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Tapping limit screw hole threads?

Old 03-14-21, 10:06 AM
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acm
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Tapping limit screw hole threads?

I bought a replacement for my worn out (and elusive!) Suntour LeTech RD on ebay. Advertised as working, pics looked good to me, yadda yadda. One I actually installed it on my bike and began setting the limit screws, I noticed the b screw just turned in place and would slide right in and out of the screw hole with enough force (pressing very lightly on the parallelogram or pushing the screw in, etc). Compared it to the screw on the one I'm replacing and it's completely different!

The screw ought to be a 4mm with a very fine pitch (too fine to measure with my vernier). The yahoo who replaced it used a 1/8" screw with a *significantly* coarser pitch.

Would tapping be an option? If so do I need to tap it to a larger size? Because the threads are absolutely destroyed.

I do have an RD I can use in the interim, but this was part of the original group and I prefer the look of it so I'd like to salvage this is possible instead if returning.


Last edited by acm; 03-14-21 at 10:22 AM.
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Old 03-14-21, 10:33 AM
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Well, since the threads are destroyed and a tap cannot replace metal, it would follow that you will need to drill and tap a larger hole. Lube the threads when inserting the screw.
Longevity hint; when adjusting a derailleur, relieve the spring pressure by hand and adjust the screw, rather than using the screw to drive the derailleur against the spring.
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Old 03-14-21, 10:35 AM
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Originally Posted by dsbrantjr View Post
Well, since the threads are destroyed and a tap cannot replace metal, it would follow that you will need to drill and tap a larger hole. Lube the threads when inserting the screw.
Longevity hint; when adjusting a derailleur, relieve the spring pressure by hand and adjust the screw, rather than using the screw to drive the derailleur against the spring.
Thanks, I figured that would be the case but I've never tapped a hole before so I didn't want to make any assumptions. Thanks for the tip as well!
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Old 03-14-21, 12:22 PM
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Well make sure you can drill it out to larger diameter. Use a tapered tap, not a bottoming tap.
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Old 03-14-21, 12:38 PM
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Do you already have the tap and handle? You might do just as well by getting a new rear DR, again. They usually aren't that expensive. Unless you have to have that exact DR.
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Old 03-14-21, 12:46 PM
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Do you already have the tap and handle? You might do just as well by getting a new rear DR, again. They usually aren't that expensive. Unless you have to have that exact DR.
No, but this is a good excuse to get one, haha. It doesn't *have* to be this specific one but I would prefer to use it. I have a Shimano Tourney I've been using on this bike that works just fine, but the bike has a new paint job and I polished all the old silver components, the black RD sticks out like a sore thumb.

What can I say, I'm a stickler for looks
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Old 03-14-21, 01:00 PM
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Originally Posted by acm View Post
No, but this is a good excuse to get one, haha. It doesn't *have* to be this specific one but I would prefer to use it. I have a Shimano Tourney I've been using on this bike that works just fine, but the bike has a new paint job and I polished all the old silver components, the black RD sticks out like a sore thumb.

What can I say, I'm a stickler for looks
If you are still here in 10 year, let me know how many times you needed that set of Tap and Die set. <grin>
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Old 03-14-21, 01:11 PM
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Originally Posted by Iride01 View Post
If you are still here in 10 year, let me know how many times you needed that set of Tap and Die set. <grin>
I've spent more on less useful things 😉
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Old 03-14-21, 01:52 PM
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Hold up here. You say you have a 4mm fine pitch screw. That's probably MF4 (with a 0.5mm* pitch, btw, in case you want to try to order one). The tapped hole that this fits into has a minor diameter of 3.459mm.

A 1/8" screw has a major diameter of 25.4mm / 8, or 3.175 mm. So a perfectly intact tapped MF4 hole could be inserted into a MF4 tapped hole without much damage at all.

I suspect that the comparator screw you show there is the other limit screw. Double check to ensure that it's not an MF3.5 screw. But if it measures 4mm (about 3.90mm typically) I suspect the derailleur started life with MF4 and 0.5mm pitch*.

If so, I'd start by buying that size screw. First, this should be a good pitch gage to compare to your other screw (To ensure it's 0.5 pitch and not 0.35mm ptich). If it has good enough purchase, great. If not, you may have enough meat left to to drill out to use an MF4.5 x 0.5mm thread. This should use about a 3.9mm drill, but check with the tap provider you buy from: they may have some suggestions.

If the threads have gone to thread heaven, you'll need the tap (tapered, I think, not bottoming style), a properly sized drill bit (and a 4 or 4.5 mm thread using a drill SMALLER than 4 or 4.5mm!), a tap holder, a drill press, a vise to hold the derailleur (this may be the hardest thing to do: hold the RD). If you're very good with your hands and you have a "sporting" approach to life (that is, if you break a tap off in the RD, you'd chuckle and say "well, I tried", rather than throwing your bike through a window in frustration) you could mount the RD on the hanger and use a handheld drill. You also want to get some appropriate tap cutting fluid to lower the odds of breaking your tap.

One key is to drill the hole straight. Another key is get the tap started in perfect alignment with the hole. That's why the drill press is easier: you drill the hole, replace the bit with the tap, and then hand turn the tap into the hole, using ample tap fluid. People tap holes manually all the time, though. That said, I've been doing stuff like that a long time and I break off small taps like 4mm occasionally.

Good luck. But check the proper sized screw before drilling anything!

*There's also an MF4 with a 0.35mm pitch. I'm guessing hard to find.
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Old 03-14-21, 02:01 PM
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Woah, thanks for the thorough reply!

Yes I'm not the type to throw a fit becuase something doesn't go my way, I like doing things like this and if it doesn't work out, hey, not the end of the world.

My math may have been wonky on the imperial screw size, but it was bigger than the b screw that should be in there (I have it pictured with the b screw from an identical albeit heavily worn RD). The 'right' screw measured about 3.89mm and the 'wrong' one was something like 4.1. Not a crazy crazy difference but the radical difference in thread pitch completely trashed the inside threads. There's almost nothing there holding it in besides friction.

But yeah, I'm not going to pretend that this is a reasonable approach to this problem when I have another working RD. I just like this one and I like learning new shop skills
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Old 03-14-21, 07:38 PM
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Most hardware stores sell a drill and tap set for a given size.which is cheaper than buying a whole set for a 1 off use.

https://www.homedepot.com/p/Drill-Am...5X-8/304746971
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Old 03-14-21, 07:48 PM
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Ah, so a 4.1mm screw may be a 3/16 inch screw. Probably not metric (an M4 would be about 3.9mm, and M4.5 about 4.4mm). So, come to think of it, its probably an #8-32 screw. But no never mind: if the hole is 4.1mm ID then you likely have enough meat needed for a 4.5mm screw. (The drill you normally use for cutting 4.5mm fine (MF4.5 x 0.5mm) threads is 4.0mm). I'd be tempted to try it. The ideal tool would probably be a what's called a form tap (they squish the threads into the metal) and not a cutting tap but... But a form tap's gonna be expensive (40 bucks) in this size. If they even make them in that size.

Taps come in different tolerances. While tapping using anything else than very precise equipment kind of negates the effect of tight tolerances, I'd use the tightest tolerance tap you can find (4H probably). You might have a hard time finding a MF4.5 x 0.5 Class 4H tap, though. 6H is "normal fit".

Anyway, run a 4mm drill through the hole and then try to tap with the MF4.5 x 0.5mm tap. Be VERY sure that the tap is aligned with the hole... Then an MF4.5 x 0.5 screw should work great (you may have to to search a bit).



Good luck.

Last edited by WizardOfBoz; 03-15-21 at 06:09 AM.
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Old 03-15-21, 06:41 AM
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Originally Posted by Iride01 View Post
Do you already have the tap and handle? You might do just as well by getting a new rear DR, again. They usually aren't that expensive. Unless you have to have that exact DR.
Lowe’s have paired small bits and matching taps - iirc well under $10 for the pair.
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Old 03-15-21, 06:45 AM
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Originally Posted by Litespud View Post
Lowe’s have paired small bits and matching taps - iirc well under $10 for the pair.
I ended up picking up a whole metric set of taps/dies/handle etc for $30 on amazon. Overkill? Yes. But I (ideally) have many years left on this earth so if I ever need a metric tap again in that time I'll have it on hand, haha. Plus I just got that stimmy check, I don't spend frivolously but I won't sweat $30 out of $1400
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Old 03-15-21, 11:02 AM
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Originally Posted by acm View Post
I ended up picking up a whole metric set of taps/dies/handle etc for $30 on amazon. Overkill? Yes. But I (ideally) have many years left on this earth so if I ever need a metric tap again in that time I'll have it on hand, haha. Plus I just got that stimmy check, I don't spend frivolously but I won't sweat $30 out of $1400
While this job is aluminum, I'd be very cautious with a $30 imported tap set in a critical application, or hard to get at spot, especially in small diametrs.
While I own a cheap set for some things, when things are on the line, it's going to be a US HSS tap or die.
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Old 03-15-21, 12:12 PM
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Aluminum actually is harder to tap than most steel. I've broken more taps in aluminum than any other metal. Even with a CNC mill doing the tapping, aluminum will just make you cuss it more.
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Old 03-15-21, 12:41 PM
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Keys to tapping aluminum are using the proper kind of tap flutes, clearing chips often. I use WD40 when tapping aluminum.
Another helpful hint is to check the tap in a drill press and turn it by hand to keep things straight.

Tap Recommendations for Specific Materials - Tapmatic Corporation
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Old 03-15-21, 07:46 PM
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Originally Posted by acm View Post
I ended up picking up a whole metric set of taps/dies/handle etc for $30 on amazon. Overkill? Yes. But I (ideally) have many years left on this earth so if I ever need a metric tap again in that time I'll have it on hand, haha. Plus I just got that stimmy check, I don't spend frivolously but I won't sweat $30 out of $1400
The good news is that you'll have a serviceable tap handle. The bad news is that set may not have metric fine in 4.5mm (MF4.5 x 0.5) which is what I think you need. You can try a cheap Chinese tap for about 8 bucks (on ebay) or get a US made tap for about 35 bucks. As others say, you can use WD40 as a a tapping fluid. And cut a little and back off, making sure that the threads are cutting in evenly (indicating your centered). Good luck.
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Old 03-19-21, 03:08 PM
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Hello folks - just wanted to provide some closure to this thread as I finally had time to finish this project today. After re-measuring everything I referenced a drill-n-tap chart and determined what size screw I had to go up to (M5 x 0.8). Yeah, the coarser thread pitch is not ideal for something like a limit screw but, like, c'mon, it's a b-screw. Not all that important to be super precise there.

Anyway, I tapped it freehand as I don't have easy access to a drill press, making sure to fixture it well in my bench vise first. WD-40 as cutting fluid. Was very ginger about the whole process and it paid off. I have a functional limit screw again!

Thank you so much to everyone here in the thread for helping me with *my* threads Your tips and guidance made the process painless and fun!
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Old 03-20-21, 08:53 AM
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I like this kind of project. 1) You salvage something that somebody made, 2) you have the satisfaction of learning and completing a project, and 3) you get to go riding.

I'm an engineer and sometimes I let the perfect be the enemy of the good. The decisions you made (M instead of MF, 5mm instead of 4mm) seem like reasonable ones to get the job done and get you back on the bike. Very cool.

Enjoy.

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