I use mostly grade 8 fastner in my engines, even where they don't really need to be, mostly because grade 8 stuff comes in cad or yellow zinc, and grade 2 either unplated or white zinc, and grade 5 is white zinc. I just like the way the yellow zinc look on the motors. 90% of my stock of fastner is grade 8. I used yellow zince grade 8 on the race car forever, they seem to hold up well cosmeticly. Somethimes you have to re-use fastners because the are unique to a chassis or engine, on those I bead blast them, mask the threads off and use cast aluminum paint or stain black on the non thread portions.