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RWS Squawk: 13B Tension Bolts

Report by Laura Crook who bailed me out when I broke down 900 miles from home)

Dateline Friday, May 31, 2002 Waco, TX: Tracy decided to attend the Waco, TX RV Fly In this year. I decided not to go as I had to be in Atlanta Monday morning (plus I had a new kitten to care for). It was all for the best, as the phone rang about 5:30 pm Friday evening and it was Tracy letting me know that he arrived safely but.. (I HATE that word), when he took off the cowl to display the engine he noticed the signs of a coolant leak.

One of the advantages of the plate and spacer engine adapter is that you can see the end of the engine, the flywheel and damper in order to inspect all the bolts and rubber dampers without removing the redrive. In this case, it made it easy to see the broken tension bolt sticking out of the engine where the coolant was leaking from (See photo below).

So here it is - 5 pm on Friday night. We live 45 minutes from the nearest UPS or Fed Ex depot. Tracy instructed me to go down in the hangar and look through one of the boxes of engine debris (I didn't call it debris! T.) on the bottom shelf until you find an engine bolt. For those of you that have visited here, you know that Tracy's shelves are my idea of Dante's Inferno (hell on earth). I had less then 15 minutes to find the parts and a 12 inch drill bit located "somewhere on my workbenches", pack them and drive to the Gainesville Fed Ex depot before 7 pm. Somehow I preformed this miracle and Tracy received the parts on Saturday afternoon. He was able to replace the one broken bolt and fly home safely.

This is the third verifiable example of this failure mode in an aircraft. This is a known problem when racing the Mazda in cars at 8000+ rpm, but it was not previously believed to be a problem at the RPMs used in aircraft applications. Now, however, we are considering it a known failure mode and all rotary engines used in aircraft should assemble the engine with a damping material surrounding each engine bolt. The problem is not that the bolts are faulty or inadequate. These bolts are almost 12 inches long and since they are only supported at each end and are under a lot of tension, they act like piano strings and will vibrate when excited at their resonant frequency. This vibration eventually fatigues the bolt at its most susceptible point which is at the threaded end where it enters the threaded iron front housing. The fix is to hold the bolt in the middle so that the vibration is damped out in the same way that a guitar string will stop vibrating when you touch it in the middle.

Mazda came out with a new design bolt with a series of rolled ribs in the center which was supposed to cure this problem. Tracy was using these bolts but this alone is obviously not sufficient in aircraft applications.

Bruce Turrentine recommends the race car solution, which is to run a bead of silicone around the bolt in a spiral pattern prior to inserting the bolt.

Tracy, the intrepid experimenter, thought this was too "hit or miss" and decided to develop something more predictable. He has replaced all his bolts and put heat shrink tubing around the bolt instead of silicone. We'll keep you posted on how well this works.

Above are the 13B tension bolts, before and after the application of the heat shrink tubing. Two layers resulted in a nice hand press fit where the bolt passes through the center housing. I also used a small amount of RTV on this area to lubricate the tubing as it is pressed in and to insure that it is in intimate contact with the housing. Be SURE and put the rubberized sealing washers on the bolts before putting the heat shrink tubing on since they will not fit over the tubing.

Note from Tracy

The reason I asked Laura to send me the long (12") #30 drill bit was that the bolt was broken off at the far end of the deep hole it goes in. This normally requires that the engine be disassembled in order to get to the broken piece, not a pleasant prospect when you are 900 miles from your workshop.

The plan was to use the long drill bit to drill a small hole in the center of the broken piece, then weld an Easy-Out to the end of a rod and use this to unscrew and remove the broken end.

As I contemplated this tricky procedure I was looking at the broken end of the bolt and noticed that it was an irregular break with a small spur on it. With nothing to loose, I removed the sealing washer from the bolt so it would go a little deeper in the hole and pressed it against the broken piece at the bottom. Crossing my fingers, I then turned the bolt head counterclockwise and was delighted to see the head of the bolt slowly move away from the engine. I was able to unscrew the broken end this way. The next trick was to use one of those magnets on the end of a telescoping stick and pull the broken piece out of the hole.

I should also thank Phil Grishem of Crawford TX for his help, hospitality and tools which made this field repair possible.