Coco, It could be close. What tyres are you intending to use ? As I mentioned above, Avons are wide due to a "tapered" sidewall.
I have a 110/90 18" AM18 on the rear of my Mk111 on a 2.50 and it measures 118mm. It has about 1/8" clearance from the chainguard.
My 750 has a Metzeler ME99 110/90 18" on a similar 2.50 and it is only 108mm at the widest point.
I've been polishing my rims this evening. A quick way to get sore fingers. Mind you, the smell of Solvol autosol, burnished alli and top-fermented ale goes some way to compensate !