Retapping derailer's cable clamp?
 

I got hold of a Superbe Pro RD that was extremely rough, cleaned it up a bit, put it on a bike and discovered a bigger problem - the threads inside the clamp for the cable are stripped. Does anyone happen to know what size thread/pitch the original bolt would have used? I'm not convinced the bolt that came with this unit is the right one anyways, would like some suggestions as to what size tap I might best use to correct the problem.

The derailer in question:
http://s8.postimg.org/j4ckwl2b9/Superbe_Pro.jpg

The pinch bolt? It should be 5Mx.8, I reckon. If it's stripped of some metal, you'll need to tap it larger (6Mx1.0) and use the corresponding bolt. You'd then need a larger washer.

It appears from your photo that perhaps there are two washers under the anchor bolt head.

There should be one washer that clamps down on the grooved (for the cable) surface surrounding the tapped hole.

Perhaps removing the thick washer will allow sufficient thread engagement? Or perhaps a slightly longer bolt?

Now, since this isn't an indexing derailer, one could tap oversize, and the larger bolt wouldn't push the cable aside so as to alter the derailer's indexing geometry, but it is a big step up to a 6mm bolt, and those 6mm threads are rather coarse.

I have long wished for a slightly-oversized 5mm bolt to become available, perhaps 5.5mm, for restoring fit to damaged threads.

I bet there's an Imperial or other non-metric size that could work as a slightly bigger than 5mm option

That would be an American #12 NF bolt size. Not common though.

Thanks for the responses, I mostly wanted to make sure I wasn't missing anything obvious. I did try to tighten the pinch bolt sans washers but the threads are too worn to engage regardless. I will try retapping the hole to 6Mx1.0 and see how that works, thanks guys!

If there isn't sufficient material to drill for 6M, (and if you have enough clearance, and can approach from the right angle) you could insert an ultra-low profile cap screw from the back side, and put a nut on it to pinch the cable. I can't tell from the photo whether it will be possible, but it's fairly likely, I think.
[edit] link didn't seem to go through correctly, but they're under stainless cap screws...
http://images2.mcmaster.com/Contents...g?ver=29925666

I think that this is another good idea. ^^^^^

I've used a similar specialty bolt with a no-wrench flat head, installed from the back, to fix a 10s Dura-Ace 7900 rear derailer.
The 7900 derailer was indexing type of course, so a larger-dia bolt would have likely messed with the cable anchor position.
The specialty bolt came from an unknown derailer, and was tricky to get fitted (until I ground it a little), but solved the problem while providing durable steel threading.

This also doesn't require the use of a bottoming tap, which the OP's derailer might need to go in far enough with the tap(?).

Interestingly enough, I just had a kid at the co-op who had the identical issue with a rear derailleur.

Th hole is not a blind one, so there's no issue with using a regular tap. He ended up having to go
up to an m7 screw and tap, because his attempt at m6 somehow ended up ovalized.

It's alloy, so unless you're really anal about such stuff, you can just slowly use the tap itself to
go one size larger in most instances, if you can get it in and started. But drilling is your preferred
alternative, and is safer. Home Depot sells a set of metric taps/dies that goes from m3-m7 that is
very handy for use on bicycles.

Be careful about drilling it out to a larger size. It doesn't look like there is a lot of metal around the hole. inserting a 5mm bolt from the rear is a better idea. It may need to have the head ground down to clear.

One of my old bosses was a machinist and now and then he would do that on a bottle boss. I think 5m corresponds to 10x32 and he would tap to like 12x24 or something. He had the tap and stash of screws in his fancy wooden tool case that we were forbidden to go near.

LOL, like if you toucha my cutting tools, I'll cut off your hand, like this.

Funny story here about an inconsiderate who once almost "burglarized" my closed-up tool rollaway, took it upon himself to procure an exact-size, new, larger-sized (expensive) drill bit so that he could add some bling to his Subaru in the form of a trendy/yuppie fog-light installation.
I think that the hole was so big because the bolt was perhaps hollow, for the wires.

The not-so-lucky bloke drilled away, and put that honkin' bit right through his alloy air-conditioner/radiator, blew out the pressurized coolant and had to pony up several hundred bucks for the new condenser, coolant and installation.
That seemed like a lot of money back then, I was still a working student. I would have liked to witness the sudden release of the freon gas coolant, figure he reaped what he sowed.

Oh and yeah, the #12 NF bolts are 12-28t, a little coarser than the 5mmX.8, and perfect for getting good purchase in soft aluminum.
The worst that you could say for this fix is that the bolt will not have a metric 5mm socket.
But a bolt that comes thru from behind the arm is more of a perfect solution, provided the thin head can be shaped and situated to prevent rotation (if not, there's Loctite).