The adjusting screw (see photo) no longer works because the threads inside the Delrin housing have stripped out. When I pull the shift lever back to move the chain onto the large chainring, the adjusting screw is just pushed out of the hole and the derailer pushes the chain off the large chainring and onto the crank arm. Starting with the least drastic measure, I tried ThreadTite, or whatever it’s called, but that didn’t do the job. Next, I’m thinking of coating the screw threads with something that will harden around the threads when inserted and adhere to the Delrin inside so that when I back the screw out after the substance hardens, the screw will break free and leave new threads behind. Adhesives I’ve considered are Gorilla Glue which expands as it hardens and appears to be some kind of resin, Epoxy, Liquid Nails, and JB Weld. The concern is that the screw might become so fixed that the Delrin will crack, as it is want to do, when attempting to back the screw out. Perhaps I could back the screw out before the substance completely hardens, but I don't know how long that would be. Having the screw permanently set wouldn’t be the end of the world in the short term as long as I have the screw in exactly the right spot. If it needed to be adjusted in the future, the derailer would be relagated to the spare parts drawer. Perhaps someone out there knows of a tried-and-true method to accomplish this repair. Thanks.

get a slightly oversize screw and re-tap the hole

perhaps it's time for a new front derailleur (delrin isn't the most long lived
compound around).

+1

The best way to repair one of those Delrin FD's is to replace it. After you fix the stripped out threads, the clamping block will probably crack.

Time for a "forklift upgrade".

> After you fix the stripped out threads, the clamping block will probably crack.

Yeah, I've got two of the swing-arm clamp types and one of the two-bolt clamp types in my parts drawer with cracked housing. Since this is the original equipment derailer on this '71 Peugeot, I'd like to keep it working just to show Delrin who's boss. So far it's me-2 and Delrin-5 in terms of cracked vs. non-cracked.

Well..... if you're hell-bent on beating it into submission, you might try an old woodworking trick..... Jam a couple of toothpicks in the hole, and snap 'em off flush. Then run the screw in. It might give you enough compression and purchase to hold, as long as you don't adjust/re-adjust too much. A little drop of Locktite/epoxy on the scew body might help to keep it set up after the adjustment is final.....

After trying Liquid Nails with no success, I ended up taking your advice, dck. The only reason I didn't right from the git-go was that I didn't know how I was going to find a tap the right size. So I took one of the cracked derailers and used my moto-tool to cut in and find out what I was dealing with. Turns out there is only 5mm of thread in the Delrin holding the screw. So I found a fine thread screw with a slightly larger diameter that was shorter than the original, but just long enough. I also found that the Delrin has a relatively low melting point, so I heated up the screw and it went in nice and firm. I think the problem is fixed. The derailer now has a non-old school screw in it, but I'm putting this one up on the Big Board as a win; me-3, Delrin-5. I'm still down in the count, but I'm not out. Thank you for your assistance.

If anyone is interested, here's a photo of the innards of a Simplex Delrin derailer showing the adjusting screw threads and the path of the lever.