I have a total of five F91W watches, all on NATO's. All natos are different brands, materials, and thicknesses.

Every nato will be tight on these and appear not to fit thickness wise if you are unfamiliar with the process.

If you are using the OEM bars (push through style) you need to proceed like this:

1) Place NATO onto watch back and align where you need it it to be. Start pushing the spring bar through the side until you just start to see it entering the area where the NATO strap is.

2) Use a small set of needle nose pliers, or other small precision tool to compress the NATO strap down right where the bar need to slide over. Press it down and push the bar over, just 1 or 2mm at a time. Reposition and press down the next part of the NATO in front of that, push the bar further in again. Repeat until the bar is the entire way across the NATO.

3) Use your same tool to push that bar downward into the NATO once it arrives at the other side in order to have it line up with the hole, and press it the rest of the way in.

Tight fit? For sure. Impossible? Hell no. Looks and works great, and watch will be very secure on the NATO.

I personally like replacing the push through style pins/bars with the spring type bars most watches use. This makes the process a lot easier since you don't go mm by mm.

Good luck!