I am looking @ replacing a OS 40 LA in my Trainer with a 4 stroke. My main reason for doing this is getting ready for a float fly, and will need the additional power to get off the water. I will also be using this plane for training others with a buddy box and want something that doesn't sound like its going so fast. What suggestions for a size on a 4 stroke. I was thinking of a Thunder Tiger 50 size, or will I have to go bigger to get more power. This will be my 1st 4 stroke.
Thanks

A 50 to 56 size would be about equal to your .40 2 stroke, but if you're wanting to fly off of water, you might want to think about a .65 or .70 or something similar. Four strokes are great off of water. Very reliable idle, and a carb that is in the back (less chance of water splashing into it). Good luck!

There's OS 52's . The OS 70 is has dimension's closer to a 91

Magnum/ASP/SC 52's and 61's. These are the same on the outside as the OS 52. The Magnum 65 & 71 have dimensions closer to that of the 91.

Saito 50, 56, 72, & 82 The 72 and 82 have a small foot print. The bolt pattern is the same as the 56 but they are 3mm wider between the beams.

The beauty of a four stroke is that you can run it at a reduced throttle setting all day and it will not load up. So you can use a bigger one for when you want a lot of power and the throttle back to fly around slow and easy.

Enjoy,

Jim

I have a OS52fs on my LT40 trainer and it's a great combo, haven't flown off water with it, but I'm sure it'll work OK.

For a 50 size fourstroke for your trainer you can for example have a look at: OS52, Saito56 or Magnum52.

Fourstrokes have great idle, nice trottling response and that puuuring sound, go for it.