It's not the 24" bit that I am interested in, It's the auger attachment. It just comes with the auger. I was just looking for some guideline as to what size bit my tractor could handle in clay soil. I need to drill some holes to accept 8" sono tube
Then I'd say you'll need a 12" auger. Be sure to check on the auger type/brand before buying to see if replacements are available. If it's a common one, you should be able to sell the 24" auger to recoup some of your cost too.