the underlying problem I think is my engine is too far back - the bottom chassis rails do not allow gearbox to drop so front of engine is higher than the rear.

this means the highest point in the cooling system will be the thermostat housing. I can raise the radiator so the filler cap will be about 1/2 way up the diameter of the thermostat outlet..... but is that enough to prevent air lock and cooling problems??

The bonnet limits how far I can raise the radiator.

An option would be to solder up the filler cap, solder in a small pipe and fit a header tank I guess.

tonight I am gonna see if my carbs fit under the bonnet .... if not I am seriously thinking of putting back the 1300.
Rescued attachment therm.JPG

do you mean there is a chassis member going accross the bottom of the tunnel at the back of the engine bay?

most chassis's don't have this, you could always cut out the member stopping the gearbox from dropping?

[Edited on 16/6/09 by omega0684]

it is these 2 rails .... that are very structural.
Rescued attachment rail.JPG

what about cutting the one on the right side and moving it over a bit so the gearbox will clear? Looks like you will clear the one on the left.

Incoperate a header tank with the pressure cap in at the highest point of your cooling system, use a non pressure cap on the radiator, and block off the expansion outlet, Then vent the thermostat housing to the top of the header tank.

I have a similar layout the cap on the thermo housing is just a flat non pressure job.

[Edited on 16/6/09 by r1_pete]
Rescued attachment Bay After.jpg

Could you perhaps raise the gearbox mounting a bit to lower the front of the engine?

Sent you a u2u.If you put a header tank in with your existing rad,make sure your rad cap is rated higher on pressure than the header tank one.

steve