So, I recently, out of boredom, decided to upgrade my headset from a beaten up Shimano 600 to a sparkly NOS Dura Ace that I got for cheap on eBay. Anyway, I didn't think there'd be any significant difference in stack height, but there is. I think the Dura Ace is about 4 or 5 more mm than the 600. I know that the locking ring isn't really necessary, so I'm not worried about that, but what I am worried about, is the fact that there's maybe 2-3mm of thread for the headset, I can turn the top nut maybe 2.5 times. If I put Loc-Tite on there, will that be enough, or should I just give up the ghost and put the 600 back on?

Thanks for all your help!

Your old 600 may have been the loose ball (or caged ball) design and the newer Dura Ace a cartridge bearing type. The loose ball Shimano headsets had a much lower stack height than their cartridge bearing models.

I installed a Shimano 600 (HP-6400) loose ball headset with a stack height of 33mm in a '92 Trek 1420 to replace the OEM Tange headset and it fit fine. When it wore out 15,000 miles later I tried to replace it with a newer Ultegra cartridge bearing headset (HP-6500) but the stack height (37.6 mm) was enough taller that it wouldn't work.

I don't like Loctite on headsets, but in dicey situations like yours where I lack good thread engagement, I use "vibra-tite" on the threads, which provides enough traction that I don't have to torque the locknut as tight as I would otherwise.

The advantage of vibra-tite or similar product over Loctite is that it allows readjustment if necessary without taking things apart to freshen the adhesive.

Umm... the locknut IS necessary, unless you want your headset to come out of adjustment every five seconds.

Don't hack job a nice DA headset. Put something else on there, do it right.

I just built a bike and had the same "problem". I counted the lock nut rotations and I was at 2.25 spinning by hand. When I took my wrench to it, it "felt" just fine. I was able to get a good solid snugging against the top race. So far, so good.

I try to remind myself that the upper nut is not holding the bike together, it's just holding a light bearing adjustment in place.

Or at a minimum, just use the Dura-Ace bottom stack and see if that gives you enough thread purchase.