Wayne Parham Messages: 18792 Registered: January 2001
Illuminati (33rd Degree)
The thread pattern used on the opposing pair of mounting holes is 1/4" diameter and has 20 threads per inch. The best length I've found is usually 3/4" because a 1/2" long bolt usually only "grabs" with one or two threads, if it reaches at all, and a 1" long bolt usually bottoms out. On some horns or adapters, you might need this long of a bolt, but then again, on many of them (like the H290), you have limited space to insert the bolt, so you must use a short bolt just to get it started.