A few notes from my research into the rocker pinch bolts. First of all, the 1964-1966 rocker pinch bolts were M5, and designed to be used with the shorter 18X48 rocker shafts. The later shafts are 18x53, and use a bolt with the following specifications.
Head diameter 9.9mm
Head height 5.9mm
Length overall 53.9mm
Length to underside of head 48mm
Thread length 20mm
Shank diameter 6.9mm
Internal wrenching feature: hex, 5.0mm
Finish: Black Oxide
Property class: 12.9
This last piece of information is interesting. INBUS is a contraction for the name Inbus™, after the company that patented the design in 1936, Bauer & Schaurte Karcher in Neuss (Innensechskantschraube Bauer und Schaurte. ) You may recognize that name, they also sold VERBUS bolts, once owned by Textron, who also owned CAMCAR, but now all those manufacturers have been rolled up by Acument Global Technologies.
Anyway, this answers the question as to why you can't use an off-the shelf 12.9 hex cap screw instead of the factory part. If you use a 45mm, it will be too short and you won't have enough threads engaged in the expander nut. If you use a 50mm, it will be too long, and you won't be able to get the 8mm hexagon bit all the way into the internal wrenching feature of the nut. This would be very, very bad indeed, you would probably strip the nut.
While we are on the subject of rocker shafts here are some dimensions.
Height of expander 8.0mm
Height of nut 8.3mm
Length of rocker shaft with expander and nut: 57.0mm
Length of rocker shaft without expander and nut: 53.0mm
Hope this helps somebody someday!
'66 911 #304065 Irischgruen
Ex-'71 911 PCA C-Stock Club Racer #806 (Sold 5/15/13)
Ex-'88 Carrera (Sold 3/29/02)
Ex-'91 Carrera 2 Cabriolet (Sold 8/20/04)