On the ones that I have done,there was the glue that they used that has blobbed out at the inside deep inside the helmet.The fix that the owner of PSI came on here and told us is to use the 8mm barrel trimmer shaft,( from the kit that you get several shafts) and stick it in there reamer end first,then turn it to allow it to scrape the junk out.It's amazing how much comes out....
PSI told me it only happens on the antique bronze ones,,but I had a couple of the pewter ones that needed it also.
You don't want to go overboard with reaming it,as you can make the bore too large so it won't fit tight on the end.If you do,careful application of blue lok-tight up at the end of the bore of the helmet will be needed....DAMHIK...
That's been my experience,anyway on the 11 that I made.
Steve