My cyma mp5k has been shooting at 340-350 for a year now. I decided to buy a bore up kit, lonex mp5k set. The stock compression was trash, new kit installed fps dropped to 260. Things I have tried and combinations of switching stock cylinder head to ZCI red v2/v3 head, didn't work. Using the lonex v3 cylinder head which didn't fit due to the over lap on the head. I took off the o ring and placed it on the stock cylinder head which did nothing either. I tried switching lonex cylinder with stock cylinder also cross with zci red head, and stock cylinder head. None of this has worked. During all of this I have replaced the piston with a shs full metal teeth piston and the lonex POM head that came with the kit. This has a good compression with these parts installed. I tried changing the spring to an m100 the fps didn't change either. Never touched the hop up it has remained the same. I do notice one thing, the CYMA mp5k cylinder head is taller than the zci piston head, no feeding issues I have test fried it. My last options is to return all to factory specs, and try changing each part out at a time. Any suggestions? was the piston head and piston rack the main cause?