BB sizes
 

So I've pretty much cleaned up all the parts on the John Deere Racer and it is looking pretty good. There was much less rust than I thought, and all the parts are shiny. Except one of the cups on the BB is kind of worn on the threads and doesn't look as sound as the other side. It's probably cosmetic, but the boy wants new cranks, and I'm trying to determine if that's possible with this set up.

I've already learned enough to know that putting new cranks on the old chain rings will be a tough retrofit, since the chainrings only have places for 3 stackbolts. (Judging from the sources at my fingertips, anyway.) So if I don't want to retrofit the cranks, that means new chainrings attached to new cranks, which then means a new BB, right?

I know the old BB is British 68 mm. But what does the other measurement refer to? 108 mm, 113 mm, etc.? Which one is appropriate here and how do I determine that?

Also, how do I determine which length of cranks is appropriate? The old ones are 190 mm from end to end but I don't know if they're measured from center to center.

Now I'm going to get my credit card and make a donation, because you all have been so helpful. :)

Karen

Not necessarily. I'm assuming it's a square taper spindle? There are lots of vintage and new cranksets that will fit square taper. There are also new sealed square taper BB available for cheap - no PITA adjustments necessary!

The other measurment is the spindle length, which will determine how far out the driveside crank sits. This affects clearance of the chainrings on the stay, and chainline, which is basically how close the chain is to running in a plane between the chainrings and rear cogs. You want as little skewing as you can get to maximize drivetrain life and improve performance. Generally, 108 is for doubles, and 113 is for triples, though those numbers vary.

I don't think it's 190. You're right, it's measured center of pedal to center of spindle. 170mm is the most common length. Some run 165 or 175 or some odd numbers in between. Most people don't see much of a difference within this range. Some do. It's your call.

Here are some Sheldon Brown webpages with related information. This is a database of BB sizes. Good if you are buying a used crankset, as many are included here.

Some info on chainline here.

Oops. They're cottered cranks, so that makes the spindle cylindrical with a flat cut-out to accept the cotter. They look like a Lincoln Log on the end. :) I'll bet that makes a difference and it would have been nice to know up front, eh?

It's either new cranks, chain rings and BB; OR new cranks to fit the old rings (with only 3 stackbolts) and new BB; OR a new set of cotter pins.

Which do you think is the easiest to accomplish?

So that's measured from the outside of the cup to the end of the spindle? No, that can't be right. The whole length of the spindle is 142 mm as far as I can tell. I'll bet the fact that it's cottered makes a difference, huh? Yep, cuz it's 113 mm from the inside edge of one cotter space to the inside edge of the other.

But the size really doesn't matter if I'm not going to use it. I just need to know how long the new spindle should be to optimize the chainline, right?

Then, if the size of this spindle is irrelevant, with this double chainring we got here, a 108 mm BB might be best? Maybe the 113 mm, since that's how long the spindle is between cotter spaces?

Or maybe I should just try to find some cotter pins. :)

THANKS!
Karen

""OR a new set of cotter pins.
Which do you think is the easiest to accomplish?""

Obviously if you can find cotter pins then that sounds easiest. It all depends on if you want it to look original/classic or you simply want good performance. If it were me, I'd go for both.......you can a nice, used Campagnolo Nuovo Record Crankset and Nuovo Record or Victory 68 X 115.5mm BB on eBay for $100. That suff looks classic, is lighter-weight, performs well and maintains value.

If you just want good performance...get a used Shimano 600 crankset and square-taper Shimano BB for $50

Otherwise, keep it totally original and get cotter pins.