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Old 10-09-2012, 06:01 PM   #1
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Angry HELP! Weirdest rear diff sound, help me diagnose it - MBX6T

I was driving the truggy on the track, then I started hearing a sound like the rear conical or bevel gear is starting to strip. I heard this when I hit the brakes, at first. Then it started getting worse, and then when I would accelerate, I hear it and it sounds like the conical or bevel gear stripped. Then it started to not accelerate as well as it was before, engine revved, but car didnt go as fast. Its a brand new diff and bevel gear though, first 10 minutes of driving it.

I opened it up and the gears look brand new. Not a sign of wear on them at all. Ok so maybe the set screw thats connects the driveshaft to the bevel gear is loose? Nope, tight. The end of the bevel gear looks brand new. Ok, maybe the CVD? Nope, its perfect, set screw in there is tight. The diff is tight in the diff case, no play, so I don't need to shim it.

I narrowed it to the rear diff, or at least, the rear of the car.

The diff gears are perfect.

I can hit the brakes and roll the car backwards and hear the sound. Its a horrible notchy sound EXACTLY how you would think a stripped conical/bevel gear would sound. But, like I said, gears are brand new and have no wear on em at all.

Anyone have an idea? I'm stumped, everything looks perfect, rear gears are tight and greased properly.
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Old 10-09-2012, 06:12 PM   #2
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you seem to diagnose everyone elses issues over the internet why cant you diagnose your own?

too tight or too loose make heaps of sound.
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Old 10-09-2012, 06:16 PM   #3
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i saw this weekend a guy had a new ring and pinion gear in the rear diff but didnt replace the diff case/housing and the gears still looked good but it was skipping because the case was worn. replace the whole diff case/housing.
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Old 10-09-2012, 06:17 PM   #4
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Originally Posted by chopper82p View Post
I was driving the truggy on the track, then I started hearing a sound like the rear conical or bevel gear is starting to strip. I heard this when I hit the brakes, at first. Then it started getting worse, and then when I would accelerate, I hear it and it sounds like the conical or bevel gear stripped. Then it started to not accelerate as well as it was before, engine revved, but car didnt go as fast. Its a brand new diff and bevel gear though, first 10 minutes of driving it.

I opened it up and the gears look brand new. Not a sign of wear on them at all. Ok so maybe the set screw thats connects the driveshaft to the bevel gear is loose? Nope, tight. The end of the bevel gear looks brand new. Ok, maybe the CVD? Nope, its perfect, set screw in there is tight. The diff is tight in the diff case, no play, so I don't need to shim it.

I narrowed it to the rear diff, or at least, the rear of the car.

The diff gears are perfect.

I can hit the brakes and roll the car backwards and hear the sound. Its a horrible notchy sound EXACTLY how you would think a stripped conical/bevel gear would sound. But, like I said, gears are brand new and have no wear on em at all.

Anyone have an idea? I'm stumped, everything looks perfect, rear gears are tight and greased properly.

Check the spiders inside the diff cases. If they are worn they can make the same noise... Wouldnt hurt to check all 3 diffs....
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Old 10-09-2012, 06:28 PM   #5
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Originally Posted by LosiPRRACER View Post
you seem to diagnose everyone elses issues over the internet why cant you diagnose your own?

too tight or too loose make heaps of sound.
ok you diagnose it then, instead of criticizing me for not knowing what it can be
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Old 10-09-2012, 06:30 PM   #6
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i saw this weekend a guy had a new ring and pinion gear in the rear diff but didnt replace the diff case/housing and the gears still looked good but it was skipping because the case was worn. replace the whole diff case/housing.
I was thinking that might be the problem. Ok, I'll try that, makes sense.
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Old 10-09-2012, 06:30 PM   #7
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Check the spiders inside the diff cases. If they are worn they can make the same noise... Wouldnt hurt to check all 3 diffs....
Ok, I will check them.
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Old 10-09-2012, 06:46 PM   #8
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Also check the gusset support in between the pinion bearings, I've seen them partially crack away from the bulkhead just enough to allow the pinion to shift when under pressure
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Old 10-09-2012, 08:26 PM   #9
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The rear bevel gear/conical gear is still stripping... even if they look new (for now).


The problem is the diff bearings don't fit snug into the diff case... If you have the entire bulkhead off the car, push and pull on the outdrives... you'll see the diff move front and back inside the case slightly. Under load it'll shift and the gear backlash will be so big the bevel/conical will slip. Even on some brand new diff cases it's still there: over-shim the diff until its notchy, then pull/push the outdrives to move the conical away from the bevel, and the gear mesh changes to something more reasonable! Only way it can do this? Loose fit! Maybe every other diff case is a decent fit new, but if you plan on frequently swapping diff oils, forget about it.

The best way I've found to correct the problem: Shim the bevel/conical as you normally would until you find the correct mesh. On the opposing half of the diff case, I use a drop of RTV (viscous, liberal hardness) onto the bearing channel on each side, then join the two halves of the diff case. Let it sit until it dries. The RTV will fill voids and enhance the bearing fit to keep the diff from moving around. You could probably use hot glue if you were cheap, though you would have to act fast.
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Old 10-09-2012, 08:42 PM   #10
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The rear bevel gear/conical gear is still stripping... even if they look new (for now).


The problem is the diff bearings don't fit snug into the diff case... If you have the entire bulkhead off the car, push and pull on the outdrives... you'll see the diff move front and back inside the case slightly. Under load it'll shift and the gear backlash will be so big the bevel/conical will slip. Even on some brand new diff cases it's still there: over-shim the diff until its notchy, then pull/push the outdrives to move the conical away from the bevel, and the gear mesh changes to something more reasonable! Only way it can do this? Loose fit! Maybe every other diff case is a decent fit new, but if you plan on frequently swapping diff oils, forget about it.

The best way I've found to correct the problem: Shim the bevel/conical as you normally would until you find the correct mesh. On the opposing half of the diff case, I use a drop of RTV (viscous, liberal hardness) onto the bearing channel on each side, then join the two halves of the diff case. Let it sit until it dries. The RTV will fill voids and enhance the bearing fit to keep the diff from moving around. You could probably use hot glue if you were cheap, though you would have to act fast.
That makes sense, I have been messing with the rear diff a lot and taking it apart and back together at least 10 times so yeah, that must be it.

Thanks for the help, greatly appreciated!
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Old 10-09-2012, 08:57 PM   #11
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The best way I have found to keep the bearings snug is to superglue a thin piece of lexan cut to fit on the groove on the diff cover. Let it dry then reinstall the cover.
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Old 10-10-2012, 06:19 AM   #12
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I have drilled a small hole and used a grub screw to keep the bearings tight in the diff case, cheap, quick, and easy
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Old 10-10-2012, 10:38 AM   #13
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If you engaged your brakes holding the front or rear up with a set on the ground while pushing .I will bet you will hear a loud clicking noise. Its a mesh and gear issue as stated.
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Old 10-10-2012, 12:04 PM   #14
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I have drilled a small hole and used a grub screw to keep the bearings tight in the diff case, cheap, quick, and easy
Cool, I'll try it. Do you think I can do that with my gearbox or would it be better to just start with a new gearbox, If I need a new gearbox I have to order one and wait, I've been waiting to get on the track for almost 3 weeks and I need to practice, would suck to have to wait another week before I can get on the track and start setting up my car and practicing.
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Old 10-10-2012, 04:55 PM   #15
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Cool, I'll try it. Do you think I can do that with my gearbox or would it be better to just start with a new gearbox, If I need a new gearbox I have to order one and wait, I've been waiting to get on the track for almost 3 weeks and I need to practice, would suck to have to wait another week before I can get on the track and start setting up my car and practicing.
It would not hurt to try, but I would certainly order a new one and replace it at next teardown.
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