Diff teardown....shim behind input gear?

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chrisexv6

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Arrma RC's
  1. Kraton 6s
Recently had the drive cup grub screw back out, which caused the input gear to hang out inside the diff case a bit too far. Gouged the aluminum diff cup a bit, but other than needing a good cleaning, all of the teeth look OK. Diff is smooth, etc.

Reassembling everything, I noticed I can move the input gear in and out a bit. In far enough and it will bind while turning the diff. Out far enough you feel something similar to correct motor pinion to spur gear mesh.

Which is correct? Im guessing not binding, but is a motor to spur gear mesh what is correct for the diff input? There were no shims behind it during teardown and it survived this well so far. Just curious if I should buy shims and shim it myself (need a new input bearing anyway, so Ill had to tear it all down again)

The vids I see on shimming the diffs seem to concentrate on whats inside the diff cup, not much said about the input to ring gear.

Thanks.

-Chris
 
Hey Chris.

Just because one guy said he installed a shim, doesn’t mean your diff(s) will require it. You’ll need to take apart your diff assemblies and examine them carefully. Visually examine the depth of the pinion gear in relation to the crown gear and adjust accordingly if needed. Use these diff shimming videos merely as a guideline/reference as it may/may not apply to you until you examine your own diffs.

To bind or not to bind: Keep in mind that these are cast gears so their quality is not as precise and smooth as a set of machined set of gears. Casting proedures leave irregularites which might cause binding (high spots). A slight bind is normal given the quality of these gears... but what I and another member consider a “slight” bind can differ greatly in definition.
 
I guess my question is how to tell the depth of pinion to crown gear once the diff is in the housing. Since you cant see it, you can only feel it, should there be some "play" if I turn the drive cup while holding the diff cup in place.
 
I tried shimming the pinion before. Behind it, shredded the shim, behind the bearing, but ultimately having several 1/8th scale trucks I don't shim it, yes it will have some play, but the shape of it makes it true itself under load. Also don't try to shove the input cup on so close to the bearing, you'll catch the edge of the flat spot and that's why most people have input cups let loose.?
 
Take the bulkhead off to see the diff from underneath. You should feel a tiny amount of rocking (if any) between the diff pinion and crown gear when you put the diff cover back on, as long as they turn freely and aren't too notchy. There should be less play/rocking of these gears than the motor pinion/spur, if your diff gears are spiral cut (Kraton/Outcast/Talion)

The diff input pinion and input cup should be pressed lightly against the bearings they sit against (don't squeeze them too tight! You'll feel a little resistance if you do) to stop any play that will damage your gears and bearings

You can use Kyosho 8x12x0.2 shims behind the diff pinion to bring it forward a little towards the crown gear, but not too close where it will bind against the diff cup. I've not had a lot of luck with these as these shims can shred and end up leaving slop, so I've given up using these shims

But you'll definitely want 13x16 shims on the diff bearings to mesh the crown on the diff pinion
 
Thanks everyone! Going to leave it alone for now...it was built and working fine before, so it should be good now (no evidence of a shim being there but getting chewed up, etc)

Did not even open the diff cup itself, so those shims remain. Did make sure to keep the single shim that was on the ring gear side of the diff as it went back into the case. It spins smooth in the case and out, so Im guessing that part is good.

Overall it feels and sounds like the rear which was never taken apart, so I think Im good! After running the truck and losing front drive I was worried the diff itself was toast but it looked to be just the drive cup loosening up and letting go of the input gear.
 
Thanks everyone! Going to leave it alone for now...it was built and working fine before, so it should be good now (no evidence of a shim being there but getting chewed up, etc)

Did not even open the diff cup itself, so those shims remain. Did make sure to keep the single shim that was on the ring gear side of the diff as it went back into the case. It spins smooth in the case and out, so Im guessing that part is good.

Overall it feels and sounds like the rear which was never taken apart, so I think Im good! After running the truck and losing front drive I was worried the diff itself was toast but it looked to be just the drive cup loosening up and letting go of the input gear.
If I ever get a loose input cup, I take the bulkhead off so I can access both cup and pinion for this reason ? but you might be alright without doing that. As long as you can feel there's no play back and forth at the cup it should be fine
The diff input pinion and input cup should be pressed lightly against the bearings they sit against (don't squeeze them too tight! You'll feel a little resistance if you do) to stop any play that will damage your gears and bearings
 
If I ever get a loose input cup, I take the bulkhead off so I can access both cup and pinion for this reason ?

Stupidly I did not think of putting the input cup onto the pinion while the bulkhead was still disconnected. I did try turning the front wheels while pulling and pushing on the input cup to make sure there was no binding, which there was not.

Its supposed to rain every day until we get the new bearings, so I might be tearing this apart again without using it anyway....at that point I will put it back together correctly (input cup in place before reinstalling bulkhead.
 
Hey Chris.

Just because one guy said he installed a shim, doesn’t mean your diff(s) will require it. You’ll need to take apart your diff assemblies and examine them carefully. Visually examine the depth of the pinion gear in relation to the crown gear and adjust accordingly if needed. Use these diff shimming videos merely as a guideline/reference as it may/may not apply to you until you examine your own diffs.

To bind or not to bind: Keep in mind that these are cast gears so their quality is not as precise and smooth as a set of machined set of gears. Casting procedures leave irregularities which might cause binding (high spots). A slight bind is normal given the quality of these gears... but what I and another member consider a “slight” bind can differ greatly in definition.

That! ^^
 

No doubt, I get that every diff is different (pun)....but I had never seen anyone even mention if you should be shimming behind the input gear or not. I did see a reference to visually checking the engagement of the input to the ring gear, but Im not sure how to see that when the diff is installed in the case.

I will be tearing it down again and installing the drive cup BEFORE reinstalling the bulkhead on the truck....that should help ensure I get it "just right" before reassembling everything.
 
But you'll definitely want 13x16 shims on the diff bearings to mesh the crown on the diff pinion

Are you talking about putting a washer between the bearing on the side of the ring gear and the diff housing?
 
Are you talking about putting a washer between the bearing on the side of the ring gear and the diff housing?
It goes between the OD of the bearing and the bulkhead the entire diff fits in. Some use shims between the bearing and the diff cup, but it's easier to build the diff, then use shims on the outside edge of the bearing to push the entire diff over as needed.
 
It goes between the OD of the bearing and the bulkhead the entire diff fits in. Some use shims between the bearing and the diff cup, but it's easier to build the diff, then use shims on the outside edge of the bearing to push the entire diff over as needed.

Okay what Diameter shims do I need for that
 
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