Thread stripping in crank arm?
 

I just picked up a 1977 Super LeTour for tooling around town on a weekly Taco Tuesday ride of about 10 miles. It was priced right, seems pretty bulletproof, and it's the full-chrome version, so it's nice and shiny. Perfect for days I don't want to be the guy in lycra. I'm swapping out the brake pads for something made in this century, and I'll probably recable it, but otherwise, it's in great shape.

I took it for a test spin at lunch today, about a mile in each direction. I wasn't pedaling particularly hard (it was in a spinny gear), and I'm 165 pounds, but the non-drive side pedal popped off. I went back, piced it up, and saw what looked like a teeny bit of silver metal in the threads. I was only 500 yards from home, so I cleaned off the threads on the spindle and in the crankarm with my finger, screwed it back by hand (seemed to move freely--no crossthreading), and started pedaling home slowly in my second-lowest gear. 200 yards later, the pedal popped out again, and this time, there was an obvious semicricular piece of crank in the spindle thread:
https://cimg7.ibsrv.net/gimg/bikefor...7034ecc895.jpg

The crank arm looks like there's some stripping, but not a TON:


https://cimg2.ibsrv.net/gimg/bikefor...3d20a3ea08.jpg

Any thoughts?

Options I've considered:

• Clean everything, grease everything, reinstall, and just torque the absolute crap out of it.
• New pedals?
• New crank arm (probably a whole new crankset -- there's a NOS-ish pulloff on eBay for $58 with shipping)

Thanks!

One possible reason is the pedal(s) are an uncommon French threading which is 14Mx1.25mm, these are about 0.050" or more undersized
compared to the usual 9/16x20TPI pedal thread so they screw in easily and 'seem tight' but under pedal pressure will ruin the threads in
an aluminum crank. From your pix most of the threads look ok, so you might substitute another set of pedals and see how it goes.
Fr threaded pedals are extremely uncommon these days but lurk on 60s-early 70s era French bicycles (and a few in my basement, which
is how I know this). That pedal will cut rubber soled shoes in half after awhile anyway and platform pedals are inexpensive.

One other thought that might support this opinion, if the pedals are Lyotard, risk of Fr threading goes way up, if Japanese or east
Asian then maddog34's opinion may be more likely. I personally have never had a pedal come loose when screwed in by hand.
and minimally torqued with a wrench but have had cranksets come loose from axles.

that piece of thread you saw was from the crank arm.. it would have been pulled out as the last thread of the steel pedal reached the end of the threads in the crank arm.

Pedals need to be tightened in fairly well... screwing it in by hand or even using a short open end wrench will not be enough. My pedal wrench is 16" long and is a better design than a standard open wrench.

do you feel any bad roughness in the pedal's bearings when it's not attached to the crank arm? that can help the pedal to unscrew from the crank arm.

this is EXTREMELY unlikely.

Agree with maddog, you gotta get that sucker in there tight with a long handle. If it falls out after that then I think you're looking at new (old) cranks.

As far as I know, both the pedals and the crankset came with the bike, so I'm guessing the threading matches.

OK. So based on the above:

• Step 1: Install a new set of MKS touring pedals I have in the car (because shiny) that I never got around to putting on my old Rivendell before I gave it away.
• Step 2: If there's still a problem, buy the new crankset.

I'll let you know how it goes!

If you can find a LBS that will do it (or want to try it yourself), there is a 3rd option: a helicoil repair for the crank arm.

But if done at a LBS, depending on the LBS that may cost nearly as much as replacing the crank. And many LBSs may elect not to attempt this anymore because of either concerns about liability or lack of a mechanic with the necessary skills.

With luck the new pedals will solve your issue, but I'd advise in any case that you examine the crank carefully for cracks. The cranks are presumably close to fifty years old and it would not surprise me to see that metal fatigue is occurring. If there are any cracks, they may be difficult to see at first but if you use magnification and bright light they should be apparent. Installing the pedal from the rear of the crank and stressing it during your exam may help.

Good luck.

A good shop can repair it with a helicoil

The crank arm threads appear to be mostly intact, so a helicoil repair would be over kill. The first thing I'd try is threading the pedal into the arm from the back side; this will allow the pedal axle threads to chase the damaged area and will usually clean things up enough to allow the pedal to mount properly from the front side. If this doesn't work, then try the helicoil option.

N.B. while it's possible to do this without removing the crank arm, it's much easier if you remove the arm and hold it in a vise to chase the threads.

Superglue.
Gorilla glue.
JB Weld.
There isn't much better surface for gluing something than threads.
I probably will use JB Weld for attaching a nut to the spindle of Milwaukee's cordless mud mixer, for my next electric bike. The glue being necessary because the drill will run in reverse, on the left side of the bottom bracket. I will correct this if it doesn't work. I used JB Weld once before, but the lesser drill burned up, so it was a short test.

^^^ This forum needs a "dislike" button.

Quoted for truth

This LeTour crankset holds pedals fine and the threads don't look that much better than yours, maybe the thread depth on the pedals you're using?
https://cimg0.ibsrv.net/gimg/bikefor...bdd0b5455e.jpg

Not sure. They're the original pedals, but anything could have done wrong in 45 years.

New MKS pedals are on, and they have enough of a similar look to the old ones (in my hand) that I feel pretty good. They SEEM to be in there solidly. Planning on riding a couple miles tomorrow to confirm or deny.
https://cimg6.ibsrv.net/gimg/bikefor...372e789fb.jpeg

Terrible advice. Are you going to pay the medical/dental bills of anyone who takes your suggestion to glue in a pedal?

(I don't like saying this, and while I have thought about saying it a few times in the past, I have always refrained.
There's a first time for everything, I guess.)

Please stop posting responses to questions in the bicycle mechanics subforum. You have no idea what you are talking about and your advice is bad and dangerous.

Of course not, nobody does.
Grown-ups are responsible for their own actions.
You get what you pay for.

Oh sure you have. More likely that's a personal problem. Seems the poster wanted a cheap solution, so that's what I offered. My advice is "dangerous" while some people say riding without a helmet is hunky-dory. Time to grow up...

Looks like the lesson is to inspect bikes and test ride them before you buy them, and after you buy them check them over to make sure the previous owner did not do something like take their clipless pedals off and put the old flat pedals back on the bike without tightening them etc.. The reason for the different threads for the pedals on the two sides of a bicycle is they are supposed to be self-tightening, but if the bearings in the pedals are bad in some way then they switch to being self-loosening.

Forums fascinate me. It has been four months and the OP hasn't indicated that he solved his problem, but someone dug this back up to defend their highly questionable advice.

I'm sorry, I really wasn't looking for a response. I was just writing something to warn people new to the site that your advice is moronic.

Unless the injured party decides to contact 1-ILL-SUE-THEM. You may not lose the case, but your legal fees may well bankrupt you.

Forgive my ignorance but why is glueing the pedal in so dangerous? (Not that I’d want to do it)


and yeah, I really want to know if the OP got on ok with his other pedals.