Issue with the arrma alloy diff cup... seems too long.

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olds97_lss

Bash Break Repeat!
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Arrma RC's
  1. Kraton EXB
  2. Outcast 6s
I don't know if anyone else had any issues using these cups, but it almost seems like they are too long to allow you to shim them properly. Maybe I can take an exacto and separate the seal/gasket and only use half of it to give me some wiggle room for proper shimming... just had that thought as I was posting this. However, that would possibly make the internal gear lash too tight...

As it is, I cannot fit a shim on the ring gear side at all. It just won't go in without destroying it. Even after assembling it, I pried on the gap between the case and ring gear with a screwdriver to see if it would move at all, it doesn't. My only option was to shim the pinion and it grazes the surface of the diff cup. Even then, the mesh isn't as tight as I'd like it.

You can see how much I turn the input back and forth to see the gear lash about 3/4 through this. It's a brand new bulkhead, ring/pinion/internals/shims internally.

I upgraded the cup in the center too. I didn't bolt it to the chassis, I just put the top cap on and tightened it up and it almost seems like the bottoms of the legs are going to be too far apart and when I screw them onto the new chassis tomorrow, it's going to crush the bearings.

I'm using the gaskets that came with the cups... perhaps they are too thick? Can't imagine I'm the only person having this issue. Or I'm just doing something really wrong.

This is the case:
https://www.arrma-rc.com/part/ARA220050

Also, in the diagram, they show that you should use 2 shims under the larger sun gears. I have the mugen big shim under there and there's no way another shim is going to fit on top of that. Not without distorting the shim once I hammer the pin in there.
 
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Shims are used to set gear lash. And take up slack. My gear lash is too loose in my opinion, but there is no slack to allow me to adjust it.
 
Sorry, I was only halfway through my first cup of coffee ?
 
Sorry, I was only halfway through my first cup of coffee ?
I could have typed it wrong... lol! I've been up since 5am and need to hit the hay. Kind of wanting to get up in about 4 hours to charge batteries to get out in the morning and get a bash session in with a couple other trucks while I wait for the new chassis to show up.

I just have a feeling I should have not bothered with the "metal" diff cups and stuck with plastic... I just figured I was making things better, but so far... it's just irritating me and costing me time and money.
 
I put the Arrma dif cup in my rear bulkhead and realized the same thing. No way to add a shim to the outside of that bearing. I thought that was a good thing, as the external shim was an add on used by many to help with a problem created by the stock cup/case setup. My conclusion is they made the tolerance of the aluminum cup more precise, negating the need for that external shim. As far as inside, I used the shims they included to cover the o ring under the larger sun gears inside the cup. So far, with this setup, I've had better luck than stock, but after taking it out yesterday during a bulkhead swap, I noticed a few teeth broken on two of the smaller satellite gears. Took an hour to completely clean and remove the debris from inside the cup as each time I reassembled, some little bit was getting jammed in between gears and binding the whole thing.
 
I put the Arrma dif cup in my rear bulkhead and realized the same thing. No way to add a shim to the outside of that bearing. I thought that was a good thing, as the external shim was an add on used by many to help with a problem created by the stock cup/case setup. My conclusion is they made the tolerance of the aluminum cup more precise, negating the need for that external shim. As far as inside, I used the shims they included to cover the o ring under the larger sun gears inside the cup. So far, with this setup, I've had better luck than stock, but after taking it out yesterday during a bulkhead swap, I noticed a few teeth broken on two of the smaller satellite gears. Took an hour to completely clean and remove the debris from inside the cup as each time I reassembled, some little bit was getting jammed in between gears and binding the whole thing.

I was able to add 1 shim on the ring gear side in my old diff case, which was old and worn, but with the brand new one, I couldn't add any. Without any, the gear lash seems way too loose. Even with the one shim in the old one, it still skipped teeth... I'm not 100% sure the bearing was bad. The inner seal had popped loose, but I pushed it back in, then wiggled and spun it, it felt tight and smooth still. Which tells me shimming is definitely needed with the alloy cup.

Is anyone blowing the diff cup in the F/R? I have only seen the center blow apart. If you are not melting the plastic diff cut, I figure the metal is not needed?
I only did it because I figured it would help disipate heat and would be stronger in general. If I fry another ring/pinion... I'm going back to plastic. I still have one of the cups in the package.

I'm really considering pulling the diff back out, pulling the ends off and splitting the seal, then see how well the inner gears mesh. What sucks is I put red locktite on them... lol! When I pulled the diff from before, all 4 of the screws on the ring gear side were loose. They are so short that blue locktite didn't seem to work.
 
Is anyone blowing the diff cup in the F/R? I have only seen the center blow apart. If you are not melting the plastic diff cut, I figure the metal is not needed?
I changed mine because I bought one and already have two HR ones that I use for the center. I was going to save it, but when i started having issues with my rear dif trying internal gears on a regular basis, i figured I'd just give it a shot. Gotta do some research on hardened gear options, but for now, the gears are holding up better inside the aluminum cup
 
I'll see how mine does next weekend. I was able to install the M2C chassis, had an issue with aligning the motor mount and I should have used new screws because just as the last one tightened, it stripped the head. So I had to dremel it to get it out... the first ding on the new chassis... :(
2019-0907-Outcast-M2C-Chassis-Bottom.jpg


With all the screws tight, it feels too tight on the center. There is zero wiggle room, just like the diff in the rear. I modified an old hpi dogbone I had to grind it down so I could spin the diff with my dremel. I ran it at 10k rpm's for a few minutes and monitored temp. It went up a degree or two... so will see if it cooks the bearings. I dropped from 500k to 300k in the center as well. Now that I'm on 6S, 500k was a bit much.

If the rear diff goes... I'll either try rebuilding it by cutting the gasket in half so it's thinner so I can add shims to set the mesh better or just go back to plastic and consider the arrma cups a total loss.
 
Well, Horizon thinks I put the diff together wrong and I have a bent center diff mount/motor mount. I don't think either is true. The diff/motor mount seemed nice and flat and worked fine with the plastic cup, even had some wiggle room that required a shim or two so it didn't move around. And really... how can I put the diff together that wrong that it won't fit?

Maybe I'll open the 3rd diff cup and without any internals put one of the old ring gears on it with bearings and see if it fits any different. I couldn't tighten the screws down any further on the cap or spur without stripping them and the gears inside aren't binding at all, if anything they are loose even with the 4 mugen satellite shims and large mugen shims under the sun gears. I'm still of the mind that the gaskets are too thick. I'll have to fiddle around with it a bit. I think I even have a new diff/bulkhead spare that I could tear down/rebuild off the truck completely and see what I can sort out.

Might try removing the cap end gasket all together and just put some RTV in there and lock the screws down. I have a digital caliper now... suppose I could measure the OE diff, then build with the alloy cup and remeasure... but we are talking a few thousanths of play. Not sure how accurate my $35 mic is.

This is a stupid problem! LOL!
 
Confirmed that it is nearly 1mm wider from the outside edge of the bearing to bearing. Stock diff from jennyrc, 31.80mm, with alloy cup it's 32.65mm. I literally had to force the diff into the bulkhead and seat the bearings in the bottom of the channel with pliers. As before, the gear mesh is very loose.

On the new cup, I didn't install any of the internals. Just the 2 gaskets, ring gear, end cap and bearings. 1mm is a lot when your talking about shimming gear lash.
 
Well, I took the diff back apart, removed one of the gaskets, then reassembled it dry to check fitment. I now have enough room to add a single arrma shim to the ring gear side, which gives me as good as or better mesh than the single shim behind the pinion. This is on a new spare diff/bulkhead outside the truck. I'll likely pull the rear diff out of the truck and do the same to it. Just hoping rtv is good enough to keep the juice in. I do need to add an additional shim between the outdrive cup and the case/ring gear to remove some of the pressure the o-ring is putting on the internal spider gears as they don't spin all that smooth now. But if I pull the cups out by hand, they spin smooth and quiet.
 
Well, I took the diff back apart, removed one of the gaskets, then reassembled it dry to check fitment. I now have enough room to add a single arrma shim to the ring gear side, which gives me as good as or better mesh than the single shim behind the pinion. This is on a new spare diff/bulkhead outside the truck. I'll likely pull the rear diff out of the truck and do the same to it. Just hoping rtv is good enough to keep the juice in. I do need to add an additional shim between the outdrive cup and the case/ring gear to remove some of the pressure the o-ring is putting on the internal spider gears as they don't spin all that smooth now. But if I pull the cups out by hand, they spin smooth and quiet.
I would try anaerobic sealant.
 
I would try anaerobic sealant.
I've never had much issue with it not curing when using it for engine carb necks, cinch bolts and back plates. Maybe that's more of an issue when what it's sealing is wider. Considering the contact area is about the same as an engine backplate, guessing it should be ok.
 
I pulled the rear diff and replaced it with the one I just built using rtv instead of a gasket on the cap side. I left the shim on the pinion, with and and the additional shim on the ring gear, it still spun nice and smooth after putting the cap on. I spooled it up on the bench for a few minutes and other than the lack of slop noise from the dog bones (I put new center bones in as well), it spools up nicely again. Hasn't sounded this smooth since I got it. Pesky maintenance... lol!
 
I pulled the rear diff and replaced it with the one I just built using rtv instead of a gasket on the cap side. I left the shim on the pinion, with and and the additional shim on the ring gear, it still spun nice and smooth after putting the cap on. I spooled it up on the bench for a few minutes and other than the lack of slop noise from the dog bones (I put new center bones in as well), it spools up nicely again. Hasn't sounded this smooth since I got it. Pesky maintenance... lol!

This is the shim package I was able to get to work. Sounds like you are getting close.

 
Those 8x12x.3 would help push the pinion out. However, I put a shim behind mine to the point where it was lightly touching the diff cup, and it didn't help much with mesh/slop, but it did engage pretty much the entire pinion tooth face with the ring gear. I only had a couple of those shims and didn't have a clue where I got them. I'll order up some of those.

It's tough searching for shims on sites like amazon... their search logic is abysmal sometimes.
 
And... the rear diff is done. Not sure if it's from the impact that shattered the outdrive cup or what. I didn't notice it until I was replacing the hinge pin braces and was spinning the diff to check the bearings. It caught in a spot. Upon further inspection, the tip of one tooth is gone and the one next to it almost looks sheared. The shims behind the pinion are gone/ground up into almost nothing. I wasn't even really on the throttle when it broke.

Definitely tired of arrma diffs... I've gone through more diffs in this thing than I have my other 3 electric trucks combined. Almost glad there's snow on the ground as the outcast is going on the shelf until spring... unless it warms up and the snow goes away.

Going to order another diff and just go back to stock cup on the rear, shim it and just not think about how annoying these alloy cups have been.

Anyone tried another companies gears in these diffs to see if they fit? Tekno, mugen, losi? Just curious...
 
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