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I've tried an Allen key 7/64 but that didn't work and the others on my Allen key set but none seem to fit this shimano on a Schwinn Ranger.

One of the derailleur cogs broke in half.

Here is an image of a similar bike with the same Shimano drive train.

http://www.target.com/p/schwinn-mens-ranger-26-mountain-bike-red/-/A-15287654

Thank you!

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    What "plastic gears" are you talking about? Are you trying to remove the spoke guard on the wheel or the chain guard on the crank set? – Nate W Jul 7 '16 at 15:26
  • @NateWengert it's not those. It's the gears that are on the Shimano mechanism themselves which hook into the bike chains on the back tire. – Foo Fighter Jul 7 '16 at 15:30
  • The cogs of the derailleur? What is the purpose of the change? If the derailleur is the issue you may just want to replace the whole thing. Or have it properly adjusted. I can't imagine that bike having overly high end parts. – Nate W Jul 7 '16 at 15:43
  • One of the plastic cogs broke in half and to undo the cog there is an allen slot – Foo Fighter Jul 7 '16 at 15:55
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    Bike tools need to be metric sized. On Shimano components all screws are metric without exception. Using imperial sized wrenches will wreck the screw-heads. – Carel Jul 7 '16 at 16:08
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The allen is almost certainly metric. Both 2.5mm and 3mm are reasonably close to 7/64 inch. 3mm is a very common size on bikes. If you are going to do any work on a bike you need metric allen wrenches.

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  • What's not 3mm is mostly 5mm. And my pedals are 10mm. – andy256 Jul 8 '16 at 1:01
  • Turned out to be 3mm. Thanks everybody! – Foo Fighter Jul 9 '16 at 14:13
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The rear der on that bike will cost you at most $20 US, but if you take what's left of the jockey wheel to your LBS (local bike shop) they should be able to hook you up with a new or used part for less than $5 us.

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  • The question was about allen key size. – ojs Jul 8 '16 at 17:25

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