ahson

I am trying to tighten this old French headset on a vintage Peugeot but I realized this headset has only 1 of those hexagon shaped adjustable locknut on it. How would I adjust these kind of headset? I thought most threaded heatset has 2 of those hexagon locknut.

fietsbob

There is a particular type of Grip Pliers, to hold the top race, while you tighten the locknut.

the French like being Unique..

Add:
someone mentioned that due to the anti rotation washer being somewhat effective,

Adjustable race is not so inclined to also rotate as the top nut is tightened..

bigger flat surface across the back. /D shaped washer hole ..

Fork, rather than the grooved slot on other nationalities machine practice. [UK/Italo/Sino].
with tabbed washer , which does commonly rotate .

cyclist2000

My older campy head sets have lock washer and the locknut.

JohnDThompson 

VAR makes a special pliers for that:


But a regular wide-jaw pliers will do in a pinch:


You're just less likely to mar the cup with the VAR tool.

Some older French headsets had holes for a pin wrench on the side of the cup. For those you'd use a different VAR tool:

reptilezs

i use channellock v jaw instead of the straight jaw. also euro style tongue and groove pliers, like the knipex cobra

Grand Bois

I've never found it necessary to use two tools on any of my French headsets. The top race is knurled because it is intended to be turned by hand. The no-turn washer prevents turning of the race when the top nut is tightened. My older Stronglight headsets have a toothed washer that positively prevents turning. I'd never even consider using pliers on an expensive vintage headset!