Rich Grise wrote:

Is it possible to put black oxide on just one surface of a part,
or does it have to be the whole thing? Do they mask them, or
what?

I've got a client who wants to paint black and white stripes
on a modified VW flywheel - he's had the ring gear ground off,
since he's using an electric motor; and we've already got an
optical sensor on order - yes, I know about gear oil leaking
into the bell housing, but we'll cross that bridge when we
come to it.

So, there's this machined surface that we can point an optical
sensor at, but I have to think of something to make it
black with 6 white stripes.

Would epoxy paint do the job? I.e., paint the ring black,
then paint white stripes, or would it be better to black oxide
it, and then paint white stripes, or even mask the oxide and
leave six shiny spots?

Thanks,
Rich

There is a reason that there are few to no optical sensors in an
automotive engine - they aren't reliable in that environment. Try
looking at magnetic / inductive sensors such as used for crankshaft and
camshaft position sensors, vehicle speed sensors and wheel speed
sensors. Indeed put a proper flywheel with ring gear back in and pickup
a VSS pickup and mount it to sense the flywheel ring gear teeth.