Pre 1950s rides of LFGSS: old bikes, vintage rats, classic lightweights

I hope this will be my final comment on cotter pins.

I regret having to say this, but cottered cranks are not really ideal for use with fixed.

Why? Because the cotter nearly always makes the chainset slightly oval, since it inevitably pushes the crank away from the spindle. Although the crank may seem to be a good fit on the spindle there is nearly always a bit of slack which is noticeable as a tight spot in the chain.
This doesn't matter with a freewheel since chain slack is more acceptable, but with fixed you really want to get the chain as tight as possible (allowing for free running of course).

As we all know, millions of miles have been done on fixed with cottered cranks, but there must have been many occasions when chains came off because there was too much slack at the slack spot. It has happened to me!

Here is a tip: When adjusting the back wheel for chain tension, set the right hand crank at about
2 o'clock, which will probably be just before the tight spot. If you get the chain right at that point you should be able to get it as near as possible to perfect at the tight spot itself.

Always keep a close eye on the chain and re-adjust when any slack develops - this will happen very quickly with a brand new chain.

It should also be mentioned that not all rear sprockets are as round as they should be. The late TDC sprockets were pretty bad in this respect and there are plenty of those still in circulation.