I love all my OS 4-strokers as well as my Saitos. Both are quality engines. To replace a .46AX the rule of thumb I use would dictate a .70 four stroke. The Saito .72 is a great engine, comparable to the old OS .70 Surpass of which I have 3.

However, I recently compared my Saito .72 to a new OS .56a in the same airframe. I did this because the specs looked similar on paper and the OS is much lighter. Guess what - that new little OS is about 99% as powerful as the Saito .72. This is with them both running inverted, unpumped. The new OS 'a' series seem to change all the rules as the new .82a is at LEAST as powerful as the old OS .91 Surpass.

The YS guys are right about the power of those engines but I've really never seen one that didn't ask for a lot of tinkering. With the OS or Saito it's set it and (almost) forget it.

Good luck with your choice. There's nothing like the sound and reliability of a four stroke-unless it's twin four strokers!!!!!