Porsche never spec'd or offered 18 inchers for the '97, in fact they have cautioned against them.
In '98, after adding additional structural bracing to the chassis to better withstand the stresses of larger wheels, Porsche offered them for the first time.
There are several people running 18's and larger on '97s, apparently without issues.
So, if like some others, you wanna play tag with the Devil and go with them, I believe you'll need 5mm spacers (minimum) to get the offset right if using 996 wheels.
I have original 18" Turbos (993 wheels) on my '99 and these match the required offsets w/o spacers, so you could always be on the lookout for these too.
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