Kraton A(nother) reason to rebuild you diffs from new

[Paul777]

I recently brought a new diff + housing from an unused disassembled V5 kraton and today I set about my rebuild process. But this time I found a new assembly fault. An excess shim - one of the shims used to shim the diff into the housing had been placed between the input pinion and it's bearing. Not only is that the wrong place to put a shim but the size of the shim meant it would have caused all kinds on damage if it had been left there either chewing up the bearing seal, or slipping over the input gear at getting chewed up in the gears. When I first saw if I thought it was some strange bearing design (1st photo) - until I moved it (2nd photo). (As it happens the extra shim was handy as it was exactly what was needed to shim the diff into the housing properly). The bearing was also not seated properly as it had plastic shavings stuck under it - I take it out as I add 2x0.2mm shims behind it.

If anyone is interested this is my rebuild process for the diff internals - all parts are cleaned and thickness measured, then grouped into sets to give most equal thicknesses of the 3 parts combined. The diff housing is also check to confirm that the widths in each direction are equal - this one was, not all are. This hopefully ensures a nice even balance on all 4 spider gears. I have found that I can then add an additional 0.2mm shim on each gear to run as I like it. You can see from the measurements (photo) that putting them together randomly could end up with >0.1mm difference between the largest and smallest set which may be significant if you are trying to run your diffs reasonably tight. When filling the diff oil I also place the output drive gear into the diff and wipe away excess fluid (avoid the screw holes) to get the fluid level just right, then place the crown gear onto it. Some will be interested, others will think I'm just being pedantic and going OTT.

 

[Performance nut]

Blueprinting is something many folks feel is useless because there is no obvious, measurable gain. But many master builders would not have it any other way. OCD... maybe but it might also be the difference between good enough and a diff that works exceptionally well.

Looking forward to seeing the results. I never rebuilt one so I wouldn't even know where to start. I got a recommendation to mess with the rear diff in my Felony... I am not sure I want to mess with it though

 

[IrieWheeler]

...as if we needed more reasons.

 

[extremeengineer]

What were the different measurements inside the diff cup? I mean if we are being pedantic

 

[Paul777]

What were the different measurements inside the diff cup? I mean if we are being pedantic

2 out of 3 of the cups that came with my V4 had a 0.15mm difference across one way compared to the other. In those cases the parts also had significant variance so I was able to counter most of it with the way I grouped the parts. Once assembled I check the internal gap between the pairs of spider gears is the same (or as close as I can get it). I then dry fit the output gear and make sure it sits evenly and has no rocking.

(I brought the new diff to let me have it setup with a different weight oil - the original diffs are still going strong, although I think the input pinions will need swapping out soon as they are showing signs of wear)

 

[extremeengineer]

GD&T, the next big advancement in RC diff maintenance

 

[Paul777]

Looking forward to seeing the results.

I swapped the new diff into the car (trying some thicker oil), I swapped the entire housing so as to maintain all the related shimming. So I thought I'd take the opportunity to inspect the one I took out. I pretty happy with the results after running about 30packs/6S/punch-6 for a mix of jumping, bashing, high speed grass running. There is no detectable wear on of the diff gears, the only sign of use is some polishing on the cross axles and shims and the pin behind the output gear on the crown gear side was broken into 3 pieces This bit is annoying as it maintains a need for more regular inspections so I need to look into options for stronger, but not brittle, pins
Even the input pinion that I thought was getting worn when looking end on in the car is actually fairly negligible when cleaned up and compared directly to the new one. (The V5 input pinion is a bit different, it is darker, seems harder and has more visible signs of being cut rather than cast, this has been highlighted elsewhere as a good upgrade for V4 owners as the part ARA311054 is compatible, V4 and 5 both use the same crown gear )

Old parts on the left , parts from new unused diff on the right. The left includes the additional CoreRC 0.2mm shims I use.