One of the best (or maybe the best) connectors are the well known heavy duty circular style connectors. These are very common for aviation and military use.
The quality is very good and also known as MIL spec connectors. (Meets the military specifications.) Here are some characteristics:
 Sealed (Moisture, air, ozone, sand, dust and magnetic permeability.)
Caps for unused connectors.
Mechanical lock/mating. (Bayonet, thread lock, set coupling forces, collet retention, resist physical Shock)
Position keys. (Master key with alternate keys/pre indicative numbering.)
Crimped contacts! (Insert retention, set contact insert force, high electrical potential and ground resistant.)
Heat resistant. (Withstands up to 200 degrees Celcius for 1.000 hours and flame proof.)
Variations
There are a lot of variations possible. Like:
Shell size
Number of contacts
Contact size
Key orientation
Contacts: Solder/crimp
Lock: threaded/bayonet
The list of possibilities is huge! This results that there is a connector for every purpose, but for hobby use it's almost impossible to find the desires one for a low price. These connectors are rather expensive, so second hand use is the way to go.
Definitions
Receptacle is the fixed connector (panel side)
Plug is the movable connector (cable side)
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 A requirement for 'MIL spec' connectors is that each pin/socket is identified. The coding of these connectors is quite interesting. The identification of each pin/socket is als follows:
First all capitals: A, B, C...Y, Z.
Then lower case: a, b, c, y, z.
Then double upper case: AA, BB, CC... YY, ZZ.
Remind that the letters I, O, Q, l and o are never used. The I and l look similar and O, o and Q look also rather similar. To prevent mix-up, these six letters are never used. The exact number of identification labels correspond with the number of pins/sockets used.
 Depending on the connector shape/style the contact coding may be different. The connectors shown above have an individual marking per socket. It's also possible that the identification is done by 1, 2, 3, ... numbering. Sometimes pins 10, 20, 30 and so on are graphically marked. On the image here is a 'snake' drawn on both connectors to identify the contact order. For the left connector every tenth pin is indicated with a circle around the pin. Not every pin is marked, but only the significant pins like the first and last of the 'line'.
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 I prefer the solder type contacts since no special tools are needed. Otherwise contacts have to be crimped to a wire and the wire has to be inserted in the shell. Check if the connector has solder contacts or at least if it's the 'loose contact type' if the contacts are provided.
Remind that for some official purposes soldering is not allowed. Crimping is considered more reliable. For hobby use the result is that contacts can't be reused. Solder contacts (or soldered crimp contacts) may be therefore more convenient.
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 There are several contact sizes. The most used for 'regular avionic instruments' is size is #20 as shown on the adjacent image. The lower the size number, the larger the contact (and maximum rated current). #20 size is rather common and #16 is the next bigger one which is also rather common used. Contact sizes #20 (top) and #16 (bottom) are shown here.
For official use, the batch/lot number of the contacts must be known! So keep the documentation in place where needed.
The contacts are marked with colour rings to indicate the model. For example contact numbers 357 and 358 are shown. Based on the resistor colour table the colour rings are orange (3), green (5) and violet (7) and orange (3), green (5) and grey (8).
There's also a small hole in de contacts so it's possible to see if the wire is fully inserted before crimping.
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The connector has usually five keys. One 'double width' master key and four other keys. Usually the master key is located at pin A; the first pin. On the photo is visible that the left two plugs have the master key at the default 'A' position. The right connector has the keys at an alternative position. The master key is placed near contact 'U' in this case. There are several possibilities of key placement. These keys are used to prevent mix-up of connectors of the same pin count, shell size and contact size.
 | For hobby use its thinkable to file away the 'wrong' keys to make it fit. Remind that it is in this case possible to install the plug in the wrong position... Remind also that some connectors are cadmium plated and cadmium is highly toxic! If you want to file away keys of cadmium plated parts, be sure to 'wet' everything to prevent inhaling dust and be sure to collect all debris and rinse everything to precent contamination with cadmium. |
On the image below are for example the four regular keyway rotations shown as used by Amphenol for the SJT series connectors.
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 For testing I don't want to buy the original plugs since this is usually more expensive than the surplus instrument itself... I also don't want to modify or damage the original pins like soldering wires to the contacts. The solution I chose is to buy only the contacts. By looking on eBay I find once in a while a small lot of contacts of 25...100 contacts. The contacts are never cheap, but much better affordable than buying new. I soldered wires to the (crimp) contacts to create test leads for bench testing. This works rather well. A price below €1 each contact is considered well priced. You may be lucky to find a well priced large lot like on the adjacent advertisement. $140 is still a lot of money, but considering this is a lot of 1.000 contacts $0,14 is a bargain! For the record: M-39029/5-115 is a size #20 MIL-DTL-26482 series 2 socket that is usually priced new €1,47 each. So 90% 'discount' is great.
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 For testing I made some jumper leads. There are crimp sockets soldered to the wire ends. Every wire has a coloured ring to distinguish the wires to prevent mix-up. The socket and coloured ring are insulated using a piece transparent shrink tube. The result are rather neat test leads. These leads fit on most of the avionic instruments and the synchro source for example. There are also a couple of test leads with a bare wire end for connecting alligator clips for example.
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