hi i have a wr450f o7 i have just put a big bore piston and cylinder in my bike (athena)when i put it back together it started fine but i had a small oil leak from top of cylinder and head so i took it all apart and tried again there was still a smallleak but slight improvement someone then told me the head might be warped so i took apart again wet and dried head and put back together now it wont start and i can hear a slight hissing from head can any one help pls

air leak at the head = low compression = no starty.

shouldn't have sanded the head really, try using a slightly thicker gasket and see if that fixes anything, alternatively i would think you need a new head