I suspect that what you need is a non-magnetic feeler guage. I used to
use a brass set to set ignition air gap in old Chrysler cars of the 70s
and 80s. The sensor is a coil built around an iron core which will
become magnetised and drag a steel guage.
I'd bet money that Clymers is wrong here.
Jim G

HG wrote:

I want to adjust the sensor air gap in my Volvo-Penta I/O. The Clymer's
manual says use a non-metallic feeler gauge (note: non-metallic, not
non-magnetic). I've checked all around and cannot find one. A google
search of non-metallic feeler gauge bring up references to it's use for some
autos, but no sources. Snap-Ons online catalog does not list one.

Any idea where to find one? Thanks.