Yes, it's 3.5mm. But no need to search high and low for a 3.5mm hex key because the more common size 9/64" hex key (available at your local Sears, among others) fits nicely (even if a touch on the snug side) and works fine.
Lee Berg Palo Alto
Mark Bulgier wrote:
> I'm no Delta expert, but I know at least some iterations of Deltas took a
> 3.5 mm hex key - a very rare size, not included in any set of hex keys I've
> ever seen. Rather an annoyance; anyone who has Deltas or works on them
> needs to hoard any 3.5 mm wrench they can find.