PART NO.  SPL.PAGE  DESCRIPTION                                                           
52K531    15.A.5    Stud for water outlet elbow (3) -(cyl.head, engine unit 1500)
53K531    sup.5     Stud -water outlet elbow (3) -(engine unit, sup)
53K531    16.A.5    Stud -water outlet elbow (3) -(cyl.head, engine unit 1600)
53K531    16.A.26    Stud -water outlet elbow (3) -(cyl.head, engine unit 1622)

52K531 - Stud -Stud for water outlet elbow (3) -(cyl.head1500, 1600, 1622, 1800)
Length: 2"
Top end thread: 5/16-24-UNF x 3/4"
Bottom end thread: 5/16-16-BSW x 15/32"

High Strength Stud

Alternate part numbers:
CHS2515 - BMC standardized stud pattern
328-605 - Moss Motors, USA
C3 - Scarborough Faire

This one is a little strange, as it has UNF thread on top end and BSW thread on bottom end.
You cannot screw a 5/16-18-UNC nut onto the bottom end of an original factory stud (maybe).

Notice: Studs procured from Moss Motors in summer of 2022 were soft steel (not properly hardened), and stripped the thread when tightening with normal torque.

Addendum Feb 7, 2026:
Now I have some conflicting information. I have modern replacement studs procured from Scarborough Faire in Rhode Island, USA, generally considered to be a vendor of good quality MGA parts. These are correct 2" long, but the top end thread is 5/16-24-UNF x 1" long, and the bottom end thread is 5/16-18-UNC x 5/8" long. Notice how short the center shank is. You can screw a 5/16-18-UNC nut onto the bottom end of this stud. For additional check, I found a decent looking cylinder head and removed one of the three thermostat cover studs. This new stud screws right into the tapped hole in the head with minimal interference. This implies that my prior information above is wrong, and the original studs did not have BSW thread. Otherwise if the original hole did have BSW thread, then I don't know why the UNC threaded stud would fit without interference.

The other problem is extra length of the threads on the new studs. Longer top end thread should not be a problem, as it just means more threads hidden down the hole in the thermostat cover. But longer thread on the bottom end is a problem, because two of the three tapped holes in the MGA head go through into the water jacket. Screwing the new stud in until it runs out of threads makes it go farther down into the head, leaving male threads protruding into the water jacket (maybe not in issue of itself), and the top end of the stud landing lower, which may not leave enough threads to install the washer and fine threaded nut on top. The solution for that is to back the stud out of the head a little to bring the top end high enough to install the fasteners above the thermostat cover. As always, do apply thread sealant to the bottom end threads, so water (coolant) will not weep up through the threads to leak out the top around the washer and nut, and cause rusting of the stud inside the hole in the cover.

I did run a sharp corner file over the center shank on this part. It removed the black oxide coating see picture), but did not remove any metal, so this checks to be properly hardened high strength stud.