Too wide to fit berween the fork legs.
Mmmm ... you might
find a 100/90 too wide too ...
. Triumph fitted specifically a 4.10x19 Dunlop K81 TT100 (or, very rarely, a similar-size Avon AM9 Roadrunner) between those legs.
. Tyre companies say 100/90 is "equivalent" to 4.10. It's a porkie. All 100/90's are wider overall
than 4.10x19 Dunlop K81 TT100. Mostly it doesn't matter but Triumph and BSA were particularly stingy with spare space around both front and rear wheels.
. In addition, afaict at some point in the 1980's, all tyres became wider overall for the same section widths moulded on the sidewalls. If you wish to compare overall widths of the 4.10 TT100 and, say, current 100/90 Avon AM26 Roadrider, TT100
and click on "VIEW SPECS", Roadrider
and click on "Show available sizes". The Roadrider page is just a for-example, ime 100/90 Michelins, Continentals, etc. are all either similar or larger overall widths. Fwiw, although I don't have to on the T160's because the disc-brake fork legs are further apart, I've preferred the 3.25x19 Roadrider because its overall width is closer to that of the bikes' original TT100.