mnmrmstd

I'm installing a set Ford shorty headers (M-9430-P50) on GT-40 heads (1989 GT). I've seen the header bolt size for this application listed as both 3/8-16 " x 3/4" and 3/8-16 " x 1". The key dimension here is the underhead length: 3/4" or 1" ? Thanks for the assistance.

PRO50SC

Just look at the mounting flanges of both headers. If they are the same the stock length bolts will be fine. If the new header flanges are thicker, use the longer bolt.