I have not been able to find any resource regarding converting the bolt grades of Japanese cars to modern bolt grade systems. There are many grading systems, ISO, ASTM, SAE, etc. and none of them are consistent with the bolts found on Japanese cars, even through 1993.
I am finding bolts marked with the number 7 and the number 9 on the heads in the suspension of a 1969 Japanese car. I have already read one warning to use strong bolts from someone who came close to a serious problem when his car started steering funny, and upon inspection, found half of the shiny new bolts he had recently installed were broken or missing.
The best explanation I have found so far is from 1984-1989 shop manuals:
4 = 4T, Low Carbon Steel
7 = 7T, High Carbon Steel
9 = 9T, Alloy Steel (The number nine has a straight line for the back/lower part, so that it is not confused with a number six).
This does not give any conversion, rating, or detail at all.
A second explanation from 1990-1993 shop manuals:
4 or No Mark = Class 4.4
8 or Long Slash From Hex Point And Dot = Class 8.8 Refined
8 Dot or Short Slash From Hex Point And Dot = Class 8.8 Non-Refined
9 or Slash From Hex Flat Side And Dot = Class 9.8 (Again, the number nine has the straight back).
I know that this system is not SAE or the American system of bolt classing.
And the lack of a 7 and 9 together in the more recent classing system with translation to what appears to be the modern decimal based "property" classing system (8.8, 10.9, 12.9), worries me.
In my experience, I have not seen many bolts come off a Japanese car with a number 9 printed on the head, and this seems to be rare and indicate a rather extremely high rating. I'd like to get a solid conversion of the rating system, because fasteners get pricey in the modern 10.9 class, and even more expensive as well as unavailable in a lot of fine thread sizes in the modern 12.9 class.
Is there a chart or web page that I have missed in months of research? Or anyone who has investigated this subject more successfully?