I get my hardware from https://military-fasteners.com/ (recently needed a ton of stuff for an ultralight)
vic wrote:
Airbus
Plenty of imperial fasteners on Airbus. Partly because when it was setup all the titanium fastener manufacturers were only catering for the US market.
Airbus had to “go American” on fasteners, hoses, etc, to sell the planes outside Europe; everybody in the big plane business, especially Americans, knows about the “metric part ripoff” 
In the meantime Socata managed to go for metric airframe fasteners and metric (ISO thread) hoses, so it costs 2k-3k to change the engine compartment hoses at engine overhaul time. Actually Saywells can do them for much less now but not many people know that.
tmo wrote:
Do tell, are the metric LN screws really direct replacements for the imperial AN/MS screws? Genuine question, as I am sure I’ll be missing a size or three anyway, and getting stuff from Spruce takes time and too much money.
Specific to the OPs question 8-32 AN screws are very close to the same diameter as 4-mm screws, making the metric washer ID useable for the AN screws also referenced in his post. I believe that may have been the intent of the slightly odd washer spec quoted. If you were manufacturing or maintaining his aircraft in 1960s Germany this may have made your job a little easier.
Slivaire.
Correct 8-32 is almost perfect for 4mm, I once used two 8-32 aviation screws with those dreadful tri-wing heads to make a Honda motorcycle Headlight theft proof.
A_and_C wrote:
Correct 8-32 is almost perfect for 4mm
Certainly it’s true for interchanging washers… Beyond that, I hesitate to post the following but FWIW here is an 8-32 pan head stainless aircraft fairing screw (widely available at airports and low profile) and a 4mm x 0.7 pitch commercial screw. Both may have been removed from my aircraft within the last five minutes for the photo op…
The diameter of the 8-32 screw is 7 thou or 0.18 mm oversize and the screw pitch is about 10% coarser. Thin wall nut plates of the type used to retain fairings etc are generally sufficiently soft that the 8-32 screw will form the nut plate as it’s screwed in, seemingly no metal cut and the female thread remains intact. It’s not as bad as it may sound and was locally inflicted on my plane years before I got it, which is my excuse for knowing this
It is extremely unlikely that a certified aircraft allows the substitution of metric fasteners for imperial fasteners. They are not interchangeable. Use the fasteners specified in the parts catalog for the aircraft, and do not substitute without an approval. There are a myriad of fasteners which look like they would work, and might fit, but are not suitable. Sure, there are also some very high quality commercial (non aviation) fasteners which would be suitable for aircraft use. Rarely are they less costly than the proper hardware.
Imperial fine threads are used for most structural fasteners. Coarse threads are occasionally used when threaded into tapped holes in cast aluminum, they match the joint strength better (I’ve done the math, and it’s right, but it’s a long explanation), so yes, you can buy engine studs which are male coarse thread on one end, and fine thread of the same diameter on the other. They will also come in oversized threads for a tight fit, so use the right ones. Airplanes have crashed because fasteners were wrongly substituted.
Never allow a structural bolted joint to bear on the threads of a male fastener, it must be the shoulder of the correct length, with at least one washer (though usually not more than three) then the nut.
Yes, my friend once stripped the Whitworth bolt on the cylinder sump of his Gypsy Major – it was a problem. I did a temporary repair, which got him home (1500 miles).
Meccano made its own thread, which does not match any known commercial thread, however a 5/32BSW is workably close. It’s okay to substitute threads on Meccano projects.
Replacing specially made metric fasteners or Whitworth with AN/MS is more often ‘the thing’ that’s done, not vice versa. Otherwise, with reasonably available hardware of any type or grade there’s little point.
My IA and DAR friends across the field are busily replacing and reworking their Canadian built Chipmunk to remove most of the remaining Whitworth fasteners, most recently in conjunction with a tailwheel assembly rebuild and rebush. You can’t get rid of all of them, granted, but the more the better. It’s not an original plane anyway, having long ago lost its boat anchor original engine in favor of an IO-540. Climbs at 2000+ fpm now, Art Scholl style.
That one has long been Experimental Exhibition, as with Yaks etc but in the case of my certified aircraft, nobody knows or cares about miscellaneous screws and washers as long as it isn’t structural, and e.g. 4-mm to 8-32 AN/MS can be useful for non structural applications where metric commercial grade fasteners were originally used to save money. The parts catalog lists AN/MS hardware where applicable for airworthiness but doesn’t list non structural fastener specs, so its archeology to figure out what was there in the first place. Both metric and AN were used, metric as mentioned mainly in non-structural locations but also for high tolerance ground fasteners like the wing attach bolts. No, fairly obviously you don’t stick a substitute fastener in a reamed high tolerance hole, or on a certified aircraft anywhere else where significant loads are applied.
BTW and for obvious reasons, AN/MS, SAE etc fasteners are not properly termed “imperial” fasteners (that bunch was demoted out of the US aircraft business circa 1776) 
What happened to @Maoraigh? I was hoping he’d come back with fasteners successfully ordered or in hand.
