I've just purchased a new headertank for my build......said tank has a large diameter outlet on the bottom, a midium sized one at the top with a small diameter inlet next to it...question is...do I need the small inlet, or can I just block it off?. The cap is a pressure release type, and I'm not running a heating system.

The bottom join goes to the low point on the hose that goes to the rad, the top join goes into the head.........

Small ones usually for the radiator bleed, if you haven't got one just block it off

Cheers...blocked off it is then..

You need to have a bleed into the top of the tank from either the radiator or suitable point on the engine, preferably both. Without one trapped air cannot escape and will cause localised boiling, hot spots and eventually damage. That is why production cars have pipes to the top of the expansion bottles.

Without knowing your exact set-up and what tank you have bought it is impossible to say. It could be a pipe to vent water that bypasses the cap or a bleed pipe. Some pictures might help

Here is a pic...see blog.


http://paulsj15.blogspot.co.uk/

I you are running a thermostat you are better with a small diametrehose acting as a bleed from the rad top or radiator side of the thermostat or top hose.

The medium size inlet can act as a by-pass hose in place of the heater connection.