Finally got round to removing the exhaust manifold on my SD1 for the purpose of fitting a Lambda sensor for the Megasquirt conversion. Only to find it's not quite as simple as I thought.
There are two visible bosses - undrilled - for sensors as fitted on (I guess) federal spec cars with cats. One presumably for the oxygen sensor and one for an air pump? But that's a guess as I can't find any details - my workshop manual doesn't cover US spec.
There is one boss close to No1 cylinder and one at No7. But neither is designed if drilled to poke into the actual downpipe inside the manifold. Instead they'd go through the casting to the back of the manifold where there appears to be what could be a plenum chamber - a sort of wall with bosses for fixing screws for a cover. That's where a sensor would end up if drilled square from the bosses. And I suppose holes drilled into that plenum from each downpipe? I did wonder about fitting it after the manifold - but the individual pipes from that (one per two cylinders) are rather small and was worried the sensor would restrict them. And where they come together beside the gearbox there simply isn't room. It could easily go in the Y piece that joins the downpipes from either side together - but that's at the back of the gearbox so a long way away?
I have a later manifold from a 3.9 Range Rover (totally different and couldn't be used) where the sensor boss shows it would just go straight into one of the pipes within the manifold. And assumed mine was similar - since this is the first time it's been removed.
If anyone's interested and confused by my description (very likely) I could post a link to some pics.