Many times, fine thread bolts are Grade 8 versus Grade 5, although Grade 5 are also available in fine thread.
Frame bolts would also likely be Grade 8 for strength in critical areas. I would be reluctant to replace a fine thread bolt with a Grade 5 coarse thread. Unless you can be sure, I would treat all coarse thread bolts as Grade 5.
Check to see if there are head markings on the bolts you remove to determine the the grade.
Attachments
SAE Grade Markings.jpg (85.04 KB, 382 downloads)

Last edited by klhansen; 03/29/2020 4:22 AM.
Kevin
1951 Chevy 3100 work truck
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1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.