The guy at my local hobby shop told me that the Thunder Tiger 91 FS has a bolt hole drilled through the cylinder wall (Below the top of the piston). Can anyone verify this? Do you think it will cause a problem down the road? Does anyone know if TT plans to fix this? I was thinking of ordering one, but I can not find one I can put my hands on around where I live.

Thanks

I don't know the answer about a bolt hole.
If this is a brand new engine we are talking about lets say just for the sake of conversation that it is true.
What would make you think it is a problem that TT needs to fix ?
This engine has been on the market as is for a long time, it is unlikely any tweaks are going to be made to it at this point.
All reports of the TT91 FS that I have seen have been positive, about smooth running, good power, long life.
It is reported that the TT91 FS is heavier than the other 91 four strokes, but if your plane needs nose weight then this is not a drawback either.

I'm the happy owner of a Thunder Tiger F-91s. There isn't anything about this powerful, smooth running, easy-to-tune engine that needs to be changed.

I've never even seen the TT 91FS but how sure are we that the hole being referred to isn't the one used to remove the connecting rod pin during disassembly. It would be on the back side of the cylinder and you would see the cylinder liner blocking it when you look into it.