Hi I have a OS.65LA using a 12x8 prop and want to switch to a 3blade prop where or what size prop could be used and is there a conversion chart for prop swap with engines size thank's Frank

Although not perfect, the general rule of thumb is to go down one inch in pitch or diameter. If you use a 12x8 two blade, try an 11x8 or 12x7 three blade prop.

You maybe correct Jim. I was told down 1 in in dia up 1 deg in pitch so a 12x8 2 blade would be a 11x9 3 blade. I don't know for sure that's what i was told

I changed a 14x8 to a 13x7 MAS on a 90 4S, works great.

Thank's Jim Jet and scratchonly I wrote to OS but they haven't reply in two days guess best thng to do is try one of each size down and one with the pitch down too ...[8D] see how she act's Frank

For what it is worth (and its not much) here are RPMs on a Magnum 61 four stroke comparing 2 and 3 blade props.
Test was done using 15% fuel, both props tested on the same day, thrust measured with digital fish scale, RPM are sustained (not peak).
Master Airscrew Scimitar 2-blade 12x6 10k RPM 5lb 5oz thrust.
Master Airscrew 3-blade 10x7 10500 RPM 4lb 12oz thrust.

I have not tried to replace a 12x8 but here are some conversions I have used:
10x4 2 blade= 8x6 3 blade
11x6 2 blade= 9x7 3 blade
12x6 2 blade= 11x7 3 blade
The 11x7 3 blade loaded more than the 12x6. Maybe the 11x7 would work for you?

Using “Prop Selector,” a 12x6 three blade should load an engine about the same as a 12x8 two blade.

If you’re wanting to change because you need additional ground clearance, an 11x7 three blade would load the engine slightly less than your current prop; and an 11x8 would load it slightly more.

Sometimes I think prop selection is voodoo or something. You buy every size imaginable. You try a bunch, and only like a few. Then you go back to the one you originally started with.

The more accurate rule of thumb is "down one pitch AND diameter for every extra blade." It's worked just perfectly well for everyone I know, and didn't really reduce performance much at all. 4-stroke engines may be different, due to more torque...but that's the way 2-strokers work!