If the TH 40 MKII is anything like the original (not sure), BB engines are not a problem. Move the battery back some to offset the weight. I have the original TH 40 Trainer and have flow it with a OS .40fp, OS 40max-h, YS .45(WOW!), Enya .45X, OS .46FX and a TT .46 pro. This plane is my .40 to .50 size engine test bed. The .40fp flew it but left nothing to be desired. The .40max-h was much better, it is still an air-bleed carb but just had that much more power and it's a BB engine. The others were great but the YS was too much. The.46LA would be fine, I won't argue that. My only point is why pay for OS's lesser engine when for the same price yet can get a better value engine to move into other planes down the road.