Article by Jim Neubert
I recently had the threads strip in one of the 2 holes on the cylinder where the exhaust manifold attaches. I believe the engine vibration had caused the attaching nut on one side to loosen and this caused the threads on that side to eventually fail. A better preflight check would probably have discovered the lose nut before it became the bigger problem it eventually did. But that is hindsight.
I now had to deal with the issue at hand. After researching online and talking to my flying buddies, it seemed I had a few options with varying degrees of difficulty and cost. One option was to purchase a brand new cylinder. This would involve, at the least, getting a set of piston rings and doing another break-in period. It also seemed to me to be the most expensive and time consuming option. At the other end of the cost spectrum was using a tap to tap out the threads to the next bigger size and use a bigger bolt in that hole. I didn't like the idea though, of using 2 different sizes of bolts. Another option mentioned to me was to use a helicoil to fix the threads. This seemed like a reasonable choice and I was pretty much set on going this route until I heard about Time-serts. Time-serts are steel inserts that you install much like a heli coil but with several advantages. To quote their web site; “TIME-SERT is a thin wall, solid bushing, self locking insert with positive placement.
I found a new Time-sert installation kit on E-bay for the size I needed for around $60 with shipping. I ordered the kit and it arrived in 3 days and it came complete with a drill bit to drill out the hole, a counterbore drilling tool, a special tap, an insert installation tool, 5 inserts and clear, concise instructions. The web site (http://www.timesert.com
) also has several installation videos. The entire procedure took less than a half hour and would be even less if I had to do it again. I truly believe I could make this repair in less than 15 minutes. In the first photo you see the kit.
In the next photo you see my cylinder resting on my trailer fender with one stud in and the other is the stripped out hole. I could probably have completed this repair even faster if I had left the cylinder on but I wanted to make sure I got the new threads in square with the head.
I took the cylinder to my work shed and put it under my drill press. I put the drill bit that came in the kit in the drill press, added a little oil to the bit and proceeded to carefully and gradually drill out the hole.
Then, I put the counterbore tool in the drill press so I could make the nice inset you see in the photo. This leaves an area to accept the flange that is on the insert. You could also use a regular tap handle for this process. A drill press is not needed to make this repair. But it did make it a little easier and perhaps faster.
After the counter bore, I added some oil to the special tap that comes with the kit and using a tap handle, I tapped the new threads.
Then I added a drop or so of oil to the insert installation tool, spun on the insert and screwed it in. The insert starts easily but then after a few turns, it starts to be harder to turn. This is the bottom threads, which are a little different from the rest, locking down. Here is a quote, again from the web site; “On installation the bottom few internal threads of the insert are cold rolled to expand into the mating external threads of the base material locking the insert in place.” I continued turning the insert tool, per the instructions, until it once again became easy to turn and the insert was installed.
In the final picture you see the cylinder head with the insert installed, ready to be installed back on my engine. I reinstalled everything and went for a nice flight. If you run into this type of repair situation I highly recommend taking a look at Time-serts.