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enter image description here

Belongs to a fairly basic kids bike. Bought second hand and was able to remove it using almost my fingers alone, with a little assistance from a flat head screwdriver.

After replacing the bearings, wondering what tool should actually be used to tighten this up, and also how tight?

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The proper tool to interface with that bottom bracket is one like this:

Bottom Bracket Wrench for One-piece Crank

The pictured tool is a Park HCW-18 and runs about $15 USD.

There should also be a large nut that threads on top of the bottom bracket piece (an adjustable bearing cup) in your picture. This nut is used to jam (tighten) against the adjustable cup and lock its adjustment in place. Without a jam nut, the adjustment you make to the bearing cup will not be maintained.

I was going to find an image of this jam nut on a one-piece crank, but I actually found something better: A link to a Park tool webpage that includes a video on how to do the adjustment you are seeking.

Bottom Bracket Service - One-Piece Crank

The way to adjust this bottom bracket is a little trial-and-error to get it just right as the tightening of the jam nut always imparts some change in the adjustment. When it is correct, the bottom bracket should have no play in it, but NOT be overtightened such that there is perceptible drag due to increased friction from the over tightened bearings. Also note that some of these one piece cranks/bottom brackets may not be of the highest quality and sometimes you cannot get a perfect adjustment in that you would either have to accept a tiny bit of over-tightened bearings or just slightly loose. Personally, when dealing with this situation, I would prefer slightly tight over loose bearings, and I am stressing "slightly" as in barely perceptible.

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    That's a great video, very informative. I do have that large nut and washer, just out of shot. It's interesting how the adjustable cup is used, probably explains why I was able to loosen it with very little effort.
    – Chris
    Dec 4, 2022 at 20:24
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    @Chris, thanks. The tool Criggie suggested (a pin spanner) would also work in a pinch if you did not have the tool I suggested. I believe I have both somewhere in the bottom of the tool box. I am a tool guy, so what can I say...
    – Ted Hohl
    Dec 4, 2022 at 20:54
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    While watching the video, I was waiting for the magic trick to remove the z-shaped crank from the tube. Flawlessly performed :-).
    – Peter - Reinstate Monica
    Dec 5, 2022 at 14:02
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This is a one-piece crank, also called an Ashtabula crank. They are not high quality nor high precision.

The normal tool would be a pin spanner, but with shallow recesses like that it is likely to slip off.

So the standard followup is a small punch and a hammer alternating between slots until the correct tension is achieved. Or worst case is a hammer and flathead screwdriver. You're not putting a lot of force into this lockring, so is not as bad as it sounds.

enter image description here

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    Ashtabula crank - I learn something new every day!
    – Ted Hohl
    Dec 4, 2022 at 18:17
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    @TedHohl I got curious -- it's a native American name that became the name of a town in Ohio and later of a steel factory there which also produced bicycle parts. Here is a one-page showing BMX bicycles from them. This forum post claims that the crank they produced didn't have a good reputation.
    – Peter - Reinstate Monica
    Dec 5, 2022 at 14:12
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    @Peter-ReinstateMonica that is some interesting history there. Interesting perspective on the BMX racing community of the day and their perception of the cranks being low-quality compared to the cranks they typically ran. The quote of someone "Referring to them (the lighter cranks they were running) as Ashtabula would have been fighting words!" says a lot.
    – Ted Hohl
    Dec 5, 2022 at 15:23

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