Bought a new rtf with a OS 65la engine, the book says break in prop 12/6 the one in the box is an 11/6 and clearly labeled as such. Can I run the 11/6 on it just set a little bit more on the rich side ?

Yes to break it in on the ground if you are careful not to over speed it.

But fly the plane with a 12-6

The 11X6 is what I fly most of my .46 engines with so it may be a tad small??

Lately have been working with several students using LA 65's on Kadet Senior ARF's (an excellent combo) and 12x5 APC has worked out well indeed.

I run in the engine using the 11x6 prop, after the last tank of fuel you could still lay your hand on the head with not to much discomfort, but I kept it a tad richer than I normally would for final break in. I am using a 50/50 synthetic/castor fuel. It ran out really well. Thanks for the help.

I will fine tune and run through at least 1/2 a tank of fuel with the recommended prop before I fly her just to be sure.

Sounding good.

The LA 65 is a great sport engine

The smaller prop is so the engine will not get too much load while not broken in yet. There are a lot of threads on how to break in an engine but I would not fly one without at least a couple of tanks of fuel through it on the ground. And I seldom reach max rpms till at least the 3rd tank. I like to break in less than full throttle. It should get hot enough with a proper tune.

Breaking them in on the bench gives you a chance to get to know the engine also

I have "gotten to know" a lot of engines lately.

Won't be long and you'll be giving all kinds of advice?

Sounds like you are having fun

I ain't up to 2000 posts ,,,,,,yet.
I was selling new and used engines on the bay to make ends meet due to my fixed income and I ran as many as I could since I didn't want to sell no junk . I think I learned a lot. [8D]
I learned I am good with engines,,,,,,,,,,,not so hot on the sticks.