I have a '77 DT250 that time and vibration eventually took its toll on the rear signal stalks.  I ordered a set of K&S Technology aftermarket signals from OEM Cycles and they arrived very promptly but the threaded portion beyond the base is much too short.  The threads don't clear the inner fender so there simply is no way to attach the new signals.  The OEM Yamaha stalks have threads 26mm beyond the base of the flasher stalk while the K&S flasher has a threaded portion only 17 mm.  So today my brother fabricated a threaded adapter for me that allows me to mount the aftermarket stalks on my monoshock DT.  It did require me to very slightly enlarge the hole in the metal inner fender and to enlarge a portion of the hole in the pot metal spacer the flasher stalk goes through.  If anyone is looking for a way to mount aftermarket signals to their DT, reach out to me and I'm sure I can get my brother to manufacture a few more.

EDIT: I tried attaching a picture of the adapter but I can't get it to upload.  If it's allowed by site rules contact me and I can emai/text pictures to you. 

Thanks for posting.

You need to use a third party host such as Imgur to post photos.

Yours is not an uncommon problem nowadays. The threaded hollow stud is usually removable and, here in the UK, Yambits sell an aftermarket part that might solve that sort of problem.

Original indicators came with different lengths of hollow stud, depending on where on the bike they were fitted - this sometimes differed from country to country, even on the same model.  

This is an old and well-known issue... I ran into it about 15 years ago.  K&S makes two versions of that signal, only difference is stud length, and their catalog is wrong for some monoshock models.  The part number "4015" comes to mind as the correct version with the longer stud (which are "cast in" and not easily removable on this style signal) 

A forum search might yield the answer, or I think might have pics on my old computer still.  But it sounds like you have it fixed already!  Great job!

Thanks for bringing this issue up again, it hasn't been discussed in years.  I found the answer by searching ebay. 

K&S shows the part number as 25-4016 for the DT monoshock...  This version has the short stud and is not correct!

What you need is K&S part number 25-4015 which comes with the long stud. 

Side note...  in the past both versions came with 12v bulbs and not the correct 6v bulbs. 

Well that is some good information to have.  I didn't even think to try to remove the old stud from the broken flasher.  Could have saved myself some time and avoided prolonged contact with my brother

I did look up the longer K&S but I wasn't able to find a vendor with them in stock.  Fortunately, I just converted my DT to the 12 volt VAPE system so the 12 volt bulbs will work swell.  Thank you all for the information.

Studs are only removable from the early "metal stem" signals...  you would have destroyed the turn signal trying to swap...  don't ask me how I know.

The long stud 25-4015 version can be found on ebay or many retailers, even Partzilla has them at a great price if you ever need another one or want to get a spare.  www.partzilla.com/product/aftermarket/2F6C-K-S-TECHNOL-25-4015