Why instructions about fitting this tiny pin? It's because the tolerance range between pin and the hammer and strut hole is enough to give you a loose fit with one combination of parts and a snug fit with others.
The fit between the strut pin & the hammer should be a light interference fit, i.e. it should require light taps to insert the strut pin into the hammer. I normally use a 4 ounce ball peen hammer for this job. The fit between the hammer strut pin and the link should be free fitting. If you have a loose fit between pin and hammer hole, go to the bottom for those details.
I start by laying the hammer flat on it's side on a bench block or such surface that the side of the hammer is supported all the way around the strut pin hole (unlike straddling a gap between two vise jaws, etc.) to evenly distribute the force from the strut pin being lightly tapped in. Again, let me stress gentle taps! Insert the strut pin into the hole in the strut and using the strut like a handle to hold the pin and to help align the pin with the hammer hole. If you start the pin straight, you have fought half the battle. If light taps won't start the pin, stop and remove the pin.
I prefer the strut pin start into the hole by hand at least enough to hold it in the mouth of the hole If you find the pin to hammer fit is too tight, the easy way to fix it is to remove a little from the pin's OD. The easiest thing for most anyone to do is to hold the pin in the chuck of your electric drill, leaving just a bit more than half the pin's length sticking out of the chuck's jaws. Tear off a strip of 220 grit sandpaper and lay the sandpaper down on the edge of your work bench or other flat surface, rough side up.
Turn on your drill and lay the side of the pin against the sand paper making even contact along the exposed length. You can also use a fine cut file instead of sand paper if you feel comfortable. You can also put a small chamfer on the end of the pin to help you start it into the hole. Check the fit into your hammer as you go and as you feel it getting easier to start into the hole, switch ends of the pin and work the other side. It won't take much sanding before you have removed enough material, so check often.
Be careful of two things - don't beat the end of the pin so much that you mushroom it. If it's mushroomed now, start with that end exposed in the drill chuck and use a file to remove the mushroom. The other thing? Don't beat on it so hard that you crack or break the side of the hammer! Cracked or broken hammers only happen from abuse and will not be warranted.
Have a loose fit?
Assemble the hammer, strut and pin as normal and lay the hammer flat on a steel bench block. Use a center punch and hammer to make a punch mark fairly close to the edge of the hole, so that the edge is slightly collapsed, just enough to retain the pin. Now turn the hammer over and repeat. You only need one punch mark per side, just enough to keep the pin from falling out when the hammer and strut are removed from the pistol. When the hammer & strut are in position in the pistol, the pin is trapped in place.
