Very inexpensive, simple, well made, I use it as a preliminary gap checker. Edge gets progressively thicker along the circumference with thickness markings. Plug electrodes can be placed between the edge, then the device is rotated until mild resistance is felt. The gap can then be read. The surface of the device is a template against which the negative electrode can be pressed to reduce the gap, but this is crude. Better to confirm the gap with a wire style gap checker, make adjustments with a thumb screw device made for that purpose.

I have used these for years but couldnt find replace ments for old ones I had. After years of use my old ones were wearing so the were not accurate anymore and the hardware store I bought them at no longer carries them. These appear identical I gapped a plug for my SR500 and checked the gap with feeler gauges and it was spot on.

Simple, effective, cheap, weighs nothing and fits in any tool kit.