Brass plugs for engines usually have NPT threads. These
American tapered threads are used on pipe fittings where the
inch sizes refer to the diameter of pipe the fittings were designed
for. When used to hold anodes they are bored with UNC/WHIT
threads instead.
We keep one metric plug as used on heat exchangers.
CME-MBP 18 x 1.5 metric fine . anode 10mm . Price £6.53

. . Zincsmart home
(An approximate outside diameter is shown, measured over the threads)
CME-00BP 1/8" x 27 NPT. . . . .anode 1/4" UNC/WHIT
Wt 10 g . . . . .(dia 10mm). . . . .. Price £3.75
CME-0BP. 1/4" x 18 NPT, . . . . . . . 5/16" UNC/WHIT
Wt 15g . . . . .(dia 13mm) . . . . . . Price £4.90

CME-0/BSP . 1/4" x 19 BSP tapered . 5/16" UNC/WHIT . . Price £5.04
(This BSP plug (9/16" AF head)and CME-0/20 anode fit BETA)
(Alternative with 11/16" head CME-0/BSP-H . . Price £5.29)

CME-1BP . 3/8" x 18 NPT, . . . . . . 3/8" UNC/WHIT
Wt 25 g . . . . . (dia 16mm) . . . . . . . . . . £5.51
CME-2BP . 1/2" x 14 NPT, . . . . . . 7/16" UNC/WHIT
Wt 55g . . . . . . (dia 20.5mm) . . . . . . . . . £8.77

CME-3BP . . 3/4" x 14 NPT, . . . . . .1/2." UNC
Wt. 100g . . . . . (dia 26mm) . . . . . . . . . . £14.08
This plug is also used with CME-4 and CME-5 zincs. Larger plugs
are available. CME-7 takes a 1" NPT plug and CME-6 needs
1 1/4" NPT
(British Standard Pipe is not always the same pitch as NPT)