Derailleur hanger thread size?
 

I was trying to straighten the brazed on derailleur hanger on my old steel bike today, and I couldn't get my Park DAG-2 to thread into it. Was there a different standard back in the 80s? The derailleur is the classic Campy NR that I had put on the bike when it was built. I just figured this was a standard that hadn't changed.

I have a 10sp Shimano wheel on the bike and I can't use the largest cog because the DR hits the spokes. I can see too that the hanger is slightly angled in. It could well be that the old style derailleur just won't work in that cog. For the rest though it works really well. 10sp cassettes and chains shift nicely, and there isn't much space between cogs so the shifts are quick with the downtube friction shifters. Everything is nice and quiet again after lubing the pulleys and chain. This was my bike from the 80s that I had custom built, and my son has absconded with it but brought it back because it got noisy. (I keep telling him to get a modern bike but he doesn't see the need.)

Modern hanger thread is 10mm x 1mmpt. This is virtually the same (for the 6 or so threads engagement) as the older Campy thread of 10mm x 26tpi (and note the combo of metric and standard units, ask why and I'll tell you). It's quite possible that the hanger threads are distorted. The hole might not be round. The hanger could no longer be flat (thing potato chip like). The initial threads might be squished. I would consider chasing the threads only after confirming the hanger's shape condition. Any "blacksmithing" you need to do should go before the thread chasing. Andy

I guess any of the above could be true, but that Campy derailleur has been on that hanger since the bike was built. This was the first time it had been taken off since the bike was built in '81. In the meantime I'll just set the limit screw so I don't destroy the wheel, although I've managed to avoid doing that in the 14 years it has been on that bike. Since the hanger is a braze on and integral to the frame I'll probably just leave it the way it is.

For the long time BF'ers out there that may happen to be reading this, the wheels were purchased from the infamous R600DuraAce that frequented the forums. I guess I got a good deal as I'm still using them (or my son is), a Velomax Tempest II pair. The front wheel was on my brand new Kuota when I crashed it hard in '05 and broke the frame. The wheel didn't have a dent.

And why the combo of metric and standard?

Run the tap through from the inside out to chase the threads. If the starting threads are damaged running it from the backside will catch good threads first

Given the greater story I suspect that the threads are "fine" but need chasing out. Running a threaded something from the backside can give more data on thread condition, like is only the initial threading the issue or is the complete hole in need of attention.

Many of the Euro bicycle dimensional standards were developed during the hectic (economic and political) times of the 1920s-1950s. Between wars, market depression, trading partners changing, national alliances shifting basic manufacturing also shifted about. Each country had it's own threading standards and it was only as exporting became more viable (compared to only distributing within one's country) did these conflicting standards compete in the greater market.

I have heard that Italian bicycle industry came upon (bought outright or acquired by other methods) English or US machine tooling (lathes). These are not cheap to replace and changing the screw cutting (threading) gearing that were present on these non metric machines determined what thread pitches were cost effective to use. But screw diameter could be changed by just specing some other major/minor diameter. So like any business trying to make a buck and not have to replace otherwise well working tools they just retained the non metric thread pitches but adopted metric where they could do so cheaply.

This story is much like why Schwinn retained their "Chicago" style of frame construction for so many decades, even after the market said thick walled and small diameter tubes were not the future. I have been told of Schwinn having gotten much of their tubing tooling (that was used until Schwinn started outsourcing) via government subsidy due to WWII. Why change it if it was still working? Andy

I did try to run the threads in from the back side but I still couldn't get the DAG-2 to go in. I just assumed that it was a slightly different thread at that point and posted this thread (a thread about threads!) As I said, I'm just going to leave it.

When I think about the thread pitch vs. diameter if it is the pitch that is different I should be able to thread it on but then have trouble at a point turning it in. In this case though I couldn't get it started at all. The bike will just have to go another 39 years the way it is.

No, the bike can be changed. Andy

try threading the derailleur onto another bike where the DAG-2 fits. If it fits, then it is the same thread.

Not sure what this thread is really about. You have the tool and frame. Why not examine them closely, inspect the threads, measure them, then you will know. You want to know, right? You want to figure out how to use low gear, right?

If I cared about the lowest gear I'd have done something different 15 years ago. Instead I was happy with 9 of 10 gears. I just figured I had the tool and I'd straighten it since it was already on the stand. I've had the tool for about 5 years and it took this long to even try it. Clearly not a big priority.