i need to drill and tap a hole in my exhaust manifold to install a thermocouple.

i expect the best way to do it would be to haul the manifold off and do it on the bench, but i expect removal would be a world of pain and seized and broken bolts so my preference would be to do it in situ.

it being a turbo diesel, the manifold is unfortunately upwind of the turbo so whatever falls into the manifold when drilling gets to take a journey through the turbo.

do you think it is worth risking drilling and tapping in situ using a slow speed and lots of cutting paste to minimise swarf and make it stick to the drill bit? or is this definately a manifold off job so that no swarf can get into the turbo.

IMO if you take care how you drill and tap either using grease, cutting compound or shop vacuum the amount of swarf produced will be minimal.
Cast iron does not tend to produce large swarf particles so I see no reason why the turbo will not cope with it. It will all be gone within the first few seconds of the engine starting.

Steve

Take it off and do it properly
otherwise you'll risk the Turbo and a larger world of misery paying for a newone

Just as Tony says, It is not worth the risk and (IMHO)precision is far harder to achieve when leaning over and at an awkward angle.

better safe than sorry ?
(not my area of expertise)

I just drilled and tapped mine. Used a bit of grease, though. That was a few years and a long distance ago. Our local so-called diesel tuners, SAC, used to drill and tap all manifolds to fit a thermocouple before rolling road testing. They would fit a brass blank afterwards. I can't see them taking the manifolds off.

When we see the brass blank we know that the chances are the engine has been tweaked and overheated. Just my personal opinion, you know.

We normally use a strong magnet to collect the swarf if doing any work like this

When machining cast iron it should be done dry.

Callum,
Just done mine after reading this thread...

I decided not to remove the manifold because it looks like it has been there for a while....

I drilled the hole dry with a magnet right beside the drill. Most of the swarf got stuck to it then once the hole was tapped, I stuck the magnet to a smaller drill (to magnetise it) and fished around inside the manifold to pick up odd stuff that had fallen in.

Seemed to work a treat. 11/32" hole is reasonable size so I could see in with a torch and it all looked quite clean.