I managed to cross thread my sump plug. It is odd that it happened. The sump plug was turned to its normal level of tightness, and did not give any indication it was cross threading at all. The next time I went to undo it for a service, the plug was spinning in place. I couldn't pull it out at all.
I ordered a thread replacement kit from online, and had to wait for it to turn up over the christmas mail rush period, so I left the plug in place until I could finish the job. I was a bit peeved, and ordered in a rush so didn't consider the options of a threaded bush. I used the same type of kit the last time this happened (in 1993!) and I had good results then.
The sump plug as I said was firmly stuck in place. I tried to slide a paint scraper and a thin chisel between the bolt and the case to start it on its way with a little pressure as it turned, but it would not budge. As soon as the tool was removed after turning it out a little, it would go back to flat against the case.
I had a think about it and decided I needed a puller, so I carefully drilled the sump plug to a depth of about 3/4" and threaded it with a 1/4UNF tap, following up with a finishing tap as well. I used a bolt and a nut with a large washer to screw into the plug through the socket, and tightened it up quite tight. I turned the socket to take the bolt out using a pair of multi gripswatching the tension on the bolt, and re-tightened a few times before finally succeeding in pulling it out. I was quite chuffed my devilish scheme worked..
The insertion of the thread repair was the next challenge. The biggest problem as others who have performed the job, is ensuring the tap is square to the face of the sump. If the plug pulls up off-square, then it will be difficult to keep a seal and oil will leak.
I did this by using a 14mm socket to engage the end of the tap, and adding a long socket extension bar. This extends the length of the tap, and makes it very easy to see the tap alignment. The socket is a solid engagement on a very large tap of this size, so it worked quite well. I used a 1/2" ratchet to turn the tap carefully through the length of the bar watching the alignment remained square. . This solved a couple of problems. Holding the tap in alignment, and supplying some torque the big tap needs in a very tight space. The sump plug on the car peeks out only just below the subframe from the side aspect, so it is very tight.
I didn't think to get a photo of the setup as I cut the thread, I guess I was focusing too much on the job to think about the camera! The end result was a clean and neat thread. It was easier than I though once the tap bit into the case. The alloy cuts like cheese. I pulled the tap out every two turns to remove as much swarf as I could, and after that I spent a long time with a kids craft pipe cleaner (they bend into hooks and suck as you require) to fish out any remaining swarf from around the hole. The tap is slow, so it doesn't really go far from where it is cut, and I feel confident I managed to fish it all out as I spent a long time after the last chip still looking. I then flushed a few litres of cheap oil through for stamps to see if there was any more. (and there wasn't).
The tap kit is quite complete, a drill is not really important here as the tap uses the old thread as a guide, and the outer pitch of the original thread is about the perfect size. The thread if you are wondering is 5/8" UNC.
You do need to trim the supplied thread inserts up to suit the hole you are using. Five turns is enough, and it is best to cut them down with a grinder to get a nice fine finish. If you use a pair of side cutters you will bend the insert from normal, which can stop it seating in the thread nicely once inserted. You also need to make sure the cut is neat as the insert will stand up in the thread if there is a little flash left and will cause the bolt to bind in the thread as it is screwed in.
Making this work is all about ensuring the tap is square as you run it through the case. Do that and the resulting thread is stronger than the original, and seals as you need it to. The next job is screwing the thread insert in using the supplied blue tool. Turn it through until the last turn of thread is below the face of the sump. The tang that the tool is secured to then needs to be removed. I use a pair of long nose pliers to pull back toward myself. You wouldn't want to leave this piece of stainless steel in the sump!
Job's done. The sump plug screw in with ones fingers. I used a new copper washer and seem to have a normal oil seal after a long drive on a hot day. No leaks, at least from the sump plug anyway
