I believe it is dryseal...3/8"-18 unless my memory fails me if it has the original.
http://www.sae.org/technical/standards/J476_196106
So it wouldn't be metric if that's the case. Many of the fittings on the car are 'American'....with some metric, but most of the engine fittings are UNF (Unified Fine) or UNC (Unified Coarse) I believe.
Mine doesn't seal very dry any more....I need a little thread sealant to stop it dripping oil.
Dryseal is just a slight variation of NPT (National Pipe Taper), that is 'supposed' to not need any thread sealant. So to the best of my knowledge you would need a 3/8"-18 NPT fitting.
Of if you want a longer version of from 'Whitworth to Wedges' (catchy title isn't it?)
http://en.wikipedia.org/wiki/Screw_thre ... ardization
"Problems with lack of interchangeability among American, Canadian, and British parts during World War II led to an effort to unify the inch-based standards among these closely allied nations, and the Unified Thread Standard was adopted by the Screw Thread Standardization Committees of Canada, the United Kingdom, and the United States on November 18, 1949 in Washington, D.C., with the hope that they would be adopted universally. (The original UTS standard may be found in ASA (now ANSI) publication, Vol. 1, 1949.) UTS consists of Unified Coarse (UNC), Unified Fine (UNF), Unified Extra Fine (UNEF) and Unified Special (UNS). The standard was not widely taken up in the UK, where many companies continued to use the UK's own British Association (BA) standard."
But British Leyland finally did take it up by the 70s to a great extent and that's why there are so many non British Association and non Metric fittings on a wedge. It always puzzled me why that was.
[:)]
