Compression fitting inserts are sized based on the tubing’s OD, not the insert body that slides inside. For example, a 5/16" OD tube requires 5/16" compression fittings, ferrules, and inserts. The insert’s inner width is 0.232", allowing it to fit inside a 5/16" OD tube, while the outer flange matches the tube’s OD to prevent it from slipping inside.
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