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I need a new chainring but when I called the manufacturer they said my model is so old they don't have the part any more.
I've attached an image of the crank from the manual and a photo from my bike. Can someone tell me if it is possible to get this type of chainring, or an entire new crank?
How do I measure it to make sure it fits my bike?
It seems that you're asking about a BMX chainring. As far as I can see (I'm no expert in BMX) they are fairly standard. To be sure you'd neet to take three measurements:
Have a look at the attached image with those measurements marked red:
As you can see, the measurement 3 is somewhat flexible with the chainring pictured.
With those three measurements you can visit couple of LBSes and confirm that you're purchasing the correct part.
You have a one-piece Ashtabula-style crank. These were commonly found on inexpensive department store bikes.
Unless you can find a donor bike or donor parts for cheap (say at your local bike coop) the bikes themselves are almost always not worth the cost of trying to repair with contemporary parts. I’d look at Sheldon’s page above and try to figure out which “standard” Ashtabula crank you have and find a replacement from a donor. Finding the right chainring is a particular challenge but someone with a drillpress or mill could mod one for you.
Otherwise, you could use a universal threadless bottom bracket (from Velo Orange or elsewhere) to put a modern square taper crankset on but it’ll cost you more than $100 in new parts and the original bike is likely not worth $100 unless it has sentimental value.
You have what is called a one-piece or 'Ashtabula' crank. Don't ask me why it has that name. Info on Sheldon Brown here: https://www.sheldonbrown.com/opc.html.
Unike cars, bicycles generally use common standardized and commoditized components that do not have to be replaced with the exact part originally used. You can absolutely find replacements for these cranks and chainring. According to Sheldon there are a couple of sized of bearings used, but the bottom bracket shell and chainring center hole and drivepin are pretty standardized, so you can either replace the whole crank with new bearings or just the chainring.
You'll need to remove the crank to replace the chainring. Park Tool has a video on how to do it:
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