Big Rock Rear diff replacement - smooth running or tight diff case....but not both

PatMarcus172

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Dear Experts

Somehow or other, I trashed the rear diff in my Big rock and sure enough the main gear on the diff and the input gear were both toast, as several people suggested was the likely source of my grinding noises and lack of power.

So, I ordered a complete new assembly (with diff yoke) from a trusted RC Breaker, here in the UK. It was claimed to be brand new and looked in mint condition but I took it apart anyway, filled it up with 10K, checked all the bearings (and made sure there was a shim on one side) and put some red lithium grease on the gear. All fine.
Until I put it back together.
Now I know you have to put it back together with the right case on the right side but even so, as soon as I did up the case screws to anything approaching proper tightness the gears start catching and wont spin smoothly.
Drive shafts attached fine and these, too, are loctited on as Ive seen reccomended in countless "set up your Arrma" videos

So, I was faced with the dilemma of having a slightly loose case with screws which are "in" (and I have loctited just in case), or a properly tight case with catching gears that Im somehow hoping will "wear in".
I chose for option 1 but I don't feel good about it. Im worried that the case is gonna somehow come apart or at the very least leak grease all over the car.

Any thoughts as to what I might have done wrong ? Ive ordered a new diff yoke just in case (as theyre very cheap) and was considering maybe a GPM or Hot Racing one as an upgrade but they seem to have their far share of critics, too.
 

FH-RC

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Dear Experts

Somehow or other, I trashed the rear diff in my Big rock and sure enough the main gear on the diff and the input gear were both toast, as several people suggested was the likely source of my grinding noises and lack of power.

So, I ordered a complete new assembly (with diff yoke) from a trusted RC Breaker, here in the UK. It was claimed to be brand new and looked in mint condition but I took it apart anyway, filled it up with 10K, checked all the bearings (and made sure there was a shim on one side) and put some red lithium grease on the gear. All fine.
Until I put it back together.
Now I know you have to put it back together with the right case on the right side but even so, as soon as I did up the case screws to anything approaching proper tightness the gears start catching and wont spin smoothly.
Drive shafts attached fine and these, too, are loctited on as Ive seen reccomended in countless "set up your Arrma" videos

So, I was faced with the dilemma of having a slightly loose case with screws which are "in" (and I have loctited just in case), or a properly tight case with catching gears that Im somehow hoping will "wear in".
I chose for option 1 but I don't feel good about it. Im worried that the case is gonna somehow come apart or at the very least leak grease all over the car.

Any thoughts as to what I might have done wrong ? Ive ordered a new diff yoke just in case (as theyre very cheap) and was considering maybe a GPM or Hot Racing one as an upgrade but they seem to have their far share of critics, too.

You put in the diff seal ?

Arrma AR310875​


Might sound stupid, but I forget it once during a diff switch and was cranking the screws so tightly to get it together, that the diff was also barely working.
The diff seal acts like a shim and provides that edge of distance needed for smooth running.
 

GRC

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One possible reason that has happened to me. Sometimes the pins cannot seat fully if the pin holes are filled with oil. Some pins/holes will let oil out, but other fit so tightly that any oil in the hole prevents the pin from going all the way in. Once this happens, the gears stop spinning properly when you try to tighten the case. Empty out the oil, clean out the holes. Insert the pins BEFORE putting in any oil. When you fill with oil, don't get any oil on the top of the pins. Wipe them off with a Q-tip if you do. Put the top cap on carefully. Try to do it ONCE. A failed attempt can end up pulling pins out, which of course fills the bottom holes with oil. You can still try to push the pins all the way down. You'll feel/hear a little pop if the oil is able to squeeze out. So in general, try to keep the holes clean, make sure the pins are fully seated.

Other possible reasons are nicks, scratches on the gears. Or a warped differential case or cap.
 

PatMarcus172

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You put in the diff seal ?

Arrma AR310875​


Might sound stupid, but I forget it once during a diff switch and was cranking the screws so tightly to get it together, that the diff was also barely working.
The diff seal acts like a shim and provides that edge of distance needed for smooth running.
That’s interesting.!there definitely wasn’t a diff seal in the “pack” but at the same time I serviced my front diff and that didn’t have one either.
However, in case I’ve misunderstood, isn’t that seal for the inside of the diff case containing all the planetary gears?
My issue is when you tighten the outer diff yoke (AR310783) and it stops things spinning smoothly.
One possible reason that has happened to me. Sometimes the pins cannot seat fully if the pin holes are filled with oil. Some pins/holes will let oil out, but other fit so tightly that any oil in the hole prevents the pin from going all the way in. Once this happens, the gears stop spinning properly when you try to tighten the case. Empty out the oil, clean out the holes. Insert the pins BEFORE putting in any oil. When you fill with oil, don't get any oil on the top of the pins. Wipe them off with a Q-tip if you do. Put the top cap on carefully. Try to do it ONCE. A failed attempt can end up pulling pins out, which of course fills the bottom holes with oil. You can still try to push the pins all the way down. You'll feel/hear a little pop if the oil is able to squeeze out. So in general, try to keep the holes clean, make sure the pins are fully seated.

Other possible reasons are nicks, scratches on the gears. Or a warped differential case or cap.
Sorry, I’ve obviously not explained this properly. Everything in the planetary gear case is turning smoothly, though I totally see your point about about the holes filling with oil because that also happened to me.

It all goes wrong when I try to tighten the outer yoke (Arrma AR310875) as if somehow the whole contents is just a tiny bit too big to fit inside it.
I’ll see if it’s better with a new one and report back. I’m getting very fast now at accessing the rear diff 😄
 
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FH-RC

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That’s interesting.!there definitely wasn’t a diff seal in the “pack” but at the same time I serviced my front diff and that didn’t have one either.
However, in case I’ve misunderstood, isn’t that seal for the inside of the diff case containing all the planetary gears?
My issue is when you tighten the outer diff yoke (AR310783) and it stops things spinning smoothly.
Yes, it is to seal the inside of the diff, so that the oil stays were it belongs.

The seal is not part of the diff replacement sets I think, but a part on its own.
I missed it during change as it blended with the diff cap and I threw it away. Noticed my mistake when the oil came out at the diff case onto the drive shafts. Yet, took me a while to figure the seal.

About your case, maybe you have a seized bearing or a bearing does not sit snug?
 

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It all goes wrong when I try to tighten the outer yoke (Arrma AR310875) as if somehow the whole contents is just a tiny bit too big to fit inside it.
I’ll see if it’s better with a new one and report back. I’m getting very fast now at accessing the rear diff 😄

This is a common problem, I think the tolerances on the whole assembly is not very good.
 

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Try loosening up the internal yoke a hair that usually helps free sticking. Also might need an extra shim on the opposite the ring gear.
 

PatMarcus172

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Try loosening up the internal yoke a hair that usually helps free sticking. Also might need an extra shim on the opposite the ring gear.
I’ve had too loosen AR310875 quite a bit, to the extent you can see a tiny gap between the edges of the outer case in some places.

Thanks everyone for your advice and reassurance. I’ll try with a new outer yoke first and report back.
 
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GRC

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Sorry, I’ve obviously not explained this properly.
Actually, I just misread it. I also recently had a new diff gear that was sticking when tightening the yoke. Shimming the opposite side worked, but I wasn't happy with that. Turns out I must have gotten a little dirt in the diff grease, or maybe just too much grease gunked on and it was getting jammed up. I used a brush and cleaned out all the diff teeth, then greased more evenly and the diff rolled smoothly without having to switch the shim. I've also had a diff gear that warped over time and a brand new warped diff cap. A warped yoke would also create a problem. Another possible reason is the bearings aren't fully seated. P.S. I like the Hot Racing yoke, I found the GPM was a poor fit.
 
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I had one from new that was catching at a specific point on the ring gear, I sent it in to Logic (Arrma UK) for "service", as far as I could tell they just loosened the screws between the yoke halves a bit and said it will wear in within a few packs. Which in fairness to what seems a bit rubbish initially it has done.
The only thing to check is that the one shim is on the non-ring-gear side.
 

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Hy guys, had that same problem yesterday, everything seemed to be fine unless i assembled all that stuff.

Now i read your comments and you are wrighting "... shim is on the non-ring-gear side..."
When i look at the arrma exploded view i see the shim to be placed on the ring-gear-side and no shim on the non-ring-gear-side. Screenshot_20210612-194856_Chrome.jpg
 

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When I took apart both of my factory diffs, the shims were on the opposite side shown on the diagram and spun freely. I put the Arrma metal setup in the rear and wound up adding three more for a total of four (0.8mm) to make it smooth as butter.

Screenshot_20210423-135443_Chrome.jpg
 

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Hy guys, had that same problem yesterday, everything seemed to be fine unless i assembled all that stuff.

Now i read your comments and you are wrighting "... shim is on the non-ring-gear side..."
When i look at the arrma exploded view i see the shim to be placed on the ring-gear-side and no shim on the non-ring-gear-side.
The shim allows adjustment, you put it on whichever side gives best mesh, typically that is non-ring-gear side when new and the other side as it wears.
 
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razorrc

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The shim allows adjustment, you put it on whichever side gives best mesh, typically that is non-ring-gear side when new and the other side as it wears.

I have a random thought. Maybe if the yoke is too tight, it's a good idea to remove the shim altogether.
 

PatMarcus172

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Happy to try all those options. Took the Big Rock to the park to see how it felt with my “slightly loose yoke” and on about the second rip, one of my rear brand new Fortress tyres shredded. I imagine it was a complete coincidence but a bit depressing nonetheless , given how positive I felt to have the Big Rock back in action.

on the plus side, a new yoke has arrived in the post which I’ll try and report back.
 

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I have a random thought. Maybe if the yoke is too tight, it's a good idea to remove the shim altogether.
I did try that at one point and it seemed to give a half way between shim one side vs the other. At the end of the day it's just trial and error to see what works best for your specific parts they all have slightly different tolerances.
 

razorrc

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I did try that at one point and it seemed to give a half way between shim one side vs the other. At the end of the day it's just trial and error to see what works best for your specific parts they all have slightly different tolerances.

Yeah, I ended up sanding down the input gear and diff cup on my Vorteks to get it smooth.
 

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Hy Razor, i thought about doing that too. Where exactly did you sand down the parts? Can you pls explain?
 

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I have noticed that they’re are a couple of spots on the composite diff cup that aren’t perfectly smooth. These are probably from the casting process, or however they make them. Not coincidently, these spots corresponded to any rough spots when I spun my diffs in the yokes. I cleaned them up with a sharp knife, and now the diffs are smooth all throughout the rotation.

Sounds similar to what razor did.
 

razorrc

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I have noticed that they’re are a couple of spots on the composite diff cup that aren’t perfectly smooth. These are probably from the casting process, or however they make them. Not coincidently, these spots corresponded to any rough spots when I spun my diffs in the yokes. I cleaned them up with a sharp knife, and now the diffs are smooth all throughout the rotation.

Sounds similar to what razor did.

Yup, that and I filed down the top of the input gear because it would rub against it a little bit because it wasn't perfectly even.
 
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