Dave, you can get these the wrong way round, because this happened to me. After I rebuilt my engine, it would rev to about 4K out on the road, then backfire and go like a dog. I immediately switched the engine off, and looked under the bonnet for the obvious faults, but couldn’t find anything. Once started again it would drive OK, but when reaching 4K the same thing would happen. I tried everything to try and diagnose the problem. I didn’t even dream that I could have transposed these plugs, as I thought that they would have been designed not to. I eventually sent my Escort Cossie to Power Engineering to diagnose on their rolling road, and they found the problem. I had transposed these two plugs, and once switched round, car went like a bomb. A problem I’ll never forget.

Cheers
John