Campagnolo M10x26TPI taps were available back in the 70s but were extremely expensive. We had 1 or 2 taps that size at our shop (maybe VAR brand) but we had several standard M10x1 taps too.
I think that SunTour and Shimano used the standard M10x1 thread size.
26 TPI = .0385" Pitch (.978mm) 1 mm = .0394" Pitch (1.0mm)
The .0009" difference in the thread pitch between 26 TPI and 1 mm falls into the realm of "thread slop" or normal manufacturing tolerances. Especially so over a distance as small as 7 threads (dropouts were generally 7mm wide so 7 x 1mm pitch).
I don't know that the dropout threads were ever chased before assembly at most bike factories, even on top name bikes like Gios and Cinelli.
The first derailleur mounting bolt usually "chased" the threads in the malleable steel used in dropouts!
We chased the threads with a tap on every frame we sold or assembled. Also the 3mm adjusting screw holes (unless they were titanium or aluminum).
Chrome plating can add several .001" to the threads reducing the ID diameter of the dropout hole.
Taps used for threading holes that are going to be plated are .005" to .0055" oversize to allow for this. I suspect this was never done and that dropouts were used as they came pre-threaded from Campy or other suppliers.
Chas. Colerich Oakland, CA USA
John Thompson wrote:
> On 07/17/2010 01:33 AM, john strizek wrote:
>> Yes it may be true that a Campagnolo axle can be used as a thread
>> chaser. I n fact obtaining an axle may be more costly than the few
>> dollars for a proper tap to do the job properly.
> Although this may generally be true, finding a true 10mm x 26tpi tap for
> the derailleur hanger may be a bit of a stretch. At least I've never
> seen one. 10mm x 1.0mm are abundant and inexpensive.
>> I am fortunate that I have a VAR tap for old style Stronglight
>> threading. I obtained it from Tom Hayes at fair cost. That has been a
>> lifesaver more than once. Buggered "pulling" threads have been a
>> problem since the protective caps became out of fashion.
> Is this the VAR #23? http://www.os2.dhs.org/~john/var23.jpg
> I have one but haven't yet had the need to use it. Did you find it
> "bottomed" properly? It looks more like a plug tap than a bottoming tap
> to me.