A little while back I wrote about "One of Those Weeks", where seemingly everything goes wrong. I thought I had escaped from under that black cloud, but apparently not.

The replacement inverted oil system tank arrived on Monday. Raven Aircraft makes two different size tanks - I bought the smaller, 4" diameter one. The Raven Aircraft tank is the smaller one, on the right.


Monday evening I started to fit the new tank. I figured out where I was going to mount it, then I figured out what angle the bottom fitting needed to be at. I put some Teflon paste on the threads, being careful to not get any on the last thread, as you don't want any of this stuff getting into the oil system, where it may block an oil passage. I was so careful to not get any on the last thread, that the last two threads ended up without any Teflon on them.

I screwed the fitting into the mounting boss, and it started to come up tight just after I passed the required angle. You can't back off these fittings, as they use a tapered thread. Back them off and you risk a leak. So, I kept tightening it, needing to get it almost another full turn. But, the fitting got really, really tight before I made it to the required angle. OK, now I needed to remove it, put some more Teflon paste on it, and try again. But, it was really, really hard to unscrew, and it doesn't get much easier as I unscrew it, which is very abnormal. After getting it out, I can see that there had been galling between the aluminum fitting, and the aluminum boss. A chunk of the threads on the fitting had welded it self to the threads in the boss. I had just screwed up my expensive new oil separator tank. Down tools. Go into the house and have a beer.

I did some research, and learned that the AN823-10D fitting I was using has a 1/2" pipe thread. I called my local hardware store, and found that they had a 1/2" pipe thread tap in stock. It wasn't cheap, but it was a lot less money than a new tank.

Yesterday I was able to carefully thread the tap into the undamaged portion of the threads, and managed to re-cut the damage portions. The fact that the threaded hole is tapered was a big advantage, as I was able to run the tap in a bit deeper than the hole was originally tapped, and this put a new surface on the all the threads. I was careful to hold the tank so the boss was facing down, and the tap was coming up from the bottom, to ensure than none of the swarf got into the tank.

I pulled a new fitting from the stock, and put it into place, being extra careful. It was just starting to come up tight as I approached the required angle, and I didn't tempt fate by trying to get another turn out of it. This tank runs at low pressure, so there is little risk of leakage once the Teflon paste hardens up.