I just happen to have mine pulled apart at the moment. I put a metric caliper on it. It measures 4.5 mm. gas tube is .02 mm bigger. Not sure if this is 100% accurate. But should be within a couple points. Mine was factory, when I pulled it apart. It was very snug. I would check the measurement first. I had to lube mine, slide it in place, wiggle back and forth while tapping lightly with a light weight mallet. once you get passed the flared end it slides in nicely. JUST MEASURE FIRST! If it is a problem with the tooling NEA will fix you up promt.
^^^like he said^^^ they have the best customer service. no questions asked.