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Grinding Gears

Old 03-13-2018, 12:10 AM
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Default Grinding Gears

Hello, Can anyone please help diagnosing a problem I'm having with my Tamiya Fox? It was running fine, and all of a sudden there is a sharp grinding sound coming from the gear box. It only happens while going in reverse, and when there is resistance. When held off the ground you can go from full speed foward to full reverse without any issue, but when it's on the ground, even the slowest reverse creates a loud gringing noise. I've taken apart and re-worked the gearbox several times. There are no chipped teeth on any of the gears. I know this is an older car, but can anyone offer some suggestions? I'm at a loss. Tony
Old 03-13-2018, 06:59 AM
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I'm at a loss too. You are sure the motor screws are screwed down hard enough and are making good contact with the spur?
Old 03-13-2018, 02:31 PM
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That made me literally laugh out loud. Thank you for your reply. Tried it, and it didn't work. He was a great car for his time, and still runs like a top, dispite the grinding in reverse.
Old 03-13-2018, 06:53 PM
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Is there any side to side play in the outdrives/gearbox joints?
Old 03-14-2018, 04:56 AM
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It could be a failed bearing causing gear mesh issues when reversing.
Old 03-14-2018, 06:10 AM
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not sure if this ball diff is compatible, but MIP has addressed a known flaw in the design for many of the Tamiya cars:
MIP Tamiya DIff | Page 1 of 1

Side note, I once had a cracked bevel gear in one of my cars, the teeth looked perfectly fine but would intermittently grind and I found this which was very subtle until I applied pressure with my fingers and the grinding went away after replacing the broken gear:

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