When doing the gear oil change for the first time, I noticed that the 8mm fill plug was on very tight and I was never unable to torque it back on properly. Being overtightened and stripped, this caused the plug to weep oil over the course of hundreds of kilometres. My dealer wanted to take the whole transmission apart, which would have been an expensive exercise. As a DIY person I decided to find the simplest and cheapest way to repair the leak. I didn't like the idea of using a matching 8mm helicoil or timesert or tapping a larger 10mm hole as the drilling process of a blind hole would put metal shards all inside the transmission.
Looking into the stripped hole, I noticed that the stripped hole was deeper than the bolt and there was still some good thread remaining. Using this idea, I decided to replace the original bolt with a longer one.
The original bolt was M8 x 1.25mm and 12mm long with a flange. The new bolt to be used is a longer 16mm bolt (with a hex insert dome head because I like the shape). I also decided to use a 8mm Dowty washer, which is a special washer that comes with a bonded rubber seal, like an O-ring. If it's good enough for high pressure oil and hydraulic lines, it should be enough for a transmission.
The new 16mm bolt and Dowty washer was screwed into the stripped hole and torqued to spec (10 Nm) with no issues. I made sure that there was no metal bits or loose threads in the hole and that the tip of the bolt didn't make contact with any of the gears in the transmission. Just to be safe, I drained and replaced the gear oil to flush out any debris in the transmission.
Overall the fix cost less than $15 and all parts came from eBay. After a long ride there were no leaks, so the job is done. This may also work for engine oil drain plugs too.
