Since there are a few of you that have built a big dummy, can you share with me the bottom bracket length that you used? I am planning on using a UN54 square taper, but wonder about the length. I am really hoping is could be a 110mm long, same as my LHT.

That isn't exactly how it works. You need the correct bottom bracket type to fit your shell threading and width (which is 68mm english on the Big Dummy),

But then the spindle length is dependent upon obtaining the correct chain line for your bike with your cranks. You sometimes have to take max. chainring clearance into consideration when choosing the cranks and chainrings, however, Big Dummy doesn't have a max chainring spec: you can run whatever size you like.

For a standard bike set up, you can just go with the crank manufacturers spindle recommendations.

Try to match chain line specs too: For instance, if you're using an internal gear hub with a chain line spec of 42 mm, don't use a crankset designed for a 50mm chain line unless you know you can get a shorter spindle to bring the crank and chainring(s) in to the right spot and still clear the frame for instance. Better off to just buy stuff that was meant to play together from the get-go and save yourself trouble.

You can read Surly's talk on the subject here:

https://surlybikes.com/blog/spew/spew...bracket_sizin/

Abneycat pretty much covered it: 68mm English, and then match the spindle with your crankset.

So pick a crankset first then match up a 68mm bottom bracket according to the crankset requirements.

On my 2011 Big Dummy I used a Truvativ ISOFlow 3.0 Powerspline Crankset that required 113mm Powerspline bottom bracket. So I got a 68mm x 113mm Truvativ Powerspline bottom bracket. By the way, I actually moved them over from my previous Kona Sutra touring built. 68mm bottom brackets are very common on MTB and touring frames AFAIK and Big Dummy is essentially a mountain bike, and most touring frames use MTB components as well. I don't know much about road bikes and other bikes though.

So if you have a 68mm bottom bracket, just use that bottom bracket and the existing crankset.

Thanks for the help, I know very little about cranksets and bottom brackets. So off to look for a mtb crankset for the big dummy. I plan on taking off the Mr Whirly and replacing it with something more mainstream.

Shimano cranksets come with matching bottom brackets. Like this Deore crankset. If you want something more mainstream and less flashy, but still decent look at Truvativ/SRAM. They'll be also a bit cheaper, probably around $50 for the crankset and $20 for the BB.

This is the one I'm using and the bottom bracket. I used 113mm bottom bracket.

FYI: better cranskets have the middle ring made of hardened, even carbon reinforced, steel, since that is the most frequently used ring. Although, considering the price, which is usually 4x or more of something like the above Truvativ crankset (they're also lighter and stiffer and... shinier), you might as well replace the cheaper crankset more often. My oldest ISIS Truvativ crankset has close to 10k miles on it, or more. But I do have to replace the cheaper BBs every few years. But I often get them on sale for under $20.