MG MGA Twin Cam Workshop Manual page 21

GENERAL INFORMATION-continued
IDENTIFICATIO~ OF UNIFIED SCREW THREADS
The general standardization of Unified screw threads makes it necessary to identify all nuts, bolts, and set screws
with these threads in order to ensure their being matched with correspondingly threaded components and the fitting
of correct replacements.
Identification has been standardized and is effected in the following manner:
Nuts. By a circular groove turned on the end face of the nut or by connected circles stamped on one flat of the
hexagon.
Bolts and set screws. By a circular depression turned on the head or by connected circles stamped on one flat of the
hexagon.
Wheel stud nuts. By a notch cut in all the corners of the hexagon.
It is of the utmost importance that any nuts, bolts, or set screws marked with the above identifications are used
only in conjunction with associated components having Unified threads and that only replacement parts with Unified
threads are used, as these are not interchangeable with Whitworth, B.S.F., or Metric threads.
The Unified thread is, however, interchangeable with the American National Fine (A.N.F.) thread for all
practical purposes.
NUT
\ t'
·-·--)<:-~-/
UNIFIED
SCREW
ANF
This illustration of the Unified thread and the A.N.F. thread to the same scale indicates their close relationship
Spanners. It is to be noted that all A.N.F.- and Unified-threaded nuts and hexagon-headed bolts are made to the
standard American hexagon sizes and that spanners of the appropriate size must be used when tightening or loosening
them.
KEY TO SPANNER SIZES (Nomina/widths bet!<v'eenjaws)
Diameter of screw thread (inches) it"
i"
*"
1"
9 "
i'' i" ~II
6
TG"
8
For B.S.F. screws and nuts
·448 ·529 ·604
·705
·825 ·925
1·016 1·207 1·309
For A.N.F. screws and nuts
·440 ·504 ·566 ·629 ·755 ·880 ·944 1·132
1·320
For Unified screws ·440 ·504 ·566 ·630 ·755 ·817 ·943 1·132 1·321
For Unified nuts (normal)
·440 ·504 ·566 ·692 ·755 ·880 ·943 1·132
1-321
For Unified nuts (heavy)
-
-
-·
-
-
1·069 1·258 1·446
NOTE.-In the case of some Unified-threaded components the size of the hexagon for the nut is different from that
of the bolt. Where tbis occurs the spanner size is shown in hea''Y type in the above table.
1"
1·489
1·508
1·509
1·509
-
MGA (Twin Cam). Issue 4. 27765
General Information 9