There was a post awhile back about putting the xr400 head or base gasket on the 400ex to gain some compression, but i can't remember which one it was. Can anyone remember if it was the base gasket or the head gasket?

 

Head.

According to Honda, the XR400 has 9.3:1 compression from the factory while the 400EX has 9.0:1

The Base gasket is commonly tinkered with on 2-strokes to alter port timing.

 

The head gasket on the xr is thinner than the ex
giving you an extra half a point of compression (approx)
oz_400ex

 

It is the gasket that is between the head (where the carb goes in and the exhaust pipes go out) and the cylinder (where the piston is) which is below the head. There is a third area (the top) where the cam is and the rocker valves are. Hope it helps. I just changed mine and I think it gave me more power.