from memory a 1/8npt thread will require a 8.5mm hole for the thread to be taped

It's a size R which is 8.6mm. The hex on the sump plug has heaps of meat and depth for it..

....we make bolts and adapters for a living I think he'll be right....

You should always trust a bloke that says "trust me"

Firstly, a big thanks to Dags, a job well done!

Who said it shouldn't, couldn't be done.. This is probably the best and most accurate reading you are likely to get, as the sender is immersed with oil, rather than getting a reading from a drip of pressure fed oil off the sandwich plate.

As you can see from the pics, there is a sh*t load of meat left on the thread and hex.