'71 Mercian, '72 Jeunet, '82 Jack Taylor, '13 Rawland
This happened to me when installing Honjo fenders on one of my bikes with 28mm Paselas. The fenderline is quite tight, meaning the fender comes into close contact with the tire. The options are:
1) alter the fender line such that there is greater clearance between the tire and fender
2) file down the bolt to be flush with the nut. If the nut itself is contacting the tire, consider sourcing a narrower profile nut.