Using stainless bolts/studs into allow castings is not a good idea. They can be pretty much impossible to extract even when tightened correctly due to galling.

Buy some plated steel bolts/studs.

Manifold looks like cast iron. Exhaust is presumably mild steel. These are of course alloys, but not aluminium alloy, which I guess is what you meant.

But as I said, not set on SS, just had some. If I had some mild steel bolts that fitted I'd use them. Buying replacements that fit is likely to be tricky, but I may attempt it.

Didn't go well today,

One of the original bolts sheared off. Looked like gasket had failed to the bolt hole, torching it and maybe destroying temper. Probably have to drill that out from underneath, a quasi-impossible job, or take the exhaust manifold off, a whole other bucketful.

One of the two installed replacement studs doesn't have much of a grip on whats left of the threads in the manifold, and there isn't clearance to put a nut on it, unless MAYBE I file one side down, in which case I can't use a stainless steel nut.

I suppose Rawlbolts wouldn't take the heat?

Encouraging, but this is Taiwan, so there are various additional language, cultural and ignorance (mine and theirs) obstacles to overcome (Eg, buying RTV silicone recently, with a probably-correct translation written down and a picture, I had two places try and sell me wood glue, insisting it was the same thing.). I don't offhand know where to buy bolts and I'm pretty sure I don't know anyone who does.

Plus I have some technically-screws that I think are the right diameter and they bind, so I'm not sure the universal thread-pitch thing applies in this case.

(Admittedly I think "universal" is often a myth. This bias has been strongly reinforced by the introduction of "Universal" Serial Bus connectors, of which I have collected a big non-interchangable bag )

But mostly its because I think the tapped holes on the manifold flange are eroded so maybe even the correct fastener wont grip them adequately. I thought I could get around this with a nut on the end, but there isn't clearance at at least one of the flange holes.

But needs must. Yesterday morning I took a 1000 NT bet (Off a philosophy lecturer. Lamb-to-the-slaughter I thought) that I'd have it drivable by the end of the month. I'd expected to have it done that afternoon. Not QUITE so confident this morning.

Ed, have you actually had galvanic corrosion from Copperslip on exhaust manifold fasteners?I would have thought these were usually hot enough to prevent any moisture being present to act as an electrolyte?

I cheated. I put a couple of G-clamps (1 and 3 inch) alongside the sheared bolt. Bit of a struggle getting them tight with the limited clearance but hopefully they'll hold for a while.

The bet was that the car be "drivable" by month end. There wasn't anything specified about it not being on fire.

I've used coppaslip on sleeved exhaust joins with great success. I had one car that rotted its back box every couple of years, coppaslip allowed it to be replaced with ease.

Re regular back-box rotting, slightly OT, but I try and limit internal corrosion by shoving some crumpled beer can in the downpipe when the opportunity arises. My theory is this will sort-of flame spray the inside of the pipe with aluminium. I also usually add a teaspoon or so of Portland cement, for anti-acid buffering, but this time didn't have ready access to any.

Of course if the can rolled itself up and caused a blockage I'd look pretty silly, but so far, so-so.

Probably not applicable to a catalyst-equipped system

Since I had the front pipe section off I gave the outside my aluminium-and-sunflower oil treatment and its looking pretty shiny, but I'd guess most of that will burn off.

If there's a next time I might try wrapping it in aluminium.

I've used coppaslip on sleeved exhaust joins with great success. I had one car that rotted its back box every couple of years, coppaslip allowed it to be replaced with ease.

Coppaslip is indeed an antisieze compound but its not intended to lubricate and seal sleeved joints on exhausts. There are proper pastes for that job.