I then tempered it at 250-300 degrees (Light Straw) in the domestic oven plunging it into oil once more to lock in the light straw. As far as I know it is still running well.
One factor that made me decide to go with Gauge plate was the cross section of the rocker arm, it was extremely deep, shaped kind of like a letter T with a long vertical leg. This section would have sufficient strength even if the metal were a little too much on the brittle side.
When case hardening, unless you have decent heat treatment equipment (i.e. furnace with decent controller) it is difficult to get a good depth of coat, however if you can get a good depth of coat then it is probably the best method, use any of the case hardening steels, EN 36 ids ideal but is difficult to obtain in anything but round bar form EN 32 should be suitable too