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Depends on how big the Pike is!! I know you won't be pulling a 20lb fish through a 6" hole. Remember about the Nils, if anything happens to the blade it won't cut. The blades are very sharp and fragile... If you are going to spend the money on a Nils you might as well spend a few more bucks and buy a used gas auger.
10 IS THE WAY TO GO .remember hole will freeze,so it will swallow your hole up.
I have used a 7" strike master mora (the classic old school, blue kind) pike fishing for the last 3 years. I swap out my blades at the beginning of each season. It works fine, pretty good compramise, cheap, and available just about everywhere that sells ice fishing stuff.
Would never use a 6'' auger for pike. Heard someone say theyve pulled several 40'' pike through 6'' hole, 3 ft ice...i seriously doubt this..