Yes there is a specific gap. I cannot find the thread at present, but DaveW detailed the replacement of the sensor and the gap on a thread in TalkMorgan.

2mm rings a bell but I cannot be sure.

Also, the sensor is quite delicate.

Paul
Costock, UK
Plus Four 75th - Furka Rouge - Pip
Disco 5
Teddy - 17h1 Irish Draught cross