Hi Pete - Jason has summed up the action of the CP well enough - just like to add that the fit is critical and is difficult to achieve to be 'just' right. 'Difficult' in the sense that's its easy to be too tight or worse too loose - if that happens you can hear the CP going up and down with the piston quite noticeably. Some advocate making the side walls on a very slow taper and the side walls themselves quite thin to allow a degree of spring - personally having tried it I don't favour this method preferring to keep the sides parallel and getting the fit right - As Jason says if you follow the thread on the Eta engines on here and the Eta and Tigre threads over on HMEM fitting these is well covered (and how to reclaim it if made too loose
)
Though I haven't made a complete crankshaft for any of these 5cc engines using a cap head bolt in it's entirety I have utilised them to make the inner part of the shaft on the composite shafts I favour. The material has proved relatively easy to machine with HSS tooling but it's better to screw cut it than to use a die - well certainly on these shafts which were 1/4 dia. Your much smaller shaft may prove much easier. The material strength will be more than sufficient to stand up to the stresses involved on this little engine.
Jason am I not right in thinking that the making of this engine (or something very similar) was covered in depth by Ron Chernich and Mike Rehmus in Model Engine Builder magazine?
Good luck with your build Pete, I wish you every success - it's a nice little engine
Regards - Ramon