1/8 NPT will kinda work but it will jam and probably damage the case threads. 1/8 BSPT is 27tpi and 1/8 NPT is 28tpi, plus the taper angle and thread form are also different. Adapters are not hard to find if you want an NPT switch or other fitting. But they are close enough that screwing it in with teflon tape should work ok in a pinch... but that's still wrong.