Bolts – The right strength bolt can be critical to safety and reliability!
Bolt head markings show how strong the bolt is. Original bolts on MGB and earlier MGs carried markings on the bolt head, the most common of which were ‘R’, ‘S’ or ‘T’.
R is a basic grade of bolt
S is high tensile, used for most engine and suspension applications, 120 000 psi.
T is extra high tensile, used in particularly critical areas, 150 000 psi.
There were also numerical markings which indicated the tensile strength in tons per square inch, or higher grades again (U, V) but these were not generally used in MG’s modern equivalents:
S: SAE grade 5 (indicated by three hashes on bolt head) or metric 8.8
T: SAE grade 8 (indicated by 5 hashes on bolt head) or metric 10.9
Higher grades are also available – around 170 000 -190 000 psi, Metric 12.9