Nix the idea of wrapping a 9mm filter with scotch tape to make it fit snug. I just had a bad experience.Thank you for your reply and for the tip regarding troublesome charcoal granules -- different brands of filters do feature different number/size/shape air holes at each end.
Variation in 9mm filter width is an interesting topic. I have identified three variations, from wide to narrow:
white elephant, everybody else, and Dr. Pearl.
Dr. Pearl, being narrowest, will fit any 9mm filter pipe, but I don't like how the filter moves back&forth in pipes other than Vauen, so I wrap some scotch tape around the filter, so that it fits snug, and prevents smoke from slipping around the filter.
On the other extreme, white elephant filters are too wide for my Vauens, and my Chacom!
Smoked a large bowl, good smoke, but when I removed the bit from the shank, I found the filter paper rather wet and swollen and un-stuck from the tape...requiring a bit of effort and some specialty tools to safely remove said filter and tape from the bit.
Gimlet did warn about the swelling effect, thank you. Also, "thocking" is the very word for the sound made by a loosely-fit filter!