Gearbox noise

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Arrma RC's
  1. Fury
So the gearbox has been making a scraping noise for the last couple of battery packs. The differential action felt bound up. Not locked but certainly tight. So I decided to open it up to see what's what. First off, holy cow there's a lot of screws! I've read reports of weak pins in the differential and gear box in general. I checked them all out and they appear to be fine. The gear box oil is black. Is this normal?


This is after I used the paper towel to soak up what oil I could from the diff:


And the other bevel gear:


And the small bevel gears, shims and cross pin:


They look ok to me. The spacers show a little sign of wear but nothing untoward. When I put it all together it seems to turn smoothly now. Could something have gotten bunged up and then freed upon disassembly? Can I safely degrease it, then fill it back up with fluid and call it good? Any suggestions on things to do to it while it's out? I plan on installing the Castle Sidewinder SCT combo.

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Check the bearings, I'm upgrading all of mine to rubber sealed because of all the hair and grit that gets into them. Also if your diff was anything like mine, you probably did not have enough diff fluid in it. It should have bin almost 3/4 full of diff fluid. As for the color that is normal.
Thanks for the reply. I've never had a fluid filled diff before so I didn't know if that color was normal.

The bearings seem to turn fine. No binding or dragging through their rotations. Well my diff certainly wasn't 3/4 full. If it was 1/4 full that'd be a generous estimate. The fluid wasn't even up to the cross pin that holds the small bevel gears.

I guess I'll refill and reassemble it then and see how she goes.
I built up the diff using new fluid. Gave it a couple of quick turns then buttoned it up. Once it was fully assembled I gave it another go at turning it but it was soooo hard to turn. It wasn't locked, but darn close. So I opened it back up, poured out most of the fluid, and put it back together. I tried to turn the outdrives again and still not much movement. It hurt my fingers to turn them and it's soooo tight. I suppose if I was looking for a locked diff I'd be fine, but that's not what I want.

I've ordered up an Arrma differential rebuild kit AR310378 via Amazon Prime. Hopefully it arrives in two days like they promise. I'll update this thread when I've rebuilt the diff.
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I tore down the diff again in preparation for the rebuild kit. This time I did a full tear down. This is the spacer I that is under the large bevel gear. I don't think this is supposed to be conical in shape. Shouldn't this be flat?



You can clearly see where the cross pin that holds the outdrive into the diff has scored a groove into the spacer. Is this a design flaw that can only be remedied by using the HPI diff? Is there a more robust spacer that I can use which will not go all cone-shaped?

It would seem you have the older style diff. They upgraded it to prevent that from happening. But you can use the washers for the Team Associated SC10 diff. I'll see about finding the part number and then posting it for you.

Here is a link to what was upgraded.

Here are the Arrma part numbers for the replacement parts.

AR310378 ARRMA Differential Gear Maintenance Set
AR310399 ARRMA Gear Differential Outdrive
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Here is the A.E. part number ASC9888 or just 9888. It is only $2 and you should be able to get them at your local hobby store.
Thank you! Turns out the diff rebuild kit I ordered is the 2013 version. Thanks to your information I know I will need new outdrives as well so I've ordered them up. I like the A.E. spacer idea and I would have gone that route if were planning on staying with brushed power. I want to install this Castle Sidewinder SCT kit as well and I worry about the cross pins sheering from the additional power.
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All the parts are in.


Diff housing, new seal, and o-rings are in the top compartment. Old out drives and diff gears are in the left compartment. New out drives and diff gears are in the right compartment.


I decided to take some measurements on old parts vs new.

Old (coned) spacer:

New 2013 spec spacer:

Old large bevel gear cross pin:

New 2013 spec large bevel gear cross pin:

So now we have numbers on how much of a difference some of the new parts are in the 2013 spec vs the original.
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I do have a problem though. The new diff kit didn't include any installation instructions. The manual only has an exploded view of the original diff. There are extra parts in the kit that I have no idea where they go.


#1 these o-rings replace the black ones inside the right and left diff housing under the spacer.
#2 the new kit doesn't have metal shims for the small bevel gears. Maybe #2 replaces those shims?
#3 I have no clue where these thin o-rings go.
#4 where does this shim go?
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I called Arrma customer technical support. She was pleasant and all, and really searched but they don't have any printed instructions on the new diff kit. WTF! She even talked with a technician there, and nope, nothing, nada, zilch. So here is what I have sorted out:

The large diameter, thin o-ring seals the outdrive shafts on the outside of the diff housing and fit into a groove on the outdrive shaft like so:


Slide the outdrive into the diff housing and unlike my poor example image below, remember to also put the ball bearing on before you insert the outdrive:


The large diameter, thick o-ring seals the diff shaft from the inside of the diff housing. It slips onto the out drive shaft like so:


Place a large spacer onto the shaft and press it down evenly to seat the o-ring into the diff housing. I used AE Green Slime on all the o-rings and on the spacer to help seal and lube the assembly.


Slide the drive pin through the shaft. I had to apply a fair amount of pressure down on the spacer and use needle-nose pliers to get the pin through.

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This is what the diff cap looks like with the o-rings installed on the outdrive, the cap, and the spacer.


This is the diff cap assembled:

And then more problems. My diff kit has two small diameter, thick o-rings and a single shim left. I thought maybe the o-rings went on the ends of the small bevel gear cross pin, replacing the metal shim spacers in the original diff like so:


Once the large bevel gears are in place though the small bevel gears don't mesh with the rubber o-rings installed like that. So I removed them and tried again but there was a ton of slop and it became obvious that the new cross pin was too short.

Here is the original cross pin:


Here is the 2013 spec cross pin:


And just for giggles, here is the space in the diff that this cross pin is meant to occupy:


How well does this new, shorter cross pin fit? Not well at all as it turns out. It's not long enough to engage with both halves of the diff. This would quickly turn into a train wreak if run like this.


I firmly believe that I was shorted a shim in this kit. My solution is to use the new small bevel gears (to ensure as close of a match to the new large bevel gears as possible) on the old cross pin with the old spacers. This is that that looks like assembled:

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I filled it to even with the top of the cross pin.


Put the paper seal in place, then put the cap on and tightened the screws using a star pattern. Ensure all the bevel gears are meshing nicely before tightening down the screws.


I'd like to cry success, but I can't. This thing is a bear to turn! It's not locked but it's awfully darn close! I can't turn it with my fingers without serious finger pain. If I insert a hex wrench into each outdrive it turns easily enough. Is there a break-in period with a newly built diff? I've never had such a tight diff in all the cars I've built in the past. Either gear diffs like this one or ball diffs but to be fair, this is the first fluid filled diff I've ever (re-) built.
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Oh hey! That last pic just made a light bulb go off in my brain on where those small diameter, thick o-rings belong:


They go into the outdrive cup to help keep dog bone lash to a minimum. Duh! Now I wish I had another pair for the hubs.
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Wow, that's worse than mine by a long shot! I've got it all back together and with wheels on the diff works smoothly but certainly not freely. I'm going to chalk it up to newness and that it's properly filled with fluid. I'm going to give it a few packs of break-in and then see how it feels.


Looking at your pictures it would seem that Arrma upgraded the main diff gear as well. If you compare your diff gear to mine you will see that there is a difference between the two, and that is the reason yours is binding up. So it looks like you are going to have to order the new diff gear. Also are you sure that shim is not actually 2 stuck together? I know the ones that came in my rebuild kit were stuck together.

One little tip: always do a dry fit before installing, and if it doesn't feel right just ask and we will be more then happy to help you out as best as we can.

Part # AR310416

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I see that you have 4 small bevel gears vs. my 2 but the diff rebuild kit only came with two small bevels. The part number you provided doesn't include small bevels. Do you have a Mega or is it a BLS/BLX. Maybe the Mega only has 2 small bevels and the others have 4?

Edit: Ok, now I see what you might have been talking about re: the difference between our two diff housings. The pin channel in yours is shorter which is why the pin in my 2013 spec maintenance kit was shorter than original. This is obviously a superior design in that the housing wall is thicker on the 2013 spec housing. It makes sense now why I couldn't use the pin from the 2013 spec kit in my original diff housing. Since I'm using my original pin, I think I'm fine but ordering up the 2013 spec housing is probably the safest bet.

I keep all the parts, even the old ones and have double checked the shim. It is most definitely a single shim.

I won't run it and I'm going to call Arrma customer support back. I only bought this truck in June (just have to sort out from where) so with a 2 year warranty I shouldn't have to ante up for a whole new diff.
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I did some more research and finally found a 2013 spec Fury differential diagram. According to this diagram I was shorted a shim, and I assembled the diff correctly.


So with this knowledge in hand I put a battery pack through the truck at the local kids play park. It's a good place for testing because it has a lot of varied terrain: flats, hills, tarmac, grass, packed dirt and sand. This was also the maiden voyage of the Fury powered by the Castle Sidewinder SCT combo w/ 3800 Kv motor.

Ambient temp today was 59 degrees. Motor temps and differential feel were checked every minute for the first 10 minutes, then every couple of minutes until the 5000 mAh battery was flat. With the stock pinion the Castle motor never got above 74 degrees, well below the Castle maximum temp of 180 degrees.

After just a couple of minutes the differential loosened up a bit and it runs like a champ. It's not free wheelin' loose, but I don't think it should be. IMHO this diff rebuild is a success. I'll run about a half dozen packs through it at the park and on the track then check the diff to see how she's holding up. That'll probably take me a number of weeks but I'll report back then.
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