Guys -- my Wildcat has a random assortment of bolts holding the fender wells in - totally from the coffee can on the bench... What is supposed to be holding these things in? (I know there are a few sizes in a few places -- but what is the primary bolt??). I think it's 5/16-18 x 7/8??
If big cars are same as a-bodies, depends on where built. Flint a-bodies uses 5/16X 12, GMAD use the finer thread...
If you have both on same part (fender, inner) then someone's been in there (bodywork, resto). They're not self-tap, so any wrong ones damage threads in welded nut...
There are typically 3 different bolts used on Inner Fenders: I stock all 3 of these if you are looking for nice new replacements. The bolt shown in the middle with the washer is usually the one in the upper center section of the Inner Fender. These are all 5/16" Bolts. Standard thread and a coarse thread (Somewhat self tapping) I also stock the replacement nuts that are used in the Inner Fenders More info on these and 1,000's of other GM / Buick Fasteners on my website. Let me know if I can help at all. 1000's of Buick GS/GSX/Skylark Fasteners now available on my website. These are NEW fasteners, and Made in the USA. Also available . . . . Engine Cradles, Wiring Diagrams, Grade 8 Engine Bolt Kits, Factory AMK Exhaust Bolts, King of Torque – “You’re Gonna Need a Bigger Hemi” Banners, License Plates & Stickers, Buick Console Decals & More! 455 Engine Building Service – Frame Off Restorations – Distributor Rebuilding See our products here > D C Muscle Cars – Done Correct!.