abdon 

I think I can find a 165mm crank in any dura ace model ever built with the exemption of the only one I need, the 7800 series... Why oh why!

iab

170 is the end of the world, is it?

bulgie 

5 mm usually isn't super significant for pedaling IMHO, but it can make a big difference in toe overlap. If you have a little o'lap already (as on a couple of my wife's bikes), an extra 5 mm can take it from acceptable to really annoying or even dangerous. Laurie knows the "drop your heel" technique to make your toe clear, but not everyone wants to deal with that.

Cornering clearance can be an issue also, if you're a racer with a low BB height. Most of us will just stop pedaling through such a sharp corner but crit racers in particular often want the advantage of pedaling through.

If you don't care about the 5 mm, then you have more options. But enough people do care, which is why better cranks usually come in those sizes. And even in the halfway sizes like 167.5 sometimes. Which seems silly even to me, but it does somewhat correlate with the price of the cranks, expensive ones coming in more sizes. High-end consumers have come to expect it. "The Princess and the Pea"?

repechage

Probably no Tour contenders that wanted 7800 series 165 mm cranks.
today, 165mm is a thing.

in the Campagnolo pedal form factor dominance days, 165mm cranks and a high bottom bracket was a distinct advantage in sharp turns. I could pedal through turns that brought a dug in pedal for others.

today with modern clipless pedals, much less of an issue. The tire will slide along the pavement at a great enough heel angle.

iab

Performance questions? In C&V?

Certainly can make up the 5mm on the pedal foot position for overlap

Or I need to princess-up.

Seanaus

I'd guess Shimano sold very very few of them. Shimano documentation lists 7800 cranks from 165 to 180 in 2.5mm increments, in both double and triple.

abdon 

That's the thing; before the 7800 and after the 7800 they were a thing, but not on the 7800 series.

**** I lived in Japan for many years, 165mm is a whole lot more common there where what passes for average in the US (5'9") is considered tall there. Plus a lot of the short ones had long torsos and short legs.

The next generation Ultegria inherited the features of the 7800 Dura Ace. I may have to resign myself to one of those. Or to be more accurate my daughter. She's the one 5'2". The 165mm derailleur would dial in her frame better.

Seanaus

I know nothing about any of these sellers but they list some as in stock, which as we all know can mean something, or nothing, but worth a try.
https://orangecountyusedbikes.com/pr...peed-crankset/
Shimano DuraAce 7800/7803 10sp Crankset - $479.99 - Bike Parts 360