I would made the hole about 8,5 mm. Normally I first drill a 8 mm hole with a bradpoint drillbit. After this is done I use a 8.5 mm metal drillbit.

However, I often have problems with making the hole parallel. I would use square wood from about 20x20 mm, drill a hole from 8 mm and put a 8 mm dowel in it (tight fit) When you need 115 lenght I would made the lenght 125 and use a drip superglue at the ends. This dowel should stick out on both sides.  One side of the dowel than is placed in the drillpress chuck and the other side I would place in a ball bearing. Then I would turn it to 18 mm round with using a chisel and a chisle support. Doing it this way your hole is 100% centric. 

For a toy I would consider a 10 mm piston.

If my english explanation falls short, you can always contact me, but for now I'm going to bed.


Start at approx 6 minutes;






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