Is it critical to have the plugs over gapped and or under gapped by .03 either way and whats the gapping on a 1999 town car executive

Look under hood and look for sticker that has the specs .or go to local part store they give it to at no price

I've been tuning cars since the 1956 + models to current...I have never heard of over or under gapping the pulg electrode...? It is critical to match the heat range, as well as the factory plug gap...Also, the factory plug usually works the best...your stock Lincoln plug should be the Ford Mototcraft platinum tip spark plug...available at part stores for less money than the dealer...Hope this helps.

The gap is about 25th i say about as some modern vehicles use a a high tension compared to older models ,having the gap a bit either side will not cause any problems as such but an excessive wide gap will cause hard starting from cold ,and too closer gap can cause a slight sort of hesitancy