Hi All,
I'm riding a 70s Motobecane Grand Record with a Stronglight crankset that keeps loosening on the drive side. My first instinct was that the square hole has worn and that the crank is sliding off the taper and loosening the bolt. But here's what's throwing me off: When I screw the crankbolts into the BB (on either side), without the cranks, they have a lot of play... I can easily wiggle them up and down, or side to side, in the threads. It's almost as if the bolts are the wrong size or the that the threading has worn out.

So, is a worn crankset square visible? Because mine appears fine.

And are there different crank bolt sizes / do they tend to wear out?

A cartridge bb replacement is more desirable than a new vintage Stronglight crankset, for price and general upgrade purposes.

thanks!

Play between the threads of the bolts and tapped hole is normal, until they approach proper torque. When a crankarm is insufficiently tightened and ridden loose, the steel spindle damages the tapered hole in the softer, aluminum crankarm. The most common damage is that the end of the spindle creates a burr in the hole. This is visible and often (about 80% of the time) careful filing of the burr will resolve the problem. However, the hole can also be distorted and this is usually visible to the naked eye only in extreme cases. Distorted holes are rarely repairable, except by a machine shop, and even then refinishing of the hole often results in the removal of enough material to cause the spindle end to protrude beyond the bolt seat. The bottom line is that cotterless crankarms that won't stay tight are generally scrap metal, unless there is a visible burr in the tapered hole.

thanks... the reason behind the loose thread thing is really confusing to me. In any case I'll check for a burr and see if I can fix it. Hopefully it ends there. I'm nervous about buying a vintage Stronglight crank on ebay and running into the same problem.

Might the OP be able to get more mileage out of the crankset by running it on a JIS taper BB? If so, that would open up a smorgasbord of inexpensive cartridge units.

Do you have this tool for replacement of the crank arm bolt?
The very narrow wall socket allows for tightening of the crank bolt.

The amount a play between threads is a cost factor. Looser fit threads are cheaper to manufacture and easier to assemble. Threads typically come in three classes of fit: 1, 2 and 3. For most commerical products, class 2 is used, as it provides a good balance of cost, ease of assembly and reliability. Even the tightest fit, class 3 has some play between the threads to facilitate assembly. When the fasteners are tightened the threads walls are pushed into contact with each other, eliminating the play. Take any other fastener and loosen it several turns. You'll see that that you can wiggle the male portion in the female end.

Once the crank arm is removed, or if it has loosened up through pedaling, it is normal for the arm to come loose as you describe.
The solution is to simply retighten the crank bolt. Of course, you must have the correct French threaded bolt to go into the crank spindle. You would know this based on whether the bolt was previously okay.

And you must have the correct tool for a complete tightening of the bolt into the crank arm opening.

Not knowing the history of your bike, only you would know this.

I would not hesitate to buy on ebay from a 100% feedback seller of a French 1970's era, crank assembly seller. But, your problem may be simply lacking the proper tightening tool.

Both Zeus and Stronglight used non-standard 16mm axle bolts on their bottom brackets. With Stronglight, it's the non-standard 23.35 x 1.0 extractor thread that is the issue, not the socket clearance. With Zeus, it's the opposite. Zeus uses the standard 22.0mm x 1.0 extractor thread, but the narrower diameter means a very thin wall socket must be used.

In either case, the original 16mm bolts can be replaced with more standard 15mm bolts if that is helpful, as the 8mm x 1.0mm bolt thread is the same in either case.

Crank bolts come in French threading as well?

Absolutely not. There are no French-threaded fasteners.

Indeed. Thanks goodness.

I don't have a problem with tightening the crank bolt. My socket wrench is narrow enough to get in there. Thanks. I suspect it is along the lines of a problem with a worn taper hole in the crank as noted by T-Mar.

-There are no French-threaded fasteners.-

I thought this might have been the problem, but there didn't seem to be any evidence of French thread crank bolts out there. Yes, thank goodness.

huh. good to know.

UPDATE.

I found that the taper hole did have a bit of a burr on the outer edge - a sharp ridge on all four sides. I filed it off very carefully and put the crank back on and tightened it up. Giving the crank a rotation I found that the chainring bolts were brushing the chainstays. I guess this answers my questions.

Thanks everyone for the responses.

That's too bad, but as I mentioned before, I would try it on a JIS taper before giving up and tossing the part. There's not a lot of downside risk at this point.

Don't give up on the existing set-up without trying a few things to solve the rubbing.

Most older crankset use assymetrical spindles, with a longer end for the drive side. If the spindle has been installed backwards, this would definitely contribute towards the rubbing.

Typically, the chainring will rub before the bolts. This suggest the chainring may be installed backwards. Most better grade chainrings have recesses on one side, so that the flanges on the nuts (or bolts) sit flush with the chainring surface. If the chainring was installed baclwards, these recesses would be facing the spider and the nut flanges would stick out from the surface of the chainring, reducing stay clearance.

Finally, often it's possible to install a spacer behind the fixed cup. This will shift everthing towards the drive side and increase stay clearnace. The limiting factor on far you can shift things will depend on the amount of remaining thread engagement for the lockring on the adjustable cup, which will decrease by the thickness of the spacer.

actually...since the French invented the metric system, every metric fastener is "French threaded"...whether you like them Frenchies or not
revisionists can call them "freedom fried fasteners"...

It is a good possibility that the spindle was installed backwards when repacking the bottom bracket.
The ends of the spindle ARE differently shaped.

https://sheldonbrown.com/tooltips/bbadj.html

The section: 'Use Loose Balls' describes a recommended method to get the spindle correctly placed.

Thanks unworthy1. Makes me feel a little better after my above faux pas, about French threaded parts.

Actually, I guess just the bottom bracket shell threads and the head set are 'differently' French threaded?

I knew that was coming as soon as I typed it.

I'm surprised that it didn't come sooner.

French, Swiss and Italian bottom brackets have metric threads. British/ISO and Raleigh 26 TPI do not.

Thanks again folks for everything. I considered the spindle being backwards, but nope it is in the right way. As to bolts rubbing.. it's a stronglight/mtb chainstay thing. it's definitely being pulled on on too far.

Paid too much on ebay for a replacement - thanks to the PM re driveside stronglight offer, i got it too late. Bike forums won't let me reply (noob).

Italian is a weird, mutant system: metric diameter (36mm), but English thread (24tpi).

My LBS just retapped my bb thread if your having problems there get it retapped