Remove the pinion and put a scribe mark on the old hole where the pin went. If the pinion is ever to be used in the future the same hole for the groove pin must not be used.
Clean the small parts then remove and clean the transmission housing. Be careful not to drop the transmission halves on the bottom of a plastic tub. I did and it cracked it, so I had to grab up the cracked end to keep the kerosene from running out. When I finished I got a funnel and put the kerosene back in the jug.
As I recall, I believe I used the 90 degree and straight pick to install the O-ring. I pushed the lower side of the ring down with the 90 degree pick while pushing the upper side of the ring down with the straight pick and then a combination of the two to situate the ring in the groove below the bushing. I think it took me about 10 or 15 minutes to get the ring in place. I may have started over a second time and put a small amount of transmission oil on the ring to help it slide better. When the bottom edge of the ring was even with the groove I put the 90 degree pick below it to keep it from sliding down any further while using the straight pick to push the rest of its circumference into its resting place. The ring tends to pop into place once it reaches its groove because of the rings natural tendency to expand to its original shape. Absolutely make sure the ring is in its proper place before proceeding.