They have some rosin but they need a good amount of concentrated heat to really melt the solder. I see a lot of people have cut them apart and complain the solder doesnt flow well but they are not using enough heat in the right place, a bic lighter isn't doing it.
These do not saturate solder like a full soldered joint but perfectly well suited for stuff like car stereo, horn, and other small diameter wires that don't have tension on them, they have a very thin profile. For higher amp wires I like crimp/solder/shrink connectors.
The last thing I will use are the crimp only plastic covered connectors, I never liked them and do not use them.
The shrink/solder will have no problem with under the hood temps in any climate. These are crimp solder connectors.
Crimp Solder & Seal Butt Connectors
Crimp Solder & Seal Butt Connectors protect splices from water, corrosion, salt and moisture, and provide strain relief.www.grote.com
The shrink solders work great. If you have enough heat you can watch the solder "burst" then flow into the joint as they shrink. The ones I have also have glue to seal the wires. They cool to be very rigid and there's no pulling them apart.No but it needs high heat in a very small area, I use the butane soldering iron/heat gun as it has a very small diameter heat gun tip.
I bought a set of VACO 1902 crimpers on Ebay the other day that look like that after seeing your post. I should get them soon.I have an old VACO 1900 that I have been using for many years. It works perfectly with both insulated and non-insulated connectors and the wires NEVER pull out, corrode, or break. Unfortunately VACO is gone and the tool is not made anymore, but these can be found used. If you see one of these at a flea market or estate sale, grab it!
In it's absence, find something that looks like this...
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