OK so I have some cottered crank bikes and im confused. I can find TONNES of information about replacing them but no information about tightening them.

Do i just hit them hard with a hammer and then tighten the nut?

Look up cotter pin tool or cotter pin press or cotter pin and c clamp method.

or try this clever method suggested by a list member recently

tap tap tap tap...snug snug...TAP TAP...snug...TAP TAP...snug...WHACK...sn...

The snug part is never meant to draw the pin, only to take up the slack. The hammer part is never too hard, wild or out of control.

If this doesn't work your pins may be too distorted from being too loose too long. Find a shop that has a press. Better yet buy one here. https://bikesmithdesign.com/CotterPress/index.html

Also remember that no matter what the manufacturer of the pin they do fit differently and need custom filling with a bastard file to get the angles right...

+10
This is an important step if you want them to stay tight.

If you use the (not recommended) hammer method, you have to whack it pretty hard. - Hard enough so that if you miss you would damage your crank. (Which gets us back to the reason why a hammer is not the recommended tool for tightening cotters).

After I think I've got them tight I turn the cranks to 3:00 and 9:00 and put my weight on them and tighten the cotters again, then I turn them 180 to 9:00 and 3:00 and do it again.

They're correctly called cotters in the USA.

Intesting - So just for the fun of it heres a picture of some really really old ones, a 1st Century cotter, a 2nd century pin, and the dropouts from an ancient Saxon 1st century bronze fork - HA...

https://www.colchestertreasurehunting...archfinds.html

GB, I guess you're referring to my offering?

I've got a CCM catalogue from 1918 listing cotter pins. (I lie...it's a pdf) I know it's not USA but we do share and share alike. Even Sheldon called them both cotters, cotter pins cotter pin presses, etc.
Mind you I like derailleur over derailler and Sheldon hated that.

You say potato and I say...

Make your own cotter press for under $20.

Life is too short for messing around with a hammer and then you can re-use the cotters (and the R-nuts) if you want to.

Derailer.

Inside-out IGH.

You say potato and I say derailer?

You know this is why the British had to burn down the White House in 1814. Bloody colonies. I'm writing letters to Kate and Prince what's his name.

Careful, there, guys. This thread is getting derailed.

The angle doesn't matter as long as both cotters have the same angle. The spindle turns to match the angle as the cotter goes in. The reason for filing is to get them to seat at the correct depth.

And as for "correct depth" we are talking mostly cosmetics...

If you use a matched pair of cotters there is no need to file as they should end up automagically 180-degrees apart as long as the cotters are inserted from the correct sides in relation to each other -don't put the nuts on the same side of both cranks and both the heads on the other side. They need to face each other. or away from each other depending on how you look at it. I prefer to put the smooth head facing up on the dirve-side crank when the crank is in the "ahead" position. This makes it smoother and less likely to "catch" your pantleg if you are running a proper chain-guarded bike so you don't need a pants-cincher. But it's not really that big of a deal if you go the other way. I just like to be consistent when I place the cotters.

If you are trying to get away with just using ONE new cotter then you are going to have to do some filing and experimenting to get the cranks to match. Life is too short to mess around with a $2 cotter by filing it. Just replace them both -it's cheap and super easy to do if you have a press.

Yes - Here we are talking angles but really buying or making a good press is paramount...

My $16 press makes removing/inserting a cotter a 20-second no-fuss job. It takes me longer to dig the tool (and the 14mm socket wrench to operate it) out of the bottom of my bike tool box than it does to do the job.

Click-click-click-click-click-click-fffpt!