i have a bike, it is quite old, the BB cannot be switched out (non standard size in todays world) and it is a one piece chainring and crank combo. keep this in mind before suggesting i use chainring spacing bolts or get a new chainring/crankset.

i recently bought a fixed gear wheelset and it is coming in the mail (the original wheelset was a coaster brake wheelset) now i just realized the rear dropouts of the bike are either 110mm or 114mm can't measure exactly cause i haven't taken off the wheel off, while the new wheelset is 120mm hub. the frame is steel and was always planning to coldset/bend the rear stays to fit the wider hub. thought it would be easy and overlooked the chainline problem.

now without the wheels in hand yet, the chainline will be off by 5mm to 3mm depending on the rear dropout width, will this be ok? i was thinking either let it be, or asymetrically spread the dropout so to get a better chainline. or are there other solutions?

For frames with an oversized (American) BB shell and one-piece crank - which is still pretty common on beach cruisers and lower-grade BMX bikes - you can get a BB reducer kit and use any BB/crankset you want.

https://www.amazon.com/Truvativ-Ameri.../dp/B003KS00EU

That won't be ok. I might take a better measurement first, you should be able to sight the dropout spacing reasonably enough by sliding the ruler through the spokes at the hub flange checking the space. That size sounds odd. Some pictures could be helpful here. "Spreading" the dropouts won't affect chain line at all, it's all in the hub axel spacing and Crankset/front chain ring position. Good luck.

3mm isn't the end of the world, although 5mm may induce a few tsunamis and earthquakes.

My conversions chainline is 3mm off, I have been riding it for almost a year now and I've only broken my collar bone once! I also noticed that I failed a lot more ochem exams when it was 5mm before my new crank. I guess that small difference of a chainline had a big impact on my life.

pretty cool, didnt know such a product existed

not looking to spend more money to make my conversion work, how would symmetrical spreading the dropout work out? i don't my the slightly off center wheel however the chainline should be fine right?

last time my chain jumped off I ruined a tire. Peeled it back like a banana.

So, spend the cash now or later I suppose.

so wait, a perfect chainline isn't nesesary? is sheldon brown wrong?

"Perfect" within a few mm is "prefectly" alright.

so this is the first time sheldon brown has been wrong, and on the fixed gear article too, for shame