On both T'bolts and Lightnings, the o-ring is a good way to go BUT if you
use the triumph slightly thicker one it works a treat. Just don't tighten the
carb up too hard and either use nyloks or similar. You should be able to
rock the carb on the o-ring a little. It's excessive tightening that distorts
carbs. Getting a couple of 5/16 bsc nylok or clevlok nuts is now the harder
You can use 518 and a gasket either side of the spacer but the o-ring way
is the better insulator of both heat and vibration.
To reduce heat transfer to the carbs (causing me bother), I fitted 9mm Tufnol spacers, stainless studs, fibre washers and nyloc nuts. Any bigger the air cleaners would foul the side panels.
As mentioned, the BSC ones are hard to get so I made the studs all BSF which is the thread for the cyl. head.
The nyloc nuts can be fine tuned easier, once set never move.