Never replaced my crankset before but am looking to upgrade from the stock that came on my Bianchi. I am looking at a used FSA SL-K Light carbon crank. "BB386 with 53/39 and 170mm crank length. Good condition – about 600 miles on them" So how do I know if this will fit my bike?

Without knowing the year and model of your bike, or the model of your current crank... don't expect any helpful answers. More info needed!
I see on your profile it's a 2013 Bianchi Impulso?
The Bianchi website doesn't give much info, but I will assume you have an english threaded bottom bracket... if so, you will need appropriate cups for a 386 crank. https://www.performancebike.com/bikes...401088__401088

Sorry! I have a 2013 Bianchi Impulso. FSA ck-4000 172.5 crank on it. 50/34T

I'll be honest I know NOTHING about cranks and how they're put together. I am just really starting to learn the mechanics and haven;t gotten to cranks yet lol

I would assume Italian threaded. But I'd find out for sure before buying anything

According to Bianchi's web page, you have a BSA threaded bottom bracket. So the crank you listed in OP will not work, and your BB is NOT Italian threaded. If you are crank shopping, look for the word "external" in the BB description. None of the BBxx or BBxxx or PF30 or OSBB apply to your bike. GXP, and MegaEXO apply to you. I don't remember Shimano's name for it.

Do you want shorter crankarms and standard (53/39) gearing vs. compact (50/34)?

If you buy the correct threaded bearing cups, the crank will work perfectly. I have the K-Force Light version of the same thing on my BSA threaded BB. Great stuff. You want a BB386 EVO crank. Don't forget the EVO! Here are the correct BB cups but quite expensive because of ceramic bearings. The steel bearing model is much cheaper, but I couldn't find any on ebay with a very cursory search. I am sure there are some on there. FSA Megaevo BB EVO8200 Bottom Bracket BB Ceramic English Thread 386 EVO Road BSA | eBay

Here are the non-ceramic ones: BB EVO8681 Megaevo Road | eBay

Don't forget the cups require an oversize tool to thread them into the BB shell. Those are hard to find also and a bit pricey, maybe $25 or so. I forget exactly.

In general you are right about all this, but not in this case. FSA makes a large diameter aluminum spindle crank that is 86 mm wide also. Threaded cups with an oversize orifice convert the regular 68 mm BSA bottom bracket shell to accept this crank. Many advantages. Aluminum spindle for lighter weight, oversize diameter for better stiffness and also the 86 mm width for better stiffness. All you need to use the crank with a BSA bottom bracket shell is the matching pair of threaded bearing cups. The same cranks can be adapted to just about any other BB shell with the suitable bearing/adapters. The crank is called BB386EVO and the bearing cups for the BSA bottom bracket shell are called BB386 Mega EVO.

As you can see above, when you ask on a web forum, you don't get a definitive answer. You definitely have a threaded bottom bracket (based on the Bianchi website), and as both @rpenmanparker and I have stated, that will work with a 386 type BB (because a 386 is longer than BB30 or PF... the later two wouldn't work). However, it will only work with an appropriate adapter. In this case, English threaded. It could be Italian, but many Italian bikes are shipping with the English threads these days. Since the website is short on info, you need to either call a dealer/manufacturer or have the BB measured to verify.

Unless @rpenmanparker is 100% certain that it's an English thread? Also, I posted the stainless steel bearing version above...

I am not certain the BB shell is BSA threaded. I was relying on [MENTION=236201]RollCNY[/MENTION]'s information about that.

BTW, you are not quite right about BB30 and PF30 BB shells not working with the BB386EVO crank. There are adapters for them also that press into the narrow shell and widen it to accommodate the 86 mm BB386EVO spindle. The bearing cartridges press into the outer sections of the two adapters. Just about every popular BB shell can be used with the BB386EVO crank given the proper adapter which would be available from FSA.

This

For what it is worth, the archived 2013 Bianchi Impulso 105 link says BSA bottom bracket. I would post the link, but it is difficult on my tablet. Frustrating, more than difficult. Heck, let's try:

Impulso 105 | Bianchi USA

That looks right to me. And the crank is coordinated with that BSA BB. Its a Mega Exo compatible crank. That means it will accept the Mega EVO oversize BB bearing cups for sure.

I have NO idea what any of this means!! Would someone mind explaining what all this means? I guess I just don't really know the differrences in cranks, BB, etc

Your original post suggest to me (all of us?) that you were specifically looking for a BB386 EVO type crank. Now it appears that was just something you came upon without caring about it's particular characteristics. You really need to do some background reading. Maybe the crank topics on the Sheldon Brown site would help. You will need specialized tools as well as the parts. Do some homework before you try to pick something out.

well i suppose upon original post I was ignorantly unaware of all of the crank nuances out there. I will do some more research for sure.

Misunderstanding... I meant that a BB30/PF30 crank can't be used in an English/Italian BB (but a BB386 crank can given proper cups/adapters); didn't mean to imply how it was interpreted.

Start here:
https://problemsolversbike.com/files/..._Reference.pdf

FYI, if you're looking for a 170mm crank with standard rings then RBP has 10 speed 105's on sale for $135. Crank and BB will drop about a quarter pound off your bike and match your 105 components. I personally would rather have that than carbon cranks. I bought an FSA Energy crankset from them a couple of weeks ago, they ship really fast.

SHIMANO 105 FC-5700 170mm 53/39T Road Bike Crankset + BB Double Black | Random Bike Parts

Gotcha. (Uh oh! I mean I understand, not I busted you.)

Great deal but I think I'm gonna stick with the 50/34 I got now to make it a little easier on myself with my 12-25 rear. Advantage over carbon is...?

Why do you want to change it ??
What gain are you expecting from an upgrade ?

Durability and price. A hundred grams or two won't make you faster if that's the only reason you want to upgrade and keep the same gearing.

I was thinking weight and stiffness over the current iron-feeling crank I have now. Not worth it? $150 for used (600 miles) FSA carbon slk 2014 crankset