Last year we bought a second hand box trailer. It is small, 5′ x 3.5′, but it is being towed by our Saab 900i so I wanted a trailer a bit smaller than the usual 6′ x 4′. It was manufactured by J&P Trailers in November 1993. For light-weight, non-braked trailers like this there are 3 main types of coupling used.

The relevant Australian Standards (AS 4177.3-2004 for the quick-release couplings and AS D18-1968 for the older style) do not specify hole spacings. Those quoted seem to be manufactures’ conventions. Varous sources specify from 100mm to 102mm for the quick-release 2-hole type. Ideally the coupling plate should be drilled using the coupling as a template, so the holes are a perfect match. But I doubt that happens often. The plate on our trailer has 3 holes to fit both sizes of 2-hole couplings.

This is the older style coupling rated at 1000kg and with 2 mounting holes. Our trailer had this type. It uses a tapped star-nut for fastening which can be fiddly. There is nothing to hang onto while hitching the trailer so you need to use the trailer drawbar. The hole centres are 65mm apart.	This type also has 2 mounting holes and is typically rated at 1000kg or 2000kg. It has a quick release mechanism and provides two places to hold it. The hole centres are 102mm apart. I bought one of these from Ark Corporation (catalogue no. CAS22Z) who make good quality trailer accessories.
This quick release coupling has 3 holes. Centres are 111mm and 52mm apart.

When we bought our second-hand box trailer it had an old round plug and an adaptor so it could be connected to a more recent flat 7-pin socket. This worked ok but it was messy. The weight of the adapter meant that it needed to be secured otherwise it would hang down and sometimes drag on the ground. I decided to disassemble the adapter and replace the round plug on the trailer with the flat plug from the adapter. Fortunately it was a good quality adapter which could be unscrewed and reassembled. This meant I needed to understand trailer wiring. I used a tech tip from the GoSeeAustralia Caravanning Directory but I’ll copy some of the info here, just in case I need it again.

Flat 7-pin socket	Flat 7-pin plug

The trailer does not have electric brakes or a reversing light so pins 2 and 5 are not used.