Simon , I looked at it and thought the same thing but trying to pull the foam off I think its not possible it too well made anr the foam looks like its moulded in the polly ends. the foam is fairly open so it breaths actually to well and I would not use this filter for dirt riding by its self so the inner filter or sock doubles the thickness . The main filter is what I use on the yellow bike, with the same sock sretched over the outside as the whole set up is in a ally box that I made. Just that the air flow of the 2 is in reverse. IF I was going to use pumpers so to get the max performance and wanted to use the standard plastic air box I would also do the top air box mod as well as all of this together might affect BST carbs in there standard form Give me ring and I can explain it a bit better. .