Bruce, some small engine techs who work on Briggs will have a special tool for these that they use on Briggs electric starter motor pinion gear retainers. I don't work on enough of the electric starters so I just muddle through as well as possible. Find a socket or piece of pipe that just barely fits over the hammer shaft. Place the ring in place, use a dab of grease to hold it, and use the socket and a hammer to drive it on.

My air guns I also install a small thin o-ring beneath, then spread the ring a bit so installing a socket will be extra hard on and off. I then use a one inch extension that has a spring loaded ball retainer and leave it on the air gun.

I don't know where you got only part#9 for $2, but it is better to replace part#10 at the same time. #10 is a rubber o-ring. I used to buy 3 clips & 3 o-rings pack from snap-on tool truck for $8 per pack. But I found it cheaper to buy online.

http://impactsocketretainerclip.com/

The site below sells rings and tools to install. I hope this will help.

Here is a link that might be useful: http://impactsocketretainerclip.com/