A year or two or three ago I acquired a Campy Nuovo Gran Sport RD. I've been wanting to put it on a particular bike which has Campy DO's but it doesn't want to thread on. I've mounted a Suntour Cyclone II on the bike and it went on easily. The Campy RD just doesn't like the threads. It will go on no more than one turn and isn't happy about going on that much. I don't want to force it with a big wrench.

The threads do not appear damaged. They appear to be the same as various Suntour RD's in my parts stash, but perhaps not.

Might Campy have made a version of this RD with, say, French threads?

It's not an earth-shattering question, but that RD seems to be unusable for now.

Campagnolo dropouts use 10mm x 26 TPI, mixed metric and imperial threading. SunTour and just about everybody else uses 10mm x 1mm. Also, there's a slight difference in the angle, crest and roots.While they're close, all these differences mean they're not compatible. Installing the SunTour has reformed the threads, damaging them in the process. Ideally you'd want to run a 10mm x 26TPI tap through the dropouts to clean things up before installing the Campagnolo derailleur.

Thank you. The Cyclone went on very easily as if it had been meant to. I had tried the Campy first and it didn't go. The fact is, I have no idea what this bike's history might have seen. It is the Belgian Bertin, English BB, 26.0mm seat post.

But waitaminute. I recall doing installing a 1st get Cyclone on the Masi for a test ride and it went on easily, and then a Campy NR which also went on easily. I've hung out around (and wrenched on) little furrin' sports cars all my adult life so I do know what forced threads feel like. On the other hand, the Masi's history is also largely unknown. Evidence point to it once having carried DuraAce brakes and Suntour bar-end shifters, so the derailloeurs could have been anything.

You may be able to find a SunTour bolt which will work w/ your Campag. derailleur. Otherwise, something indeed needs to be rethreaded.

I've had this same thing happen to me. I took the Campy Bolt and ran a 10mmx1mm die. Problem fixed.

Do you have screw pitch gauges Jim? Might try threading the Nuovo Record top bolt in and see if you get the same result.

There is no such thing as a French-threaded bolt.

...and good luck finding one of those! You'll just have to force it on.

No, I don't. But a thread-to-thread matchup of the Cyclone and Gran Sport bolts sure makes them look the same. Maybe the threads aren't long enough to show the discrepancy. I'm not going to take the NR off the Masi just to check the threads. (That's what C&V is for .)

Fair enough. I was just thinking maybe the GS had come off a Simplex DO, but since I've never owned one I wouldn't know.

In fact, until recently I've never had much reason to think about it.

There's one in every Campagnolo tool kit. Almost every LBS who carried high end bicycles had one of these, as it was the cheapest method to get a complete set of facing, reaming and threading tools. Given that the OP lives in the Boston area, I imagine there are a few shops around that would have them.

As suggested by Michael Angelo, using a 10mmx 1mm die on the derailleur hanger bolt is another option, but tapping the dropout is preferable, as it is softer steel than the bolt.

Of course, it can always be just forced in, as you suggest. A lot of Campagnolo dropouts had Shimano bolts forced into them in the late 1980s, after SIS arrived. The derailleur bolt is not a high load situation where proper thread form is critical. Still, I wouldn't want to be switching back and between Italian and non-Italian derailleurs.

No, a 10mm x 26tpi tap was not part of any Campy tool kit I've ever seen (and I even own one!). It doesn't even appear in the Campy catalog.

Edit: I take that back! It does appear in the 1974 catalog, at least, but not as part of any of the tool cases. And FWIW, I've never seen one in all my years.

I used a 10mm x 26 Titanium hanger bolt to clean-up the threads on a messed-up DO hanger. I think is was an SRP replacement bolt I had laying around. They're also in a Super Record RD's.

One more thing is that there is some degree of tolerance variation on threaded holes.

I've come across more than one pair of older, BSA-threaded Campag crankarms which required a lot of torque in order to thread in a pair of Shimano pedals, even though the thread spec on both parts is the same.

I usually thread in the interfering part with a back-and-forth motion as needed to avoid exceeding a maximum level of hoop stress in the female threaded part. I've even used this method to force a BSA Shimano cartridge bb into a Swiss-threaded bb shell, with enough "loosening" back-and-forth cycles to allow a feel for the torque of the cups bottoming on the flange mating (right side) and the bb cartridge (left side).