before you start buying stuff, make a game plan. most all multispeed derailleur bikes are 3/32" pitch. a crank for a 8/910/+ speed just have slightly narrower (thinner) rings and the spacing between them is closer.
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“One morning you wake up, the girl is gone, the bikes are gone, all that's left behind is a pair of old tires and a tube of tubular glue, all squeezed out"
Sugar "Kane" Kowalczyk