Hello Stephen, it should be possible to check the TPI of the drilled out plug pieces, which you may have already done to get the quoted 19 TPI which will be a BSP or BSPT thread It should be possible to measure the tapped hole and work out the thread size and get hold of a plug or something with a BSP thread to try in the hole, assuming that you are using the existing threads.
Brian

Location: Near M1 Motorway Jtn 28

3/8" Bsp = 19Tpi
.656 core dia
Roger

Location: Haverfordwest

For info my block with the squared end core plugs is slightly later having a number somewhere in the region of 140000 stamped into the valve chest (I can't remember the exact number). It dates it to some time in 1930 if I'm not mistaken. Perhaps Austin changed over from slotted to squared end plugs round about then?

Not sure that changing the design actually made them any easier to remove once they'd rusted themselves in. The one on the back of my block looks like a practice piece for apprentice (and failed) chisellers...

Location: Gard, France 30960. Used to be Languedoc-Roussillon but now it's Occitanie

Thank you all for your input.

What you have said about the plug being a drive in one originally is what seems to be shown in the majority of the period photos. This had me scratching my head a bit but now it makes sense.

Since it is unlikely I can find a threaded plug off the shelf here in Calgary, I'll just machine a pair from brass stock.

Stephen

I have had need to remove the two brass core plugs on an early block and they are indeed threaded and then peened over on the outside.
Taking Roger's advice on size I milled the peening off flush and drilled to core size for 3/8"bsp. revealing the thread. I then picked out the remains of the brass and re-tapped 3/8"bsp.
I can understand that people think that they were peened in but this is not the case, they were threaded originally.
It is a good idea to remove them because the gallery was completely choked up with crud.
Hope this is of interest.

Location: Deepest darkest Kent