Alternate part numbers: 52K2171, 53K2171, CLS2511, TE505111 ( Moss Motors UK).

This threaded stud is used to attach the tachometer drive housing to engine block for all MGA (including Twin Cam). The numbers 52K2171 and 53K2171 appear in Service parts lists for 1500 and 1600 engines respectively. Not sure if one of these numbers may be a misprint, or if the part really did have two different but similar part numbers. It was common for the Twin Cam model to have new part numbers assigned to the special new engine pieces. Sometimes they were later reconciled and changed to a standard part number. In this case it didn't happen, so we have all of these part numbers assigned to essentially the same physical part.

PART NO.  SPL.PAGE  DESCRIPTION                                                            
53K2171   sup.7     Stud -housing (2) -(tach drive, engine unit, sup) 
53K2171   16.A.1    Stud -tachometer drive spindle housing (2) -(cyl.block, eng.unit, 1600)
53K2171   16.A.21   Stud -tachometer drive spindle housing (2) -(cyl.block, eng.unit, 1622)
CLS2511   TC.A.1    Stud for tachometer drive housing (2) -(cyl.block assy, engine, Twin
    Cam)
CLS2511   TC.A.9    Stud for distributor drive housing (2) -(case front timing, engine,
    Twin Cam)

The stud is 5/16" diameter x 1-3/16" long with 1/2' thread length each end (and 3/16" plain shank in the middle). Bottom end screwing into the engine block is 5/16-18-UNC thread, while top end accepting washers and hex nut is 5/16-24-UNF thread.

In a pnich, this NAPA Solutions part number: NOE 6051345 should work.