The Crimp: A Good One Is a Cold Weld, Not a Squeeze

In a correct crimp the pressure is high enough that the copper and the terminal metal cold-weld: the oxide layers break up, clean metal meets clean metal, and no gap remains. That is what GAS-TIGHT means, and it is the whole reason a crimp is a permanent electrical connection rather than a mechanical one. Air in the joint means oxygen and moisture, which means a growing oxide layer, which means rising resistance, heat, and eventually failure. A joint squeezed with pliers has metal touching in a few places and air everywhere else. It reads as a good connection on a meter for months — which is exactly why people believe it is fine.

Crimp terminals are colour-coded by the wire range they accept: red for the smallest, blue for the middle, yellow for the largest. The crimper's dies carry the same colours. Use the matching die. A yellow terminal in a red die is crushed and cracked; a red terminal in a yellow die is barely compressed and pulls straight off. Both look approximately right afterwards, which is the problem. Check the wire is inside the RANGE printed on the terminal packet, not merely that it fits in the hole. A thin wire rattling inside a large barrel cannot be cold-welded however hard it is squeezed, and the usual bodge — doubling the wire back on itself to fill the barrel — is a legitimate fix for exactly one gauge step, not three.

An insulated crimp terminal has one barrel doing two jobs: the front part grips the bare conductor and makes the electrical connection, the back part grips the INSULATION and takes all the mechanical strain. Position the wire so the insulation ends just inside the back of the barrel and the conductor fills the front. Too far in and the insulation is under the electrical crimp, which makes no connection at all. Too far out and the bare conductor takes the strain and fatigues. You should be able to see a whisker of bare conductor at the front of the barrel after crimping, and no bare conductor at all behind it. That is the inspection, and it takes a second.

Pull the wire firmly. A good crimp will not move; a bad one slides off and has just saved you a fault that would have appeared in service. Test every crimp on anything that matters — it costs two seconds each. Do not solder a crimp 'to be sure'. Solder wicks up the wire past the crimp and turns the flexible stranded conductor rigid at exactly the point where it leaves the terminal, which is the point of maximum flexing. The wire then breaks there, and it breaks sooner than it would have without the solder. Finish with heat shrink over the joint if it needs insulating or strain relief. Slide it on BEFORE crimping — everyone forgets this once, and there is no way to fit it afterwards.