Rich....I'm on board for 3.5!
I just resealed a Weeden 670 sightglass, 3mm,
and it doesn't really work very well to show the
water level. I suppose it is capillary action
creeping into the equation. I'll swap it out
when I get my hands on those 3.5mm tubes.
In case anyone is interested, I used twisted
teflon tape as seals. Usually works pretty well
and did so here.
Wayne