Fairey overdrive drain plug

I don't have a pillar drill. I think I could create a guide with a piece of steel tubing and a bit of plate. Then I could clamp the plate to the O/D casing, centre the tubing over the hole.. and that could keep the drill bit square on. That's my best idea so far.

Well it's not the back of a fag packet but then I don't smoke..

The original plug has a flat face and a seal that sits on gear case boss if you go bigger what type of plug are you going to fit.
What size is the helicoil drill?
Nicks landrover overdrive rebuild is a good guide think I have attached it.
Had one to bits and reassembled myself still working as far as I know.

I would take it out to the next sensible BSP size, that way you can use an off the shelf drain bolt and a bonded rubber Dowty seal.

If you try and do this by hand I don't fancy your chances of you keeping the drill and tap normal to the sealing face.

A seal kit is only around £15, I would strip it down and get it on a milling machine so the sealing face can be machined at the same time as the thread is cut. Their not difficult to strip down and you don't need any special tools.

If it were mine, I'd do as post #24.

Also I agree with Steve2268w - re-coiling the thread is not the ideal solution in this application where you'll be wanting to drain the oil on occasion IMHO - they are generally bl**dy brilliant - but Citizen Kane's idea is the way forward - BSP plugs are cheap as chips, and available in brass, stainless, steel, and even soft iron & plastic !

I'm guessing, from your username your somewhere on the Fens - If you were closer I know a bloke who could do this for you

The 5/8 UNC is standard mini sump plug
Your jig will should work ok , dowty washers good and cheap from a hydraulics shop they might even have a plug correct size cheaper than this but this has a magnet on too for collecting the bits of ally !

I like magnetic plugs, my gear box drain plug has a large magnet glued into it. I really like the ones with two magnets with connections to each, you just put a multi-meter on and if its a low resistance you know some metal chips are bridgeing the gap and its some to pull it before it get more damage.

At last. I've been busy but this was a joke, it took me weeks to get around to this. All done now and no leaks at rest so far.. Cheers all for your help and advice.

Well done

it looks like you went for the helicoil option, what size did you use and drill size
liking the new bonnet too

I used a 9/16" x 12 UNC helicoil so I could use the "right" size drain plug, but I would have gone bigger if things hadn't worked out. When I offered up the 12mm tapping drill bit it didn't look like it would take much out, I think what I was left with was not the original stripped thread but the remains of an attempted fix possibly already drilled out a bit so I tapped it straight away with plenty of oil and gave it all a good rinse through for the swarf after. I used a bit of loctite on the outside of the insert when I wound but I don't think that was strictly necessary really now I have a feel for how that insert grips the casing I know it's not going to move. Glad to have the job out the way. My brother kept telling me get on with it it's already buggered you can't make it worse. And no leaks touch wood I'm looking forward to putting it back on and getting on with things again.