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Cryptic Plug Gap Specs

What does the BMW owner's manual mean when it indicates that the
correct spark plug gap for my car (84 733i) is 0.028" +0.004"?

Does the +0.004" actually mean "plus or minus", or are they
saying that the gap should be between 0.028" and 0.032"?

I am doubly confused because when setting the gap on my new
plugs the other day, I failed to consult the owner's manual,
and instead relied on the Chilton's manual, which indicated
that the gap should be 0.024" (with no "plus" setting).

I suppose that I will need to pull the plugs and reset the
gap, but not until I figure out what 0.028" +0.004" means.

BTW, what are the effects of setting too small a gap?

Advanced Thanks,

- -Richard