I need new limit screws for my rear and front derailleur. Bike shop wants to charge $5 for them when they probably cost $0.05 each. Can I just get my own screws from the hardware store or do derailleurs require specific screws? If the former, what measurements/etc should I look for as far as the shaft and threads are concerned? Also, what kind of thread locker should I get?
I was once quoted by a retail plumbing supplier $16 per "fancy screw" for a shower fitting, and needed 6 of them. They were M3 grub screws with a pointed end, so not large.
Instead I went to a specialist bolt-and-fastener shop and got 10 of them for $0.48 each.
So your screws have certain parameters that must be met
- thread pitch
- thread form
And some parameters that are optional, or variable.
- Head format - philips/JIS or flathead or hex head or torx, or a thumbscrew, or a bolt head.... Make sure its some tool that is in your multitool
- Head width and depth - doesn't matter as long as you have enough room for it.
- Material - Stainless Steel would be advisable, but other steels could be workable too. Titanium may save a gram or so but would be expensive. Aluminium is too weak for this application
For a B tension screw, the end that faces the frame is relatively unsupported, and should be approximately rounded off so it doesn't bear on the hanger with a sharp corner. For Limit screws they press on steel lugs inside the mech.
Other than all that, any competent screw shop should be able to sort you out. Only downsides are that they may be wholesalers who want to sell by the hundred or by the box/pack. Also they may not do cash sales, so check first.
Based on the sizes listed on a couple of Shimano rear derailleurs, the limit screws (Shimano calls these "Stroke Adjusting Screws") are metric machine screws. Specifically, M4 x around 18 mm. M4 machine screws have 0.7 mm thread pitch and come with various types of heads (aka: caps) such as "hex socket head cap screw" or "flat head slotted cap screw" and others. As far as length goes, it can vary between the high and low limits (the low limit is usually about 2 mm longer). Some models of rear derailleur have identical lengths of limit screws (18 mm). Obviously you can remove the current screws to determine the necessary length, and the type of head is personal preference. Note that it's often easier to obtain a screwdriver than a 3 mm Allen key though I find the socket head cap screws don't strip out near as easy nor does the tool slip out of the business end as easily as a screwdriver.
A well stocked hardware store or home improvement department store (Home Depot, Lowe's, Fleet Farm, et al) will have these metric machine screws in their screw bins. They may be individual or packaged with 2-4 screws per package. You should find the price per single screw around $1 and far less as quantity increases in a package. Regarding length: this is easily adjusted by trimming excess length off a longer M4 screw with a hacksaw, rotary tool, or grinder. Again, 18 mm appears to cover most rear derailleur limit screw lengths. Thus, an M4 x 18 ...cap screw is what you're looking for.
As noted in some comments, blue or purple (loctite 222) thread locker will help prevent the limit screws from moving on their own but still allows for adjustment of the screws with a hand tool.
M3 and M4 are both in use for limit screws. My favorite is to use a wicking threadlocker in this application, either Loctite 290 or Vibra-tite 150. It tends to get really good penetration and works well when there have been problems with the screws working themselves loose. You apply it after adjusting and it locks things up pretty rigidly, but not so much you can't adjust later.