In article ,
The Medway Handyman wrote:

It was a blond moment John, or possibly a grey one. I didn't
know they made uninsulated crimps! I've only ever seen
insulated ones.

I have always been doubtful about insulated crimps
for solid wire. ISTM that a crimp for solid wire
should be thick-walled, uninsulated, with visible
corrugated lines inside. With the correct tool
those corrugated lines should pierce the solid
wire and form a gas-tight joint.

--
Tony Williams.