Is there a metric equivalent to AN bolts and fittings?
I don’t think so, and that is one thing which inflates the maintenance costs of metric aircraft. That is, unless the maintenance company is smart, but they have no commercial incentive to be smart because say 25% of €20 is better than 25% of €5. In the bigger picture of aircraft ownership this is not usually a big deal but it’s awfully irritating to pay €1000 for a metric-fitting hose when the American version would be €80.
I’d say the biggest potential problem with hardware store bolts is not the steel grade but workmanship – geometric tolerances, plating quality, surface roughness, etc. In such a situation I’d probably hit a good automotive spare parts dealer first – an experienced salesman may know the right kind of bolt off the top of his head, and it would be a high-quality bolt.
BTW, having recently switched from four to two wheels for driving on the roads, I was impressed with the superior quality of hardware (bolts, hoses, etc.) on motorcycles compared to cars. In particular, socket-head fasteners, whether hex or Torx, are a much neater solution than cross-slotted ones (Phillips/Pozidriv/etc.)
don´t confuse prices for industrial metric stuff and spare parts prices of specific aircraft makes. If you go and buy metric fasteners in UK you will pay a lot less even in UK than for imperial equivalents. This may be quite different in USA as those folks are still stuck in the last millennium. I guess you will be driving on the right side in UK centuries before USA will be metric – as the last country on earth.
Referring to metric aircraft hoses: I paid € 1300.- incl. shipping from Lithuania for twentynine hoses for oil, fuel, air for a totally, 100 % metric Yak, including EASA form 1. OK, they wanted the old hoses (to claim they did a “repair”) . And a common industrial metric hydraulic hose for 300 bar , burst pressure 1300 bar, is € 5.- for half a meter. Silly prices are aviation ripoff.
I guess Jan will need the M 8 bolt for a Rotax type. And I assume the bolt will not need a calibrated shaft, as that would be a precision ground type. I can´t imagine that fitted in a wood prop with plain drilled holes. So any 8.8 type screw will do nicely, no matter if you get that from Ebay or anywhere else. And don´t expect fasteners for Merc or BMW coming strictly from German manufacturers. That was a fact many decades ago.
I remember dimly an emergency landing one or two weeks ago of a Cap or so in France? They lost a prop in the air, no doubt with certified imperial bolts on the American flat engine I suppose ???
BTW, having recently switched from four to two wheels for driving on the roads, I was impressed with the superior quality of hardware (bolts, hoses, etc.) on motorcycles compared to cars. In particular, socket-head fasteners, whether hex or Torx, are a much neater solution than cross-slotted ones (Phillips/Pozidriv/etc.)
Motorcycles are an interesting comparison with metric aircraft in this respect – I’ve got experience with both. Very often, depending on the health of the motorcycle manufacturer and the phase of the moon 
you can order those proprietary bolts for a fairly reasonable price, and they are available for about 15 years. Less so now than in the past, given today’s changed expectation of a disposable European motorcycle, but still the case.
As an aside, the Italians started those nicer quality socket head cap screws etc on motorcycles in the 70s and it spread. For a while in the nineties they got silly with stylized bolts using tiny allen sizes with large diameter shanks and those need replacing pretty often, but I digress.
For aircraft, standardized AN, MS and NAS bolts and all the other standard hardware was one of the most beneficial things done for the aircraft industry by the US Government. A practical move that still benefits us tremendously 80 years on. An aircraft built with AN hardware is made much more maintainable, regardless of the business status of the airframe manufacturer. It’s therefore a significant factor in maintaining a certified aircraft without ties to the manufacturer, a basic principle of FAA certification, and was a very, very good idea.
To clarify: yes it is a Rotax, a 912, but the prop is carbon, its hub consisting of two half-shells made of carbon fibre. Should anyone wonder why such long bolts are wanted: between the Rotax flange and the propeller hub is a spacer of +/- 45 mm, if it weren’t there the spinner would touch the engine cowl.
I am still waiting for the offer from the factory – guess it will be either order from factory or buy some locally and apply the hacksaw.
This may be quite different in USA as those folks are still stuck in the last millennium.
I agree. But I think Jan’s problem illustrate the differences that really matters. An aircraft made of “AN hardware and fittings” is easy to maintain and fix, certified or not certified. It is just a matter of opening the web page of ACS or similar and you have everything you need and it is of “aircraft quality”. Cutting and threading of bolts used to fix the propeller to the flange does not sound very “aircraft quality” to me, in fact it sounds pretty far out. What happens to the fatigue resistance of a manually threaded bolt? a bolt that were never meant to be re-threaded or manually threaded at all because there is no practical way to do it without weakening the fatigue properties. The modified bolt may very well be OK, because it may have dimensions much larger than needed, but how would you know? How would you torque it?
An AN bolt is a tension bolt. The pre-tension in the bolt is very important for proper clamping of the parts and for fatigue resistance. To get the proper pre-tension, the nut is tighten using a torque wrench. This way any AN bolt will do it’s job for the entire life time of the aircraft. If you start to modify the bolt by cutting new threads, you have no idea. This is just as true for any M8 8.8 quality bolt. “Fixing” high quality standardized bolts to make them “fit” in high critical places is something you just do not do. A shop with the proper tools and know how could easily make a bolt for you according to spec, but this will be very expensive, and it is also a stupid thing to do when there are AN bolts that fits just perfectly.
The proper way to fix this is (in my opinion I must emphasize) to use the closest fit M8 bolt and fabricate thick walled spacers to put under the nut to get correct thread properties (maybe one ring/disk for all the holes?) However, the best way is to use a correct size AN5 bolt. The best practice rules for AN bolts are: Minimum one thread must show outside the nut, but no more than three threads. At least one flat washer must be used under the nut, but no more than three. I don’t know about M8 bolts, but there must be some similar rule of thumbs to assure the nut sits properly on the threads and the nut is seated properly on the material.
But this here schraubenhermann even publishes a list of DIN-norms so he must obviously rock solid reliable. And the stuff offered is DIN 8.8, so just as good as what I have now.
I bought a watch in Hong Kong that had all the paperwork to prove it was a Swiss Chronometer Officially Certified, and quoted all the specification numbers etc.
However the price of about HKD100 proves something different!
Some people can get a little obsessive about the correct hardware, but in the case of a prop mounting bolt I wouldn’t be taking any chances whatsoever.
Life is short; I prefer not to make it any shorter.
It isn’t just the raw tensile stregth of the bolt that matters. It is also the surface finish, which determines how easily a crack can propagate. Bolts for “serious” uses are very smooth and nicely made. They are completely different to the Chinese crap which is found in DIY shops, and on Ebay. Sure, the cheap bolts do work but that’s because in the vast majority of applications they are working at maybe 1% of their tensile strength and/or there is no oscillatory element in the loading.
still no news why this certified Cap lost its prop in flight
Could be any of
