Strong Steel
Edit: Very happy months later. The spring does wear out in certain circumstances re-winding is a bit slow. Also I do have some small leaks somewhere. Nothing that would let out more than 10psi in 1-2 hours & it could be my own fittings. Overall, this saves me so much time coiling hose. Very high quality. Fittings are definitely 3/8" or larger. Currently looking for ways to get to my 1/4" ntp tools or debating trying to convert everything.
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