I have a hole which has been tapped to M12 x 1.75. The fitting that I have (vent pipe) is M12 x 1.5. Given that the hole is not really 'load bearing' in any way, can I re-tap the hole, apply threadlock and be confident that the thing isn't going to just fall out?

Material is Cast Ali if that helps.

No. Especially on something like cast ali you'll end up with very little metal left for the threads to bite onto (and even that will be chewed to pieces by the cross-threading of the re-tap). You could use drill it out and use something like a helicoil (if they go that big) or you could fill it with a material like Technoweld and tap that..... Or TIG if you're fancy
You could do re-tap it to M12*1.5 and epoxy it in place but I suspect you'ld actually be gluing it in and the thread would be just acting as a key for the glue to hang onto... Obviously you'll need to use heatproof epoxy...

Cheers (Dammit!)

As it doesn't have to be strong, how about drilling it out, put metal-loaded epoxy in the hole and just push the vent pipe fitting in to get a good seal?

Alternatively can you drill out the centre of the M12 and tap it for use with a smaller fitting?

[Edited on 18-7-07 by RazMan]

If you tap it to the correct thread then assemble with something like "plastic metal" epoxy then I can assure you it ain't going anywhere!

It depends how deep the threaded hole is and what kind of vibrations etc the hose will be exposed to, whether the hose will be supported at a seperate place on the engine. If the hose is attached to the tube but also attached to the engine at some point and the threaded hole is relatively deep you might get away with it... If its shallow or you're hoping to attach a hose which goes from the vibrating engine to a fixed non-vibrating catch tank or something I wouldn't fancy it..... After all, if the epoxy bond fails you may well get bits dropping into the engine....

Helicoil?

My tu'ppence worth:

Don't re-tap the hole. It's the road to eternal damnation!!
Chemical metal, epoxy, glue, body filler, chewing gum and granny's knicker elastic are all crap for this.
I have no idea what the hole is in but I recon the options are (I'm guessing that you already know this):
* Look at modifying the pipe..die nut? can it be shortened and re-threaded? or turned around? or added to by welding a new section on the end (like a drilled out bolt)?
* Re-make the part the hole is in
* Drill a clearance hole and sandwich with half nuts & Dowty washers on the pipe.