the biggest bolt i can think of are the ones holding the front springs on, some where around 3/4 to 7/8". The rear spring use pins that are bigger yet, but arent bolts. The biggest nut on a bolt would have to the one on the pinion shaft, but the whole thing dont count as a bolt.
The biggest GMC iv'e ever seen was built for the military to haul tanks. I dont remember many of its details, expect that it burned several gallons to the mile. The next biggest thing might be the Future-liners.
There are also several large engines out there,built by GMG or GM, including a line of 12 cylinder boat engines.
