I have an old style Great Dane Surfer(fixed deck)52" and I just purchased a new set of blades again. The book says the blades are 18" long. The blades the dealer sold me last year and this year are 18" long. But the blades that came with the mower when I bought it used were about 17 5/8" long. I have to grind the new ones down to that length or they will hit in one spot on the deck and I think 2 of them hit together also. What do you think the problem could be?