By Ned McIntosh

Several months ago I recall reading an article written by a microlight pilot extolling the virtues of replacing the hang bolt every 50 hours. Airborne recommend replacement or overhaul at this time interval, so these guys are doing the right thing.

However, 50 hours only for a hang bolt seemed rather a short life to me. Could I possibly find a hang bolt that had done a lot more hours, preferably several hundred, and never been replaced? How badly worn would it be and would the wear be measurable? Eventually I found just such a bolt.
"How many hours?" I asked.
The owner thumbed through his logbook.
"About 975 as far as I can see," he replied.
"I'll take it," I said, handing over a brand new bolt by way of exchange, then hurrying back to the concealed, secret metallurgy lab attached to the privately-funded R&D section of the microlight hangar.

We sat the bolt on a table under a naked 100-watt bulb and examined it closely. It was just a darker colour and had lost its coat of cadmium plating. It also looked "polished"... and was that a trace of grease I could see?

The secret to long life for metal-to-metal working parts is lubrication. On my own hang bolts I use Remington Hinge And High-Pressure Grease. A smear on a fingertip, applied along the length of the bolt is all that is needed. If you refuse to buy yet another consumable for your trike then use a dab or two of gearbox oil. The main thing is to use a lubricant specifically rated for high-pressure use, because these lubricants are designed to remain "on-location" rather than being squeezed away from the areas where they are needed most. It soon emerged that the old bolt had indeed been well-greased by its owner.

I cleaned the residual grease off with methyl ethyl ketone (nasty stuff, handle with care and don't breathe the fumes!) and pondered how best to measure the worn areas - and where they might actually be. It was then I hit the first snag: I had no idea what the as-new measurements of the old bolt were, so I had no zero-wear reference data. However, I did have access to a brand new, absolutely pristine hang bolt, which could be measured as far as uniformity along its length was concerned. Within certain limits we could make some deductions and inferences.

The next problem was just how much accuracy do we need, and how much is enough? Would accuracy to 0.001" be sufficient? (It's American hardware, so use Imperial units.) A useful principle in precision engineering is to use vernier callipers for measuring lengths and a micrometer for measuring diameters. Since the diameter is what we were interested in, a micrometer seemed best.

Common or "garden variety" spindle-anvil micrometers resolve nicely down to 0.001". But I had a feeling this wouldn't be accurate enough, and the actual areas I wanted to measure were, of necessity, rather small and not easily measured by the normal mic. I wanted resolution down to one ten thousandth of an inch (0.0001"). Why? Because I know how to get it.

To measure to 0.0001" you need a specialised micrometer called a Blade-Micrometer. It has two flat, precisely-ground and set carbide blades which are much thinner than the round spindle-and-anvil used on the usual style of mic, allowing access to very small areas such as O-ring grooves, keyways, cartridge extractor-grooves, etc. Besides these thin blades, it also has a vernier scale engraved on the sleeve which is lacking in the other style, and the vernier is the secret of the increased resolution. Naturally, the screw-thread of a blade-mic is cut to very fine tolerances. For all this refinement you expect to pay a premium, but "you only buy a good tool once - provided you don't lend it!" Figure 1 shows the hang bolt and the blade-micrometer beneath it.

So, armed with the blade-mic and the two bolts, I set to work. First I measured the new bolt at six evenly spaced "stations" to get an idea just how near perfect a new bolt really is. Then I measured the old bolt at the same stations. Figure 2 is a shot of the technique used, in this instance one of the stations towards the centre.

Logic says the area of maximum wear on a hang bolt will be at each end where the hang bracket actually touches it. That established the first and last stations; one a couple of millimetres under the bolt-head, the other immediately before the beginning of the threaded section (determined by measuring the total width of a hang bracket). The remaining four stations were evenly spaced between these two points, giving the following data:

Station New Used Note:
1 0.3730" 0.3711" Head end
2 0.3729" 0.3723"
3 0.3730" 0.3721"
4 0.3727" 0.3720"
5 0.3729" 0.3721"
6 0.3731" 0.3705" Thread end

Figure 3 shows a detail of the micrometer and how to read it. Scale 1 gives the tenths, with hundredths on the subscale. Scale 2 gives thousandths. The vernier scale (3) is examined for the best fit against the scale graduations on the rotating sleeve (2) to get the fourth decimal place. In this case the reading is 0.3711", the measurement tabulated for the head-end of the bolt.

The used bolt shows some interesting figures. Stations 1 and 6 are the areas at each end of the bolt, where we expect maximum wear to occur because this is where the bolt and hang bracket physically contact each other. Sure enough, we see significant wear at each end relative to the middle four stations; 10/10,000" at one end and 16/10,000" at the other. In thousandths of an inch, this is 1 thou and 1.6 thou. The very thin blades allow us to measure this small, worn area with a high degree of accuracy, whereas the round spindle and anvil of a conventional mic would not do so as they cover too much area.

The remaining stations (2 through to 5) also show slight differences, although not as great as the two ends. Actually, the wear is more or less even at these four stations, and it's interesting to compare the figures against those of the new bolt for which the figures are all very close to each other, as we would expect.

I admit this is a flawed comparison because we don't know the original measurements of the old bolt before it was placed in service. For that reason I hesitate to compare the old bolt against the new one, but the wear along the length of the old bolt relative to itself is well delineated.

"Replacement or overhaul" every 50 hours says Airborne. In my book, if you removed a 50 hour bolt, did these measurements on it and found less than half a thou wear at the high-stress points either end, I'd clean it up, re-grease it and put it right back into service. Inspection, measurement, cleaning and a re-lube qualify as an overhaul as far as I'm concerned. We are talking "on-condition" rather than "time-expired" maintenance. It has a shear-strength of something like 8 tonnes... you are never going to break this bolt!

Of course, if you run your hang bolt without any lubrication at all, it might well be on the way to showing the same wear as the seasoned veteran in just 50 hours. Replacement might be your only option.

If you can borrow a blade-mic, or know someone who can, then you may find the measurements interesting. If you're dead keen, a good engineering supplies company will get you a blade-mic for somewhere in the vicinity of $250-$270 (which does buy a lot of hang bolts!). Purchase via the internet is an attractive option. Starrett and Mitotuyo are excellent brands, but there are others. I got mine for a completely different purpose, but once you own a tool, it is entirely your own initiative which applications you choose for it.