smontanaro 

I've never messed with the grub screw on NR/SR rear derailleurs, confining my adjustments to the high and low limit screws. What purpose does the grub screw serve? (See p/n 812/1A in Catalog 17, p8, for example.)

Thx,

Skip

conspiratemus1

Isn't that just the cage stop that limits the backwards rotation of the cage under spring torsion? You would remove it to allow the cage to unwind all the way, relaxing the spring completely, if you wanted to remove the cage from the body (to replace a broken spring, e.g.)

IthaDan 

Yup, Taking the grub screw lets you preload the cage spring (and remove the cage without getting a facefull of spring and gunk), to rebuild a derailleur from a pile of parts, you take the grub screw out, give the cage a full [reverse in relation to the wheels rotation while riding] turn and then put the grub screw in so the cage relaxes to where the cage is scissored on the parallelogram.

A grub screw isn't totally essential, but it sure is handy- you'd have to remember to preload the cage every time you removed the rear wheel for the RD to function properly. And without a grub screw, you'd have to remember to move your face to dodge the cage and chain as they whip around once you remove the wheel.

conspiratemus1

Sounds like you are speaking from experience....

IthaDan 

You can read all about my facial reconstruction on webmd.

Ok, it's not THAT dramatic, but it's pretty scary when you're not expecting it. One turn is really pretty minor, but all it takes is one over-zealous hipster to think "if one turn is good, then TWO must surely be better", then take that into the coop like a ticking bomb. I always make a quick check for a cage stop when taking off a rear wheel these days.

muysal

I have a SR Pat. 84 with a broken 812/1A. During last chain installation, not knowing it's function (until I read this thread which I'm thankful for), I made a foolish attempt to remove this screw and it came out with end section broken off, one half probably still lodged inside. If not mistaken the hole for 812/1A is open on both ends? Maybe it's possible to drive out the broken section with a replacement screw or it's my imagination.

I'm guessing as 812/1A turns clockwise, it must be pinching the cage spring?

I'll ask Campagnolo if they have specs on 812/1A since it seems to be as rare as a black opal. They'll probably think I've lost my mind or travelled into the future from 1984. Worse case scenario I may have to wear my hockey face mask while removing the rear wheel

rootboy 

The grub screw doesn't pinch the spring. It merely threads into the spring cage, #4009, to act as a stop, to keep the pulley cage assembly from unwinding. The spring cage is threaded all the way through to accept the grub screw, but I doubt you'll have success trying to "drive" the broken piece of the grub screw out, using another grub screw or similar.
You may have to source another spring cage from another derailleur.

What specs on the grub screw are you looking for?

repechage

Boulderbicycle has a derailleur parts lot available right now for under $15. Used but much easier than figuring out how to EZ out the buried nib and locating a set screw the right thread and length.

muysal

Thank you for your reply. For specs on 812-1A I meant length, diameter, groove pitch, etc.; everything required for a local machine shop to duplicate the screw. It doesn't have to be titanium. I just need to restore the derailleur's original functionality.

Because of the broken grub screw, the entire spring cage assembly needs to be replaced? If yes, there're quite a few parts involved; the bolt, bushing, spring, cage, cage cover and grub screw.

After broken grub screw discovery, tried removing the spring cage bolt; pushed down on the bushing spring and tried turning the spring cage bolt without success.


Is it possible to drive out the broken screw by drilling a slightly larger hole into the spring cover to restore original function?

Road Fan

Take the whole derailleur with the remains of the screw to your hardware store and ask for an EZout and matching drill, and instructions on how to remove the remains of the broken screw.

Or ask them if they can get it out without damaging the threads in the cage body. A Campy derailleur is at one level a smallish machine, and dealing with even small broken screws, so if they have a service shop they might be able to just take care of it all for you. $20 to the local guy could be a lot cheaper than buying a new cage or the overall assembly, and some new tools. Then measure the diameter and thread of the extracted stub or the part that broke off, or ask them as well to see if they can replace the broken screw for you. It does NOT need to be the original Campy screw, and a stainless replacement would stand up best to the elements. Metric fasteners are pretty common these days, even grub screws.

This is an example of "you weren't supposed to unscrew that."

rootboy 

I haven't been logged in much, sorry. Boy, this seems like an awful lot of work.
But necessary , I suppose. Unless....

Disassemble the derailleur and see if you can get the broken off piece out with an easy out, Mysal. If not, I may have one of those cage spring housings, probably used. But it would take me longer to find than it did these.
Send me a PM, if you can. Maybe not. I'll send you one.....

muysal

I looked at BoulderBicycle. It was several vintage Campy SR parts from the spring cage assembly, not complete, somewhat rusty and scratched up but included the pulley cages, spring cage bolt, cage cover and 'grub screw' all for $14. It wasn't a bad deal but I hesitated. When I went back to purchase, it was already gone

CroMo Mike 

Forget the easy out. Take the spring housing off and place it with the screw hole side down (in a vise or better yet on a drill press). Use a drill bit about half the diameter of the broken screw and drill into the broken screw from the other side. When the drill gets a bite in the broken part it will screw right out the way it went in. If it doesn't screw out, you will end up with a hole through it that can then be used to extract it, either with an easy out, or drive the pointy handle end of a small file into it to use as an easy out.

AlexCyclistRoch

Page 8 has a bunch of mid-70's Formula 1 cars....

FWIW, it looks like part # 814, the cage plate, has 2 threaded holes, so I assume that screw 812 is needed to set the cage angle. I assume the head is rounded so as to not catch anything else. A set-screw with the same pitch should work as a replacement.

muysal

CroMo, thank you for your suggestions and everyone else on the thread including the friendly reminder "this is an example of 'you weren't supposed to unscrew that'." I unscrewed, it broke, extracted and now understand better how it works. I didn't have much success with EasyOut, so drilled the broken screw with a drill-bit much smaller than the diameter. Drilled through the center all the way through then used a screw-driver to drive it out from the bottom. Like to thank rootboy for selling me one of his spare screws at a discount. I will sell the broken one on eBay for $85 dollars plus s&h .

CroMo Mike 

Ahh! The sweet sound of success!

corrado33

That exact same thing happened to me when I took a wheel off a bike that came in for service. I took the wheel off and WHAM! I wasn't sure exactly what had happened, so I ignored it. Then I went to put the wheel back on and found the derailleur was slack (wouldn't hold chain tension) until I twisted the cage around 360 degrees. I'm not sure if something was broken on that derailleur or what, but twisting the cage around then installing the wheel (which would prevent the cage from "unwinding" made the derailleur work normally, so I didn't worry about it and the parton seemed happy.

IthaDan 

What's the thread on these? The usual m5x.8? If that's the case it'd be VERY easy to fabricobble one of these by cutting the head off a partially threaded longer bolt, and then chucking up the headless shaft in a cordless drill to spin it against a grinding wheel to approximate the dome.

Use a dremel to cut the slot (or a hacksaw?) and bob's your uncle.