This is a plug that screws into a inlet that's on the cylinder head just below and back of the thermostat housing, before the #1 cylinder. Anyone know what size it is? I've tried 1/4 and 1/2 pipe size. It's no. 48 on the moss motors picture, 1275 engine external.

thanks

I could not find #48 on the Moss web site for the 1275 engine externals. You aren't talking about the bypass hose nipple are you?

You said that you tried 1/4 and 1/2 NPT taps. If it was between those sizes, the hole is likely threaded 5/8-18. BMC used that on places like the banjo bolt hole on the block and the temperature sending unit on the head.

It is 5/8”-18 (or 5/8's NF). In the head, the adapter has male 5/8” NF threads (uses a copper crush ring to seal) that threads into the head and 5/8” NF female for the capillary tube and nut.
Bruce, are you missing yours?
"Dug"

I am missing it. I'll take this info and try to pick one up at the hardware store.

Thanks for the info. Great source of info. on this forum as always.