Hello all, stumbled across this site just now by searching for 172 gearbox problems. I have a mk2 172 and have just been told I need a new gearbox at 36,570 miles.
The problem was a nasty crunching when going into 5th between 6 and 7,000 rpm. Ive had the car for 14 months and have had the problem from day 1, I kept being fobbed off with Yeah they do that mate, until I took it to a different dealership!
Anyway, I was told that there is a fault with the gearboxes and it is well known. So, if yours is still under warranty like mine is then make the bstads replace it.