The diameter will depend on your airplane.

I have a 46 on order. I think the prop bolt size is 8mm. And the hole in the prop bolt for the spinner I believe is 8x32.

I looked at TruTurn. Looked like their spinner bolt is 10x32 and I think the bolt hole is 10mm. I believe they sell bushings that might work. But you then have to either pay them $9.95 to grind out the prop slots or do it your self.. depending on your prop pitch etc. The price gets up there when it's all said and done with.

I ended up buying a MPI from maxxprod.com. The 3" i needed came with a spinner bold that was metric. but for $2 they sell 8x32 bolts. And if you call the guy he will put in an 8mm bushing with the spinner at no extra charge. I spent $20 for the spinner, $2 for the 8x32 bolts and $5.95 shipping.

they are very nice looking spinners for the money. But you will still need to grind out the slots for the prop depending on what prop you use.