vintagerando

Is there a way to figure out if a set of pedal is French thread without French threaded crank to test them on? The pedals are Lyotard, 1970s vintage.

vintagerando

extra photo

sch

Certainly not from pix, my personal stash of Lyotard Berthets in Fr and Eng threads have indistinguishable threading
as the 20 TPI of the English and the 1.25M threading is identical for all practical purposes. If you measure the
diameters the English threads will be ~0.03-.05" larger than the Fr threads in actual measure,

Fr pedal threads are 14x1.25M, English nominal 9/16x16TPI. 14mm is ~0.55" where 9/16" = 0.5625" so you
would think the difference to be smaller, but manufacturing tolerances for threading result in a larger difference.

Easiest way IME would be to put the pedals on a junker English crank set and see if they are sloppy fit or tight
when threaded in just short of fully seated, say 1/2-1 turn short. Compare with a 9/16 pedal or several.

Good way to ruin an English threaded crank is to ride it with Fr pedals screwed in. I KNOW this. Fr threaded
pedals mount easily and non-destructively into English threaded cranks, it is only under pedaling stress that
the wobble starts eating the threads in the cranks, which become severely damaged in a few miles.

A few years ago I experimented with lathe turning a new set of axles out of 4140 for my remaining set of
Fr Lyotards, one of which had a bent axle. Experiment was a success, not particularly hard to do, make a
set of 9/16x20 axles for the formerly 14x1.25M ones, hence my interest in the actual diameters needed.

cny-bikeman

Look to see if the end of the threaded portion is stamped with a D or G (initials of French words for right and left). If present, definitely French, but still possibly French if not there.

vintagerando

Thanks for response.

fietsbob

I have the same pedals in 9/16" by 20tpi .. Measure anything yet?

Troul 

what are the case nuts stating for grade & what are there pitch? Wouldnt make sense to be compiled of hybrid machinery. Either all or nothing is my thinkings.

sch

Amended my first post to actual measurements. Went and dug out the damaged Fr spindles and the
remaining 9/16" Berthets I still have. First, the 9/16 pedals do have R and L stamped on the spindle
wrench flats. The Fr spindles are blank. IIRC the Berthet pedals now sold off were variably marked but I
don't remember any of the Fr threaded ones having a mark.

Now: actual real measurements: Fr: 0.542 and 0.539" diameter threads
Eng: 0.550 and 0.554" diameter threads

My new experimental spindles I made 0.559/0.560" in size. So measuring might help, depending on
unknown variances over the decades in Fr manufacturing tolerances.

Finally, IME putting Fr threaded pedals into a 9/16 aluminum crank and riding will rapidly chew up the
threads in the crank, basically destroying it. I did so back in the '90s. If the threads in the crank were
steel, it might work at least for a time but would be risking the pedals and/or the crank threads.

JohnDThompson 

The "S" and "G" markings are a good indication of metric pedal threads. The definitive proof would be to use a thread gauge. They aren't expensive, and can be useful in other situations as well.

3alarmer 

.
...if you don't want to buy a thread gauge (they're cheap, BTW), you can use the threaded end a pedal spindle of known quantity as a thread gauge.
It helps a lot in the exercise that there are only two possibilities, so either your known quantity spindle threads match up or they don't.

Ghrumpy

Seconded. A measurement is worth a thousand internet opinions.

Get one each of metric and TPI. (If you really want to be geeky, you can get a Whitworth one so you can measure Italian threads with the correct gauge....) And get a caliper while you're at it. I personally prefer a vernier because of durability, but some new digital ones can give you the nearest inch fraction as well as inch and metric decimal.

Absent a thread gauge, you can lay the unknown thread on a known one. Many are marked, but it's almost guaranteed that any pedal made since about 1990 will be ISO thread (which differs only from BSC in thread form.) A French pedal thread will "roll" a little in a 20tpi.

vintagerando

This seems important, but I am not sure what it means.

Ghrumpy

Identical pitches will lock together, with no daylight between them. Pitches that are really close but not identical won't mesh perfectly, and the finer one will sort of roll on top of the coarser one. (I don't mean roll around, but back and forth, if that makes sense.)

Once you see it happen, you'll get it.

If you have a Campagnolo 26tpi axle and a metric 1.0mm (25.4tpi) axle handy, lay one down on the other, hold them between you and bright white sheet of paper or your computer screen, and you'll see it.

sch

Thread gauge useless to distinguish the pedals as they have the same pitch for the length of thread on the
pedal. Now if you measured out to 20+ mm long thread, then some variance will appear.
20 TPI is same as 1.25M thread for thread lengths less than 22 mm or so.

Ghrumpy

My two thread gauges disagree with you. And they are about the length of a pedal thread.

Ghrumpy

As a practical matter, what you say be true. But IMO you're overselling it. And I'm not sure I understand your math because you don't show it.

As I said already, I can see the difference over the 10 threads of my gauges and on actual pedal threads. So your first statement may just be a matter of how good one's eyes or glasses are working.

But we can quantify the difference mathematically. Over one thread, I calculate it as 0.020mm:
1/20"=0.050=1.27mm.
1.27-1.25=0.02mm
That's about a 1½% error per thread:
0.02/1.25=.016 or 1.6%
That error accumulates for each added thread. ISO spec (and what most modern pedals have) is 10 threads. So the total for a pedal thread is 0.20mm (or about 0.008") and a 16% error:
(You don't really need me to do that math, right?)
I'm no engineer but that seems like that might be a problem. I'll add that after your 22mm length, the threads will be about ⅓ off each other.
Older pedals for steel cranks may have a couple fewer threads, so the error is of course less.
If I have made a mathematical or logical error, please correct me!

Whether that difference is enough to cause fit problems is another matter that depends entirely on tolerances, and as we know, they can be loose. And frankly the diameter difference between French and British pedal threads is usually more of an issue. But pitch is not a non-issue.

dedhed

Go to a good hardware store and buy a .10 9/16-20 hex nut

sweeks

For years, I've used a set of metric taps for this. It wasn't the reason I bought the taps, but it turns out I use them more for measuring than tapping.
I'm going to add a gauge to my tool box now, thanks!
Steve

Hoopdriver

Not quite. A single thread gauge may seem to fit both 20 tpi and 1.25 mm but if you have both English and Metric gauges (and you should) you will see that one gauge is clearly a better fit than the other.

vintagerando

So, ....uhh....a thread gauge will or will not solve the question: are these pedals French thread?

sch

Waited til this am to try the thread gauges and found them to be nearly useless as expected over
the 6 threads that actually exist on the pedal spindles. The metric might have shown a little light
at the end on the 9/16" spindle but the 20 tpi gauge fit both Fr and Eng thread spindles
equally. Now if you had a larger # of threads to run the gauge over it might be more discriminating.
The discrepancy over 6 threads is difficult enough to discern that I don't think this is a valid
method of discrimination.

Ghrumpy

Will the answer determine whether you buy one or not? Because you won't know until you try. Theoretically, of course, it should, but in this case, as we know, the pitch is very close. Because of loose manufacturing tolerances and a small measurement length, a thread gauge might not be as conclusive as it could be otherwise. If you don't want to spring for an actual thread gauge, then a bolt of the same pitch will function adequately (however, it is subject to the same tolerance errors that the pedal threads are. Thread gauges are made to tighter tolerances than most screws you'll find in a hardware store.)
Or of course you could compare it to any other pedal threads you can find, including ½", and see if there's a conclusive difference.

Threads are defined by form, diameter, and pitch, which is why I recommended you get thread gauges AND a micrometer caliper. (Form we need not worry about here, because Metric and BSC have essentially the same form.) If you can't conclusively determine whether it's French or not after making both measurements, then it probably doesn't matter anyway.

JohnDThompson 

In that case, what reason is there not to simply force the pedals into the crank, regardless of thread spec? IIRC, TA actually cut their pedal threads such that they would work with either metric or English thread cranks.

Or, you could try a fancier gauge, like the VAR #167:

sch

Well the point is Fr threads very easily go into English cranks, but being a bit loose
they quickly tear up the aluminum crank threads. Vice versa I have never tried
but putting Eng threaded pedals into a Fr crank would probably work, require only a bit
less than Schwartzenegger torque and permanently rethread the crank to Eng threading.
Most people by the 3d turn of the pedal into the crank would conclude something
wasn't right.

wrk101

In my limited experience, french pedals go into English cranks, but English pedals did not thread into a French crank.

I like to start any threading by hand before using a tool. Use a tool and you might be able to force it in.