What are you measuring to get 7.6mm? The screw head?
Don't you have a local store that specializes in threaded fasteners?
Take the screw in and let them you their thread gauge.
Even I have a thread gauge. I can't believe your LBS doesn't.
You can just use a high strength bolt & nut. Best solution if the hole isn't threaded or threads are damaged.
You'll want the theaded end to point out, so it doesn't interfere with any rotating/moving parts.
Probably the best, considering your skill level.
Personally, I would drill it out to the next larger size and thread it with the appropriate tap.
Some of your terminology is confusing.
"Back wheel holder"?
Pictures could be very useful!
Last edited by Bill Kapaun; 05-06-16 at 10:29 AM.