Welch plug is the term for the dished plugs with ground edge, the others are cup plugs.
They have nothing to do with releasing pressure if the block freezes, just part of the process of casting blocks. Some engines have screwed plugs.
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Richard
1943 Bedford QLD lorry - 1941 BSA WM20 m/cycle - 1943 Daimler Scout Car Mk2
Member of MVT, IMPS, MVG of NSW, KVE and AMVCS
KVE President & KVE News Editor
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