When he says its too big, is it way too big or just a few mms? ie. a tight squeeze?
I say this because in my experience and from what I've read, Pipercross filters are always a tight squeeze (in other words they are manufactured a few mms too large

) and you haev to really squidge it into the airbox and then clamp the lid down on it before it can pop up again. Thats what I had to do on my mk3 16v.. tell him to try that if thats the case
