I have a 4*40. It is running with a .40 Super Tiger and is running somewhat sluggishly. I had an idea. I have two Super Tiger engines. Both are the same physical size, so they should be interchangeable. One is the .40 size which is currently on the 4*40. The other is a .51. They are the same except that the .51 has been bored out. The .51 has never been run and is a spare. I was wondering about replacing the .40 on the 4*40 with the .51. What do you think?