My 666 had the pin (17) dropping out mostly when I tried to shift to 4th and 5th gear. It dropped onto the battery, and I was able to find the pin each time. I finally gave up and ordered a new shaft (21) that has the pin pressed into it. I replaced the shaft and after some tight shifting, it has loosened up allowing me to use 4th and 5th. The diagram shown earlier in the thread shows how it all goes together. It is a puzzle to put back together, and when you pull the shaft out, several pieces will just drop into the battery area. The pieces shown as number 42 are a challenge to line up with the shaft as you reinstall. It takes some wiggling of the shaft and eventually it will go back into place.