If you have a compression fitting that you need to block off either temporarily or permanently, these teflon PTFE plugs are just what you need. To be
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RFS Plug, Stainless Steel SS316 Material, 1/2OD (12.70MM OD), Double Ferrule Compression Fitting : : Everything Else
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Plumber said plug with 3/8 caps but caps too big : r/askaplumber
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Swagelok SS-810-P Stainless Steel 316 Female Plug for 1/2 in. Tube Fitting
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Compression Tube Fitting 1/2 OD Tube Plug End Cap Stainless Steel 316
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