Showing a rod inserted in the drain hole in relation to the grill apertures - 1973 model.

Showing the rod going through the drain hole at the bottom of the heater box (false colour as a result of the flash being very close to the painted body and chrome grill).

Earlier cars had a sort of 'leaf-trap' as seen here on the BMIHT Gaydon MGB cut-away. This restricts access to the drain hole somewhat ...

... but it's still just possible.

Showing the original 'flat nut' (arrowed) pushed onto the peg of the grille to retain it. These are virtually impossible to remove except by levering the grille up, which makes them ping into the bottom of the box. These have been replaced with plastic sockets which push into the holes from above, then the grille pegs push down into them. Similar sockets are used for badges on the boot lid. Much easier to deal with, but unfortunately it does need a bigger hole as it has to accomodate the socket plus the pin. A bit of a beggar on a freshly repainted body if you didn't open them out before painting.

'Tom's knob' (arrowed) as seen from below (image 'borrowed' from www.vord.net).

The BMIHT Gaydon MGB cut-away showing the metal spigot for the drain tube, the double-skin over the transmission tunnel, and the hole in the lower skin the drain tube exits through. If the metal spigot rots away water will tend to lie in the double-skin and eventually rot through to the cabin. If that happens probably the easiest solution is to drill a drain hole at the lowest point, although that will require engine and gearbox removal. It may be possible to repair the bottom of the air-box and the drain tube by removing the demister tube and trim panels behind the console.

Showing the plastic blanking plug that gives access to the clip securing the drain hose to the bottom of the air-box.