It's 8mm, but 5/16" works fine. If you are running VO, I'd go larger. For my conversion I used 3/8", and 1/2" inside my heat exchanger.

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'83 240D with 617.952 and 2.88
'01 VW Beetle TDI
'05 Jeep Liberty CRD
'89 Toyota 4x4, needs 2L-T
'78 280Z with L28ET - 12.86@110
Oil Burner Kartel #35

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