[Keith1968]

I was a designer for a forklift company many years ago so built up some good knowledge of fittings in my head, although I can't say I'm remembering so well now...but here goes and apologies for any mistakes......

As Wilf says the dash 6 number is the pipe size in 16ths of an inch, so no 6 is 6/16ths or 3/8 dia, no 4 is 4/16th's, no 8 is 8/16ths or 1/2" and so forth.

These is a standard "english" hydraulic fitting nomenclature, not just helicopters but anything hydraulic.

The "english" standard thread form for hydraulic are either JIC (your pipe fitter will say something like "no 6 JIC mate..") but which is actually a UNF thread form, or the other type is usually BSP.

Whitworth was never a pressure thread.

Just to confuse the issue further, a UNF thread form will have a -dash extension after the number which anotates the thread per inch, not pipe size.

What I think you 'might' need is a BSP to JIC adaptor to go into the cooler, then a JIC to JIC hose then another adaptor to go into the engine.

Of course whatn you should REALLY go for is metric face seal adaptors instead which are far far superior and universally used nowdays. Ask a local hydraulic van man to make up for your car.

Hope that helps.