I am looking at changing my crank set and I am getting conflicting information. My BB is a shimano BB-UN40 but I am having trouble finding a crankset that goes from a triple to a double. I have sent emails to eBay sellers and they tell me I cant make the conversion so I was wondering, If I buy a crankset WITH the BB, what can I use? are they interchangeable or does the BB have to be a shimano BB-UN40. I dont know anything about thread size and that stuff....sorry

Changing from a triple to a double is a bit more involved that just changing the crank, there a the shifters and front mech to think about, these may or may not be an issue..

For the Bottom Bracket, the spindle length you currently have probably won't work, as triples require a longer spindle than doubles, so staying with your current BB will give you a too wide chain line.

For thread TPI, if you currently have a UN40, this will be an English thread, and either 68 or 73mm width.

Would be useful if you post up what your intended crank will be, as this will be helpful for advise on what BB you will need.

Thanks, something like this

https://www.ebay.com/itm/Campagnolo-C...item1e71e6a68b

You cant put an ISO crank like that Campy on a JIS BB like the shimano. The two use differently angled tapers on the BB spindle, so the interface will not be as solid as it should be. Your best bet is to buy a crankset and BB together.

Thanks, I'll start looking. Do I need 68mm or70mm?

You need to measure your frame for that.

68mm is normal for road bikes with English thread

70mm is Italian thread for road bikes

73mm is normal for MTB's

get yourself a vernier caliper to confirm.

No, both ISO and JIS use the same 2 degree taper, but JIS spindles have a shorter tapered section and don't get quite as narrow as an ISO spindle:

https://sheldonbrown.com/bbtaper.html

Would this fit?

https://www.niagaracycle.com/product_...ucts_id=507361

That depends. What's the make/model/year of your frame?

Hard telling. It says 170mm BB recommended, but normal BBs don't get that wide. Could be a typo of 107mm. Unfortunately Origin8's website is kinda borked so I couldn't find info there.

If you're wanting to keep your current BB, but fit a new crank you need to remove the cranks and measure spindle length or remove BB also and look at markings on the BB to determine spindle length, most Shimano BBs from the past 15+ years have a sticker on them indicating spindle length.

Its a 2001 Lemond Nevada City. I still have all the paperwork so I can give more information if you need it. I thought they may mean the crank arm length as mine is 170 mm? If I do buy a whole set with bb, what size do I need?

I found this on ebay

https://www.ebay.com/itm/2010-SHIMANO..._qi=RTM1084480

before they jumped on the external bearing band wagon ,
Campagnolo redesigned the right crank arms, to sit in different chain-lines
depending on being a Double or a triple crank.
the square taper BB was a 111.. for fat seat tubes they made a 115.

Only got the race triple , but the arm came in close enough to slip an O ring
onto the BB axle end, and make an extra seal around the shaft.

My square taper Chorus 10-speed double requires 102mm BB. But that's neither here nor there...

I'm pretty sure Lemonds were all English but unfortunately Bikepedia cannot confirm.

Measure BB shell (68 is English, 70 is Italian), remove drive side BB cup (if it comes off normally then it's Italian, if reverse threaded, it's English.), if English thread specs are stamped on cup then... (I forget the thread specs, but they're on Sheldon Brown somewhere.

You don't have to worry about spindle width for them new-fangled Shimano dealies like that last eBay link.

Wow, this is alot more complicated than I thought.

Not really, a few variables but any decent mechanic/LBS can help you get what you need.

Brian

It's really pretty easy. I just gave you three easy ways to determine what threading your BB shell has.

I'd bet $42 that it's English but you never know ...