MightyTour 

Picked up some NOS TA double cranks in a lot a while back. Trying to figure out pedal threading without forcing anything too hard. Standard 9/16" pedals didn't want to go in. Reverse of DS crank is just '170' which, in the absence of a 'W' denoting English threads, suggests that it's French, correct? The NDS reverse markings are weirder: 'F D 170'.

Anyone got an idea?

spclark 

French for RIGHT is Droite but on NDS side a D makes no sense. For LEFT it's Gauche so the F is still a mystery to me.

I'll dig out my '70's era pair of Atom pedals after b'fast, see how they're stamped. The Stronglight 49D crankset they came with also.

bulgie 

Right-hand thread in French is filetage ŕ droite
Or "FD" for short.

I'll bet what you have is a right-front crank for a crossover tandem.

Can you tell right threads from left by looking? It is possible, but hard to explain in a forum post. Apologies if that's an insultingly basic question for you! No insult intended, we all started there.

spclark 

Think of threads as a ramp as you peer at one open side of the bore.

Right-hand thread drops from left to right, the beginning thread open to the left. Left-hand thread's opposite, open first thread starts from the right, drops from right to left.

(Maybe not so hard to explain but sometimes hard to discern depending on lighting and your eyes.)

I've never encountered a tandem or parts appropriate to one so Bulgie's suggestion I think is spot on. And yes, we all start knowing nothing at all.

scarlson 

I have a TA crank marked FD and indeed it is a tandem right crank.




The opposite is also true, a tandem left crank will be marked with a G at the very least. I've had left tandem cranks that are marked with up to three Gs of different sizes and an F, and I've had some that are just marked G and left at that, no F. Why? Who knows. Probably different years of manufacture. TA has been in business a long time.





I know it is possible to tap out a French threaded crank to 9/26x20 English pedal threads. I've never had the occasion to do this operation, but I have heard it is fairly straightforward and even could be considered easy. I assume this is because the English thread tap can start and self-align on the French threads, which it almost fits into. I am fairly certain that tapping out a French right side tandem crank for left hand English threads would be harder, and might result in misalignment of the tap and resultant threads.

spclark 

100% agree with that, and why I'd approach such a task firmly committed to use of a drill press (or other appropriate fixture) to ensure proper alignment.

oneclick 

Don't bother - or at least don't try to tap right-hand in a left-hand hole (or v.v.) of the same diameter.
There will not be enough thread remaining to hold the pedal.
Use an insert-type repair kit instead.

Reynolds 531 

I've always had bad luck with any of the repair inserts. I would rather use two right hand pedals.



(for the grumpy old men: before you claim "omg it will come unscrewed and you will die", go ponder the left side wheel bearings on your car that goes 65mph daily)

oneclick 

For any decent pair you'll have the pickup-tab and the strap guides in the front.

What you need for that attempt is two right-hand axles (i.e. a re-built left pedal with a right axle)

bulgie 

I have known at least 2 or 3 people who rode bikes with wrong-threaded pedals (left thread on the right, or the converse). They all had problems with loosening; two peole I remember riding with a pedal wrench to tighten mid-ride. I doubt they tried green loctite, but then you'd need heat to disassemble.

scarlson 

Remember Jobst Brandt's solution to the pedal threading problem? He investigated using a conical seat in the crank and a tapered collar attached to or machined into the pedal shaft. Like this:


This is similar to modern cars' conical seat lug nuts/bolts, now used instead of the reverse-threaded-on-one-side flat shouldered lug nuts once used. He reported wrongward-threaded pedals staying put during the course of his tests.

MightyTour 

Thanks guys! I should be able to figure out the pedal threading visually as you mentioned.

So, what you're suggesting is that I have a normal, DS French threaded crank with rings, and then (seemingly randomly) a right side stoker crankarm as its mis-matched pair?

bulgie 

Yes, that or it is mis-marked. Finding out the "handedness" of the crank will settle the issue.

oneclick 

I do, and it wasn't exactly that issue he was concerned with - it was cranks cracking, said cracks initiated by fretting at the flat interface between the pedal shaft and the face of the crank.
The fretting is caused by the pedal moving ever so slightly - with a flat face there is only friction to limit that; the conical surface prevents it.

It's all on rec.bicycles.tech somewhere.

scarlson 

Yes, I remember. I omitted that from my post because it didn't seem pertinent to the discussion in this particular thread. But it is another good point in favor of the conical seat.

It is a wonder manufacturers haven't picked this up. In the past decade, they have eschewed just about every other old standard that allowed us to build old bikes with modern components. Why not pedals too? Think, they could be split between true conical seat and "ball seat", just like lug nuts are today. You'd have to buy new cranks just to buy new pedals half the time. And now since the cranks are integrated with the bottom bracket, you'd have to buy that as well. The bean counters and execs would be thrilled at the cash they'd be able to milk out of the public.