Kronos Stripping Diffs

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Stormwind said:
Update time!

I have just changed the shims in the rear diff over to the Kyosho ones mentioned above, 2x0.15mm instead of of the 2x0.1mm ones from the factory. I did also notice that I had obviously put waaaay to much grease in there the last time I tried to fix this issue.
I cleaned off all the grease and put the diff in dry just to rule out any slippage being caused by the grease.

The damn thing is still slipping. I hold the front tyres up and lightly step on the rear of the rig so those back tyres are going nowhere and I can still see the rear axle hub slip occasionally along with that god awful noise of metal on metal that is not supposed to be there.

I am running out of things to try here other than just chucking a whole bunch of money at it replacing it everything: diff housing, diff spur, new drive pinion (again.... both are relatively new and don't really look worn), possibly even the rear->center drive shaft. First up I'm gonna see if I can dump a third shim in it perhaps.
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Please post photos of your 13t input gear and 43t Diff bevel gear.

If they're pointy or missing teeth entirely - they're no good.

These should not be slipping as described, regardless of how you use shims/no shims.

Shimming correctly ensures the right backlash ie - virtually none - and ensures smooth running. If the gears are already destroyed having been subjected to slipping, no amount of shimming will help.

From what you've described - slipping with no load - it sounds like the actual diff housings are cracked or broken.

Get some decent light on the subject, take photos, and let me see, please. 👍💪
 
It's a bit of a head scratcher, but a couple of things spring to mind in case you've not thought of them yet.......
  • How does the gearbox case look, particularly around the input bevel gear? There's a recessed bearing seat which of damaged would result in poor mesh under load.
  • Do the gearbox halves seat together correctly?
  • Are all the necessary screws in place holding the gearbox halves together?
Happy hunting!
 
I've just stripped the backend down and took some photos.
Originally I thought that the gears looked in good condition when I had them out but checking the photos again now, possibly the input gear is looking a bit work, it's hard to tell since I didn't do a very good job of cleaning the grease off.

During the disassembly, I noticed that the diffs bearings were ****ed leading to the input gear having a bit of play.
Luckily I have plenty of spare bearings kicking around so replaced those removing all of the play.
I put it all back together (dismantling and cleaning a shock in the process) with the hope that this would now be sorted but nope, still there :(

Again I couldn't see any cracks or misalignment or anything obvious in the diff case but I think it's gotten to the point where I am just going to replace the lot like VaporTrail says: a new diff case, input gear, diff gear.

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Stormwind said:
I've just stripped the backend down and took some photos.
Originally I thought that the gears looked in good condition when I had them out but checking the photos again now, possibly the input gear is looking a bit work, it's hard to tell since I didn't do a very good job of cleaning the grease off.

During the disassembly, I noticed that the diffs bearings were ****ed leading to the input gear having a bit of play.
Luckily I have plenty of spare bearings kicking around so replaced those removing all of the play.
I put it all back together (dismantling and cleaning a shock in the process) with the hope that this would now be sorted but nope, still there :(

Again I couldn't see any cracks or misalignment or anything obvious in the diff case but I think it's gotten to the point where I am just going to replace the lot like VaporTrail says: a new diff case, input gear, diff gear.

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Here's a used one I keep around as a spare, very noticeable difference in comparison to yours, I think you may have found your issue

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@Stormwind

Your gears are toast. The input pinion has nothing left to give! 🤣 I would change the 13 tooth input and the 43 tooth bevel gear together. No point mixing up new and old gears. The source of the problem was the worn out bearing, but the result is knackered gears.

Replace the input bearings, the diff bearings, input gears and bevel gears and you're good to go again. No more slipping.

In better news, your diff cases look ok. 👍

Edit: I quoted the wrong post in error - corrected. 🤪
 
Thanks guys. I'll spend some time trying to track down a diff gear again. Modelsport is out of stock again 😭

I'll let you all know how I get on once I manage to get everything changed over.
 
So I've just spent another 3 hours or so stripping down the back end again to replace the 2 gears. Replaced the diff bearings as well and changed the oil again whilst I was at it.
I bought a replacement diff case as well but didn't manage to fit it since this ****ing screw has stripped
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Since the screw is buried deep within that rear tower, I don't see any way I am getting that out other than by sacrificing the tower and taking the Dremel to it to make the recess more shallow. There is no way my screw extractor is gonna reach otherwise :(

That aside, I kept the existing case and re-assembled. Took it outside for a quick shakedown and good news!!!! The issue has gone completely as expected.

Fingers crossed now that the new gears will last more than a couple of packs like the last time I changed them.

I think I've learned a very costly and time consuming lesson here. Look after your bearings!
Now I just need to maintain all my shocks, check my bearings in the front diff, move my cap pack over from the old ESC and the thing will be as good as new :p
 
Stormwind said:
So I originally thought that this couldn't be the case since I had vented the wheels.
Just took it out again to try and diagnose again and found that I had only vented two of the wheels....
Possibly I did all 4 and then replaced 2 with new ones and forgot to vent them or possibly wanted to see how venting would work without doing all four, can't really remember.
This was definitely the case though. I vented the remaining two and ran up the wheels in the air to drain them. You were correct, absolutely soaked. After draining them, it now seems way more stable on the road now.

I'm gonna charge some packs and hopefully give it a proper run at the weekend to see if the diff issues have gone away due to this change now. Thanks very much for the reminder!
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Ah.. the super soaker!! LOL. I was at my brothers apartment about a year ago, His Xmaxx tires were water logged.. we used a soldering iron with pencil tip to "poke" some holes through the tires from the outside..so, what's he do? Put batteries in and give the tires a spin..inside his apartment!!:unsure::rolleyes::ROFLMAO: Hey bro, uh, you might wanna clean up the ceilings in this place?!:rolleyes:😂🤣
 
@Stormwind - has the hex stripped or are you saying the threads have stripped? There are a couple of tricks you might want to use if it's just the hex. Sometimes, you can just cut a small slot across the head of the hex head screw and then use a flat bladed screw driver to remove it.

Hold off destroying the shock tower. Don't do that. That part the screw goes through is only the wing mount, made of composite plastic. There are 2 nylocs on the other side I think. There's more than one way to approach this. 👍
 
Horatio said:
@Stormwind - has the hex stripped or are you saying the threads have stripped? There are a couple of tricks you might want to use if it's just the hex. Sometimes, you can just cut a small slot across the head of the hex head screw and then use a flat bladed screw driver to remove it.

Hold off destroying the shock tower. Don't do that. That part the screw goes through is only the wing mount, made of composite plastic. There are 2 nylocs on the other side I think. There's more than one way to approach this. 👍
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Yeah it's not the shock tower but the wing mount like you said. The hex has gone but I'm not gonna be able to get anything in there to drill it out or anything since it's in the recess of the mount.
I mean the old diff housing is fine at the minute so I'll likely just leave it as a problem for future me to deal with.
 
Yeah, all is not lost (well yet anyway)!

Have you got any Torx bits? These can be used to get a better grip into stripped heads.
Next up for future you to try is a "left handed" drill bit - more often than not, these will catch in the head as you're drilling and spoon the screw out.
Nuclear option, drill the head off with a 3mm drill bit, the head should pop off, wing mount off, leaving just enough to get a pair of grips on to unscrew the remaining stub.
 
Jeffwazz said:
Yeah, all is not lost (well yet anyway)!

Have you got any Torx bits? These can be used to get a better grip into stripped heads.
Next up for future you to try is a "left handed" drill bit - more often than not, these will catch in the head as you're drilling and spoon the screw out.
Nuclear option, drill the head off with a 3mm drill bit, the head should pop off, wing mount off, leaving just enough to get a pair of grips on to unscrew the remaining stub.
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Yes, torx bits have saved my bacon with stripped out hexes many times. The bit should be tight, like hammer it into the screw head tight. Being that it's a recessed screw, IIRC buried pretty deep too, it may end up requiring the wing mount to be sacrificed if all else fails.. would be fairly easy to cut away the plastic if need be to deal with the screw. I'm pretty sure the composite wing mount is cheaper than replacing the shock tower.
 
KnowAir said:
Yes, torx bits have saved my bacon with stripped out hexes many times. The bit should be tight, like hammer it into the screw head tight. Being that it's a recessed screw, IIRC buried pretty deep too, it may end up requiring the wing mount to be sacrificed if all else fails.. would be fairly easy to cut away the plastic if need be to deal with the screw. I'm pretty sure the composite wing mount is cheaper than replacing the shock tower.
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I have a big old set of interchangeable bits for my driver which I have used a torx head a couple of times too.
However, you are remembering correctly and it's buried super deep and narrow. The interchangable bits will not fit at all in the same manner as my screw extractor (left handed drill bit). The only thing that fits in the tube is my dedicated 2mm hex driver.
When it does come to it, I'll likely do as you are suggesting and cut away the plastic. I suspect that a replacement is likely less than £10.
 
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