Version 2 Update: I tweaked the geometry to reduce the probability of breaking off the tip of the plug while extracting. Should be much sturdier and easier to use.

I had to make a minor change to the code from http://www.thingiverse.com/thing:34027 so I have now uploaded the modified file as well.

I haven't done much testing with other sizes beyond my immediate need. Please leave comments if you find any issues with it scaling to other sizes.

This is a tool I made based on the idea I saw at http://weldingweb.com/showthread.php?168191-Internal-o-ring-inserter-tool

My '71 John Deere 4320 tractor has some hydraulic dowel pins (JD part R41511) that (during a clutch replacement) require a new o-ring (JD part R56464) to be inserted inside a small shaft in the support unit (JD part AR40572). I spent hours unable to get even one in with the tools I had.