new head core plug popping out

[Stuart Giles]

If the core plug keeps popping out it's because the hole is out of shape. The hole where the core plug seats was undercut when originally machined. So the hole is slightly larger at the base than at the top; The taper is only a few degrees, but it stops the core plug just popping out without deforming. If you were to go up to the next size, the hole would need to be re-machined, I've done this with badly rusted core plug seatings using an adjustable boring head in my milling machine. But drilling and tapping the hole for a BSP plug would be a good permanent "on the car" repair. Screwfix sell Brass parallel threaded BSP flanged plugs in 1/4, 3/8 & 1/2" BSP at a reasonable price. Using these would be a lot kinder to a relatively thin area of the casting than a tapered BSP bung.