Stainless manifold crack

[Mr Spigot]

I have a stainless 3-into1 exhaust manifold with a metro alloy inlet manifold on a 1275 midget engine. Going up to the national rally last week, after cruising around 65-70 up the A34 and pulling off near Oxford, the engine suddenly started making a loud 'blatting' noise, best described as that sound a clothespeg used to make on the spokes of your bike as a kid. It rose and fell with engine speed. The sound was coming from around the exhaust manifold area, but no obvious signs. The head gasket looked fine and no signs of the head gasket blowing. Having tightened up the manifold to head nuts as best I could, the car ran to the rally and back, a round trip of around 500 miles, running fairly well, if rather noisy. For a while on the way home, after tightening the manifold nuts when cooled, it seemed a little quieter, but the next day was noisy as before. Having loosened off both manifolds and checked the gasket, they seemed fine with no obvious signs of leakage. However when replaced again, the noise was still there. On further examination, I found that the central exhaust pipe support strap had broken, causing the exhaust pipe and silencers to hang down and I wondered if this had accelerated the problem. The noise seems to be coming around the central exhaust pipe flange where it is welded to the pipe. There is a small amount of soot on the engine facing area of the inlet manifold as shown in the photo.
It therefore seems that there may be a crack or hole in this area which is the cause of the problem, possibly caused by the stress of the weight of the exhaust system after the strap broke. I have also now noticed a small crack at the front pipe as well.

My question would be whether it is worth getting the flanges repaired with some weld, or if I would be better off getting a new manifold. This one is about 30 years old so it does not owe me anything. Am I better off with an ordinary steel pipe or keeping to stainless?

[King Kenny]

I was a welder for thirty years and it looks like the stainless pipe is welded to a steel flange. If specialist welding filler wire such as AWS309 was used it should be OK. However if an ordinary welding wire was used the joint will be prone to cracking as the stainless part age hardens. Stainless expands more than steel and will eventually stress fracture. It look like this is what has happened here. There would be no problem ig getting it rewelded if that is the cheaper option to you. It is what I would do.

[Mr Spigot]

Many thanks for that informative answer. I guessed something like that might be the case. While welding might be a slightly cheaper option, a new manifold might be a better long term option, certainly to give me peace of mind and it might take me some time to find someone who could do a decent welding job on it.