I have a SS mtb, and want to change it to a dingle (double single) speed. This would require me to replace the bash guard with a larger outer ring on the cranks. And add another cog to the rear. To make this work correctly, i would make sure that the number of teeth was the same for both combination (ie 32x18 / 34x16 or something like that). This will go on a regular shimano freebody hub.
Since is has been working with the inner ring and spacing on the rear cog as a ss, i would just remove the outer spacer, add a thinner, the take care of the crank thickness, add the smaller cog, then spacers to space out the rest of the cassette.
Is this the best way to do it? If so, what would be the spacing/spacer size between the two cogs for correct chainline?