Jim, et al,
Pitch has to do with the number of threads as in TPI - THREADS PER INCH in the inch system or the width of a thread in the the metric system - a 1.0mm thread is 1mm wide from the bottom of one "sharp" V to the bottom of the next.
PITCH DIAMETER is a measurement of the diameter of the threaded area used mostly during manufacturing. It's measuring a theoretical point halfway between the Crest (top of the V) and Root (bottom of the V) of a thread with dead sharp V shapes.
Dead sharp V threads are rarely used on mechanical fasteners. They're mostly found on pipe threads and similar applications that require an interference fit.
Since most mechanical threads (bolts, nuts, studs etc.) have either a flat or radius at the crest and the root of the threads you need a specialized Thread Micrometer with sharp V points or even better, one with a V point on one anvil and a V thread shape on the other.
>http://www.google.com/products?q=+thread+micrometer&hl=en&aq=f
Pitch Diameter measurements are part of a system of Tolerances. You need a certain amount of "slop" in the fit of fasteners to allow for easy assembly and disassembly. These tolerances are referred to as the "Class of Fit". Most classic bike components fall of the scale in this area!
You can't accurately measure thread diameters with a caliper or micrometer. All you are measuring is the Major Diameter of the threads (or Minor Diameter in the case of ID threads).
Getting back to bikes, there's a widely held myth that classic bikes are precision machines. THEY ARE NOT!
Prior to the late 70s, most European bike and component manufacturers used antiquated manufacturing equipment and processes with some machinery predating WWII!
There are thread dimension standards for bike components but I doubt seriously that they were ever checked very closely. I've rarely seen accurate threads on older bike parts!
Recently I was building up a French frame and installing a Stronglight BB. I couldn't get the fixed cup to start in the BB. I chased the threads with a VAR tool but still no go so I grabbed another Stronglight metric fixed cup. Same thing, they were both marked 35 x 1.
On closer examination they were both left hand thread Swiss fixed cups with French markings.
Out of curiosity I measured the thread diameters of about 20 British and metric fixed cups. They included different model Campy cups, Stronglight, Shimano, Sugino, TDC cups plus some old cheap cups. I used a Mitutoyo digital caliper.
As most of you know,
ISO 1.375" x 24 TPI (34.93mm) British 1.370" x 24 TPI (34.79mm) French and Swiss 35mm x 1.0mm (1.378")
The OD of the threads on the 20 or so fixed cups ranged from one cup at 34.69mm to the majority of them around 34.79mm. French or inch size, it didn't matter!
I've had similar results measuring British and metric freewheels and hubs.
As I've said before, you can't accurately determine the threads of these parts by measuring the major diameter (or minor diameter on ID threads) of the threads. They were never made that accurately!
You need to use a Thread Pitch Gage:
>http://tinyurl.com/ybj9v7r
Even with one of these tools you need to carefully eyeball the fit because the inch and metric size threads used on bikes are pretty close and the first few threads will match up.
Theoretically, hubs and freewheels had tight to slight interference fits so that they wouldn't come loose. Same thing with BB cups.
British treaded freewheels on French hubs has been well covered by others.
Chas. Colerich Oakland, CA USA
jim abt wrote:
> Here is a question: When you measure the outer pitch (the very outside diameter) of a French threaded hub with a caliper what is that diameter suppose to be and give the tolerances if you know them? On the same token when measuring the inside diameter of the freewheel with a caliper what is that suppose to be on a french threaded hub. Oh...while we are at it how about the rest of the "standards" as well. If you tell me to reference Sheldon Brown, I have....thousands of times but it seems as though he is giving the minor pitch diameters and that is not the readings that I am going to get when measuring with a caliper that I purchased from the man in a trench coat on the corner....and you thought all they sold were watches. The other thing that you may want to share is what is an inexpensive way to read the threads per inch...is there a cheap gauge out there somewhere or do I need to go back to the corner? Seeya
>
> Jim Abt
> Wausau, Wi. USA