etheir 15 or 20 amps will work .
the american fuse rating is kind of common britbike knowledge , ( there is a triumph factory Bulletin )
American fuses are sold at there continuous rating ( roughly about half the blow rating )
fuses were sold at their blow rating ( roughly twice their continuous rating )
this can leave some confused and fitting fuses way to large
to safeguard the wiring .
the factory would have fitted a Lucas
35/17 fuse .... with the 17 amp part being
the continuous fuse rating you are trying to match . ... ( 35 is the blow rating )
( these fuses were never that common in the USA )...but if you want to pay to much
for a fuse they are available online ... while the equivalent American fuse is already in every parts store
so to fit todays normal glass fuses you
need to round-down to 14 or 15 amps
or round-up to 20 amps ,.. either will work ... unless you are trying to fit a Lucas
glass fuse holder
in which case .... the length of the fuse is as important as its ampacity .
a 14 amp SFE fuse will be short by 3/16" ( which can cause overheating from inadequate spring tension )
a 20 amp SFE fuse will be a good fit ( SFE uses the lenght to also connote ampacity )
a 20 amp AG fuse will be a good fit
there are also "combo"
SFE/AG fuses that are all 1 1/4" long ... either a 15 or 20 amp will work .
these fuses eliminate the length differential associated with SFE fuses
if you fit a modern mini-blade fuseholder with either a 15 or 20 amp fuse
you wont have to remember any of the obscure glass-fuse length minutiae .