Hi everyone

I am making a model of a petrol engine, I am doing the cylinder head and I am stuck on making holes for the valves etc, I am using the Boolean difference command, but once I get more then about 5 holes if I do the sixth hole, the head just disappears, in total I have between twenty and thirty holes I need to make. I have tried making the head up from four blocks and joining them together, but then it only lets me make about three holes and when I go to make the forth hole the same thing happens.
If anybody has got any ideas I really need to hear them, there is a link below if anybody would like to see the file.
https://drive.google.com/file/d/0B9h...it?usp=sharing

Thanks for reading.

Trikeman