chain/sprocket question

[JohnnyBertone]

why is it that you shouldn't use a new sprocket with a used chain/ new chain with used sprocket?
i see a lot of people say you should change them all at once but i haven't had a problem with my chain or rear sprocket. i upped my front sprocket to a 14t and haven't had a problem without changing the chain. is it that all 3 parts sort of mate together? just curious I'm new to the quad world and would rather ask as many questions as i could and get experience from others that have been in my position, rather than go and wing it myself and explode everything.

 

[Herpderplol]

old parts are stretched and worn out. if you put a worn part with something new and in-spec, there can be problems

a new sprocket could snap and old chain. and new chain will over stretch with worn sprockets

 

[Awk08]

lets simplify this with my awesome MS Paint skillz

the rollers of a chain are a specific distance apart center to center, designed to exactly mate with the correct sprockets..........

a new sprocket for that chain has it's teeth peaks and valleys that exact distance apart for the chain rollers to engage smoothly and fully

you can clearly see the very worn sprocket on the left has wider valleys and greater distance between the teeth peaks and will not engage with a new chains rollers or spaces cleanly, which will quickly stretch and wear the new chain to fit the incorrect peaks and valleys of the worn sprocket.
as will a stretched/worn chain wear a new sprocket to match it.

worn and mismatched chain/sprockets is the #1 cause of snapped chains and busted engine cases when the broken chain whacks them