What is BCD?
BCD stands for Bolt Circle Diameter. It is the distance from side to side of a circle that goes directly through each bolt hole in a crankset. If you're looking at the hole in a crankset, imagine a circle that went directly through the center of each hole overlayed on the crankset. The diameter of that circle is the BCD. When you see a measurement like 110 or 130 BCD, that means that diameter is 110 or 130 millimeters, respectively.
What is the biggest chainring I can put on a 110 BCD or 130 BCD crankset?
To some degree, the sky is the limit. The issue with adding large chainrings to a smaller 110 BCD crank is that the large chainrings can flex. Or, if they don't flex, you'll have a huge penalty in weight. There are some niche manufacturers that make 54 tooth chainrings for cranksets, but you'll pay a premium for them.
What is the smallest chainring set I can put on a 130BCD crank?
38 tooth. The smallest inner chainring I've been able to find is a 38 tooth. Anything smaller and you, the chainring teeth will run into the chainring bolts.
How do I find my crankset or chainring's BCD?
The easiest way, of course, is to look on any chainrings or inside the stamping of a crank arm for a marking. The image below shows how it could be marked.
If you cannot find labels/markings, there are alternative methods. See the table below for measurements for popular BCDs. If you have a vintage or niche bike, it potentially won't be listed.
Adjacent hole to BCD conversion Chart
What about Chainring Bolts? How big are they and what are they made out of?
Chainring bolts are 10 mm.
The fancy folks use Titanium Chainring Bolts because reasons. Usually it's weight. You might shave a gram or two by swapping these out, but the cost efficiency is negligible. The more cost-conscious cyclists use steel or aluminum bolts.
Should I get a 110 BCD Crankset or 130 BCD Crankset?
In my opinion, that depends on the max tooth of your rear cassette. I have a 2015 Cannondale Supersix with SRAM Red 11 Speed and a 53/39 crankset with 11-28 rear cassette, and sometimes I wish I had more gearing. Why don't I get a larger cassette? Well, a short cage derailleur will max you out at 28 teeth on the rear cog. So max rear cassette size is a HUGE factor in deciding whether or not to go compact or standard.
What kind of hills max out a 53/39 and 11-28 cassette?
This is a climb I regularly did in Vietnam and wished I had a smaller crankset or bigger rear cassette:
I weigh about 160 pounds and could do 280 watts up the climb. To keep my cadence reasonable, I would have to go all out, and how far I could go in any given day would be limited by the fact that I was over-exerting myself to overcome my lack of gearing.