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Ok so, while looking to shim rear diff, I realized what might be a contributing factor to why some people have had problems even with 3 shims under each sun gear.
Background Info:
This pic shows the exploded view on Arrma site, notice the diameter of the shims in the diagram, it appears to me it was designed to have 2 shims of the smaller diameter and 1 shim or the larger diameter when you look at it visually.
So that would be according to the break out part numbers two ARA709059 and one ARA709060.
Now looking at this part of Jason's post on FB:
From that one can conclude that the ARA709060 are 0.2mm shims and the ARA709059 are 0.3mm shims.
From those two diagrams I reason that the initial designer targeted 0.8mm of shims under each sun gear based on drawing visual size and such.
So then I noticed that both part numbers on Arrma and Horizon sites listed the shims as both 0.2mm.
So I set out to figure out if it is a typo, so I pulled my rear diff and measured the shims under the ring gear sides sun gear, and determined that the larger diameter shim is the one that is 0.2mm and the smaller diameter shim is 0.3mm.
Therefore this part list should be this:
So then I noticed that on both the exploded diagram and on the reference image Jason posted the part numbers were backwards as compared to physical dimensions as compared to the visual representation.
It should be :
I verified that my rig has two of the larger diameter(0.2mm) and one of the smaller diameter(0.3mm) shims under the sun gear.
So my conclusion is that Arrma somewhere along the line reversed the part numbers and the bill of material then ended up with 0.7mm of shims rather than 0.8mm under each sun gear.
So that variance X 2 for the pair of sun gears, leaves a 0.2mm narrow on correctly assembled diffs based on part numbers.
Does anyone know if that is enough to cause teeth to break? (I'm not an expert on diff shimming.)
So my recommendation is to buy a pack of ARA709060 0.3mm shims and put two of those and one of the larger diameter ARA709059 which should add up to close to 0.8mm of shims (measure each to validate and also test feel of diff without fluid) which is what I believe the Arrma design engineer targeted.
Also to note, I had already ordered and received a pack of ARA709059 and measured it, it was the larger diameter like it should be and it was 0.2mm.
I have notified Arrma (JD) about the part number swaps and he said they will get it updated, he didn't really say if that missing 0.2mm would cause issues, he feels more are failing from the extra shim missing due to QC issues.
Hope this helps some of us and saves us from blowing a diff, I know I will be shooting for the 0.8mm total shims and testing the mesh before filling by feel.
TT
Updates: I rebuilt my rear today using the shims as I suspect the designer meant them to be used, (two 0.3 and one 0.2) for a total 0.8mm under each sun gear. the 0.8mm fits beautifully, and the diff felt perfect both empty and filled.
For the heck of it before filling it, I built it back the way it came to me (two 0.2mm and one 0.3mm) and it felt and sounded a little too loose, so maybe with big power motors it is enough to break tops of teeth off.
I highly recommend everyone build it with 0.8mm.
Background Info:
This pic shows the exploded view on Arrma site, notice the diameter of the shims in the diagram, it appears to me it was designed to have 2 shims of the smaller diameter and 1 shim or the larger diameter when you look at it visually.
So that would be according to the break out part numbers two ARA709059 and one ARA709060.
Now looking at this part of Jason's post on FB:
From that one can conclude that the ARA709060 are 0.2mm shims and the ARA709059 are 0.3mm shims.
From those two diagrams I reason that the initial designer targeted 0.8mm of shims under each sun gear based on drawing visual size and such.
So then I noticed that both part numbers on Arrma and Horizon sites listed the shims as both 0.2mm.
So I set out to figure out if it is a typo, so I pulled my rear diff and measured the shims under the ring gear sides sun gear, and determined that the larger diameter shim is the one that is 0.2mm and the smaller diameter shim is 0.3mm.
Therefore this part list should be this:
So then I noticed that on both the exploded diagram and on the reference image Jason posted the part numbers were backwards as compared to physical dimensions as compared to the visual representation.
It should be :
I verified that my rig has two of the larger diameter(0.2mm) and one of the smaller diameter(0.3mm) shims under the sun gear.
So my conclusion is that Arrma somewhere along the line reversed the part numbers and the bill of material then ended up with 0.7mm of shims rather than 0.8mm under each sun gear.
So that variance X 2 for the pair of sun gears, leaves a 0.2mm narrow on correctly assembled diffs based on part numbers.
Does anyone know if that is enough to cause teeth to break? (I'm not an expert on diff shimming.)
So my recommendation is to buy a pack of ARA709060 0.3mm shims and put two of those and one of the larger diameter ARA709059 which should add up to close to 0.8mm of shims (measure each to validate and also test feel of diff without fluid) which is what I believe the Arrma design engineer targeted.
Also to note, I had already ordered and received a pack of ARA709059 and measured it, it was the larger diameter like it should be and it was 0.2mm.
I have notified Arrma (JD) about the part number swaps and he said they will get it updated, he didn't really say if that missing 0.2mm would cause issues, he feels more are failing from the extra shim missing due to QC issues.
Hope this helps some of us and saves us from blowing a diff, I know I will be shooting for the 0.8mm total shims and testing the mesh before filling by feel.
TT
Updates: I rebuilt my rear today using the shims as I suspect the designer meant them to be used, (two 0.3 and one 0.2) for a total 0.8mm under each sun gear. the 0.8mm fits beautifully, and the diff felt perfect both empty and filled.
For the heck of it before filling it, I built it back the way it came to me (two 0.2mm and one 0.3mm) and it felt and sounded a little too loose, so maybe with big power motors it is enough to break tops of teeth off.
I highly recommend everyone build it with 0.8mm.
Last edited: