Hello boys and girls. I have a problem. I am rebuilding my old Peugeot PSN10 from 1978, but have run into a biggie. Some *^&%$ has thrown away my old BB, and although it needed to be replaced anyway, I now do not know the correct axle length. Any thoughts anyone?

Nobody can tell you because we don't know what crankset you are running.

Indeed, tell us what the crank is and someone can tell you what bottom bracket to use.

Ah well-call me green... Sorry guys, I did not know that the crankset mattered-I figured that a bike needed a certain chainline and that this was provided by a certain axle length that would be standard for that model bike! Hmmm-in that case, this may be trickier than I thought...
The bike is a Peugeot PSN10e from the 70s. I lost the Stronglight BB which was to be replaced anyway. I held a Shimano UN73 BB axle length 107 where it would all go, and with the 105 crankset on the square splines this seems fine, but I cannot be sure. The objective is to fit a Shimano 105 Octalink Crankset and D/A Octalink BB. Does this help? Please let me know if I am missing something...

I think your peugeot has french threading or something like that. I dont think that D/A bb will go on. you might want to check out sheldonbrown.com do a search on french bikes or something?? I think the most modern thing you can get on there might be a square tapered bb. ...unless someone else can comment on this???

It probably does have french threading. You can't know until you check the TPI, I think.

As for the spindle length, check the requirements for the 105 crankset.

Cheers for your answers guys! I have however bypassed the French BB problem. Forever! And no-one will ever know from the outside. You see, as the threads were worn out anyway, I was left with two options; hardsolder some solder into the BB Tube and rethread, or (more rigourously but certainly much stronger and safer) cut the BB tube, squeeze together, reweld and rethread. I opted for the latter. So now I can forget about all that French thread stuff forever. The bike has been in the family for almost 30 years, in my possession for 15 of those, and I was really hoping for at least another 30!

Next step was to stick in the BB, but this is where the problem now lies. I have looked at the D/A Octalink BBs, and my axle length choice is apparently limited to 109 or 118. 109 seems to fit fine if I stick in a square taper BB, but I don't know if Octalink 105 cranks fit on at the same distance from the side of the BB tube as the square taper. My usual LBS would be happy for me to try both sizes, only issue is they don't have the D/A BB in stock. And they would charge me 2x what I would pay on Chainreactioncycles for the same BB anyway, so I don't want to take advantage by trying it and saying 'thanks, but I'll buy elsewhere'...I like those guys too much for that. And I also don't see the point in spending the extra £££, as I could use this for some of the other parts I still need. So, I am hoping someone on this site can help me. Otherwise I will just have to buy both, try them, and see which one fits best, and then sell the other one on ebay or something.

Hmm.. i'm not sure if this answers your question, but I think the double cranks need 109.5 and triples need 118?? I'm doing the same pairing on my cannondale. I have a 109.5 DA and a double 105 octalink. What are the rest of yoru drivetrain components? Is the rear a short cage? If you aren't making this thing into a fixed gear, I think a 2 mm difference in chainline is not going to make a whole lot of difference. you will end up fine tuning the derailleurs??

It's a mixture of everything... Shift/brake is Shimano D/A 7700 STI 9sp at the front, Shimano 7800 10sp front derailleur (works fine on 9sp), 9sp 11-23 cassette on a 130mm rear wheel, the 105 Octalink double crankset, and either a Mavic SSC 7sp rear mech (short cage) or a Shimano XTR 9sp (long cage) depending on whether I can get the range out of the Mavic to cover all rear cogs. I would prefer the Mavic just to keep it a bit more French parts equipped.
I also thought 118 was for triple and 109.5 for double, same as you. My main worry is whether a 109.5 Octalink crank will sit on the same distance from the BB tube edge as a 109.5 square taper crank. I pressume it would, but then I am not sure. Have you installed yours yet? Does it work fine? No rubbing on rear stay?

When I had my peugeot, I recall the biggest chainring to be 48T?? ..which meant that it wouldn't touch the chainstays. HOwever if you have a modern crank with say 53T, it might actually rub.
I think if you go to the sheldon site, you can find what the distance from center of chainrings to the center of the seat tube should be. I think its something like 43mm??
I'm pretty sure 118 is for triple and 109.5 is for double. The only other measurement would be the casing width but i think peugeots are 68 IIRC. I will be installing my 105/DA crank/BB tonight and can measure up anything you might be after??? Would the distance from the BB casing to the smaller chainring help you??

What's stopping you from getting a bb other than dura ace?

you're comparing apples to oranges. different cranks are designed to take different spindles, but with the same general guidelines for chainline.

My biggest chainring always was a 53, as it used to be Eddy Merkx training bike. This was fine on the Stronglight cranks originally installed, so should be fine now.
Thanks for the Sheldon site (what would we all do without this guy?) reference. I did dig around on his site, but he is a far better bike specialist than he is a website builder...I basically could not find it. If you have a direct link to this info please include it in your reply :-)
You're right on the casing length-it is 68.
If you are installing yours tonight, could you reply with two pictures? One taken from the top down of the cranks that will allow me to see how much space between the rings and the stays you have, and one from either the front or the back, allowing me to see the chainline. I think a visual would be more help than measurements, although any measurements are of course very welcome. Thanks so much for the kind offer! Happy building!

I have generally found the D/A BBs to be smoother, stronger and longer lasting, as well as lighter, as the bearings are of a better quality. They also seem more watertight, as a 105BB (square) on another bike lasted me 5 years before getting gritty and rusty, whereas the D/A (square) I replaced it with was still like new when I took it out after about the same amount of time.

sorry, I didn't get around to installing it this week. When I do, Ill snap some pics.

https://www.sheldonbrown.com/chainline.html

No worries mate-I am still sorting out some other issues regarding this build anyway, but look forward to your reply! I will certainly do you the courtesy of posting some pictures of the final build result! Thanks