Make the bolt a group or better a component, edit it and use tape tool to scale it to fit.

http://support.google.com/sketchup/bin/answer.py?hl=en&answer=116447&topic=2458211&ctx=topic

bolt.jpg

Excellent advices !
If the material of the printed parts is good enough(won't broke or chip) you could use a tap and a die(which would be like $10) and the outcome will be great !

The problem comes from small segments.

This is the problem with RoundCorner, as it is based on a pure geometrical approach. It may also an obscure bug. The zigzag artefacts disappear with an offset under 2mm.

So there is no good solution except may be

to scale up, as suggested by mitcorb to put the option "Strict respect of offset" ON but beforehands to try to eliminate the few small segments by adjusting the upper surface.

RDC Bug WorldWiz.png