fuel pressure tester adapter for civic

[bluecivic91p]

Could anybody please, tell me where I can find adapter (and a fuel pressuer tester with it) to test the fuel pressure on 91 civic. The tester needs to be connected to a port on the fuel filter, which according to Haynes manual is closed with a 6mm bolt. I found an online store that sells fuel pressure tester with adapters for import (incl. asian) cars, but the smallest adapter is 10mm. Maybe the manual is wrong and it's not 6mm (I will need to check the bolt). Does anyone have some experience with this task. Any suggestion will be appreciated. Thank you.