if there is anyway to resize the head of the bolt eg file grinder etc so it will fit a smaller socket then heat the sucker up thats what i did it works pretty well if all else fails though you gona have to drill it

I have taken a couple of these apart and the following worked for me.
1. Buy two new bolts, one new stud plus nut and washer. New gasket too.
2. Soak nut in penetrating oil for a couple of days and remove
3. Use centerpunch to mark head of bolt and drill down to remove head of bolts
4. Separate manifolds
5. Use bottle gas torch or oxy torch to heat exhaust manifold in the area of the bolts. Work slowly so as not to concentrate too much heat on the one area too quickly, but make it bloody hot.
6. Unscrew bolts using vyce grips.

I then used the oxy to cut out the flapper valve as it was leaking badly both times, then threaded the holes for plugs. I got the exhaust manifolds sand blasted and painted them with VHT, then re assembled with new gasket, new bolts & high temperature anti-seize. Do not fully tighten untill manifolds are bolted to the head to ensure that both manifolds are properly aligned.

A little bit of work, but good results.

I like your thinking there Dats-hot, if your going to do a job might as well do it properly!

By the way (and I haven't bothered researching here) what does that flapper valve in the exhaust manifold do? All it seems to do is restrict the flow of exhaust gas...

Signed wondering

Hi Mildman,I dit the same as you sed,I took myne out,I think at idel the flap is half open,to keep in hot air and heat up the manifold in cold weather.There is no water going threw the A12 manifold to heat it up like the A14 and A15. In South-Africa the weather is not to bad,even in winter.Im not too sure,but thy cal that part in the manifold the HOTBOX,

Taking the head off the bolt was the only way i could guarantee that the bolt did not snap off inside the threaded hole in the exhaust manifold. Using high temp anti-seize meant that i could remove the bolts in the future if necessary.

The flapper valve is intended to direct some of the exhaust gasses to a hot spot under the carburettor. Not all of the petrol that is drawn from the carb is converted into a mist and the larger droplets fall straight to the floor of the manifold at lower engine speeds when gas velocity is low.
The heated spot below the carb then causes the liquid fuel to evaporate and flow into the cylinders. It's the gas that burns and makes power, not liquid fuel.

At higher engine speeds, the increased exhaust gas flow is enough to push the flapper away, against spring pressure, allowing the exhaust to flow straight down the tailpipe and not under the hot spot. This is ok because at higher speeds the carb does a better job of atomising the fuel and the gas speed is fast enough to draw all the fuel straight into the cylinders.
Removing the flapper valve can mean poor low speed running when cold, especially in winter, but it was the only way i could fix the leak past the worn bushes and shaft without buying a new manifold.