Angle grinders and cut off wheels don't work so well. They melt/burn the rubber. Try using a regular twist drill about 3/8" dia or slightly larger and make sure that it has flats on the shank. If you try to drill thru the tread, the steel belts will be a PITA. The flats allow the chuck to grab the bit firmly without you having to reef on the chuck key. I've seen chucks boogered up by too much pressure on the chuck key.

A hole saw won't allow the rubber "chips" to clear out of the hole like sawdust does.

I've tried a LOT of methods and for me, the twist drills seem to work the best as long as they are sharp. Lube, and have a drill that has a lot of grunt. The sides ofthe hole will want to grab on the drill, lube helps keep them slippery.

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3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).