I purchased a Tange Passage JIS headset to replace the (horrid) Tange Aero headset on my Centurion, but am having trouble seating the fork race. The race diameter from the old headset measured out at 27mm, but the new race measures out at 26.6mm, closer to ISO sizing than JIS. I know that some JIS headsets can come with ISO fork races, but the box says that the race size is 27mm; the frame cups measure out almost exactly 27.0mm so it is a JIS headset.

Question: Is 0.4mm off "close enough" to JIS sizing and I should just pound harder to get it to fit OR do you think that the race came out of manufacturing out of spec and I should try to get an exchange? Alternatively, since 26.6 is close to 26.4, how likely would it be that an ISO fork race got slipped in by mistake?

Thank you for your insights.

John