In changing the gearbox oil, i removed (for the first time in my ownership) the gearbox filler plug. The plug had clearly been tightened by a gorilla who had no access to a torque wrench. On removing the plug I ran my finger inside the port, and initial 2-3 turns of thread came away as a metal spiral. I have tried replacing the plug. It is firm - but I cannot risk torquing to spec. I have taken the car on the road, and there is a small leak from the plug. So something needs to be done to repair, or offset this problem. Can anyone provide advice??? It seems that the plug is not as deep as the filler port; ie, it should be possible to obtain (or make) a new filer plug that is a little longer, and so can bite on the remaining thread. Is that possible, or does the port thread taper with depth?