Bicycle Repair/Sprocket size


Ok..... So I had this old school, I want to say 1980s bmx bike.
I recently acquired from a friend a newer redline bmx frame.
When I tried to put the cranks/sprocket on the new bike,
I found it would grind the frame just behind the seat, just where the frame
Splits to hold the wheel. I found the sprocket itself to be a 42 tooth,
Whereas my friends ride verry small 25 tooth sprockets. My crank is a
3 piece set and seems to have the spider arms (do all three piece cranks
Have them). Anyway I was wondering if   (A) I could space it out and just use this
Same larger sprocket. Or (B) Find a smaller sprocket that would fit my cranks with
The spider arms. Oh it's a square shaped (Spindle?) I think.   Sorry for my lack of correct terminology. Thanks

 Your best bet is to try and find a smaller diameter chainring to clear the stay of the the frame. Spacing out the bottom bracket spindle could render your chain line unusable; the stability of the bottom bracket bearing could also be compromised by not having enough thread engagement with the frame.
 You'll need to determine the "bolt circle diameter" of your crank arm, as well as the number of mounting bolts (usually 4 or 5). Then you can match up a suitable gear for both clearance and desired performance.

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James Novak


I can answer most inquiries with respect to bicycle assembly, repair, tuning and maintenance. I am less than inclined to speak on such questions as "What kind of bike should I buy?", &/or "What is this bike worth?", etc. Any answers to these types of questions may correlate poorly to an acceptable "fact/sarcasm" ratio.


7 seasons of "Professional" bicycle service/repair, etc. 4 seasons as Instructor, Professional Bicycle Mechanic training college. n-1 seasons of riding, cracking, crushing, mutilating and breaking bikes of any & all varieties, and thus requiring intense and at-hand skills development, etc.

Certified Professional Bicycle Mechanic; Winterborne Bicycle Institute/Conestoga College

Past/Present Clients
The Hub Bicycle Shop, Hespeler Winterborne Bicycle Institute

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