Hi guys, I'm breaking in a Magnum .70 4-Stroke and swinging a 13x6 right now. I eventually want to try a 3-blade. Is a 12x7about the size it should handle? I certainly don't want to tax this motor more than I need to but want that scale look without losing performance. I wont put this prop on until I've got my first gallon of fuel in it and flown. Can I expect a big difference in thrust and speed bychanging 2 to 3 blade? Will I need to revisit my needle settings? Thanks!

Down one in length and up one in pitch. About the only difference you will notice is when you let off the throttle, the plane will slow down faster.

Thanks Gray Beard, So I'm good with my 12x7 then.

EDIT: After looking at prop sizes on Tower, I don't see a 12X7 3 Blade. Given the choice of a 12x8, 12x6 or 11x7 are al of these fine sizes for my .70 4Stroke?

Yes but like any other engine/plane combo you still need to test prop sizes to find the one that will fly the plane the way you like. Prop testing was one of those things I couldn't get into when I started flying RC, that's a big bunch of money to be tossing at them when you have something hanging off the end that is flying your plane. Then one day a friend handed me a bunch of props to try and when I hit that magic prop it was an eye opener. I tend to like my planes to fly slower but have better thrust so I use longer props with a smaller pitch. During testing on one of my planes I discovered the plane didn't fly well with that thinking so I went shorter and more pitch and the plane flew just how I wanted it to. There is a starting point but no set rule for prop size, it's how you like the plane to fly that counts. As long as the prop doesn't lug or over rev your engine it's all up to you. Have fun with it!!

I understand fully Gray beard, Thanks for the info!  I just wanted to make sure my prop choice wasn't going to kill my engine one way or another.

A 12x7 might be a little too much load. A 12x7 2-blade is approximately the same load as a 13x6 2-blade. You might want to lower the load by going with a 12x6. I'm not saying a 12x7 3-blade will kill your engine, but it won't like it.

I'd probably start with the 12x6. According to ThrustHP it takes 1.33 horsepower to turn a 12x6 three blade at 10,000 and 1.22 horsepower to turn a 13x6 two blade at the same rpm.