Kraton K4s 2.5 sounds different after a crash. Where should I dig?

[Barmaleus]

Guys, does this look/sound good?

 

[Dales396lt4]

Looks a little lose but well within spec. Don’t just check it in that one spot check it in four different places on that gear. I have found that Most gears are not round but egg shaped.

 

[Barmaleus]

Looks a little lose but well within spec. Don’t just check it in that one spot check it in four different places on that gear.

Right. Makes sense. Will do that tomorrow, off to bed now

 

[Glitch]

Guys, does this look/sound good?

No, it's definitely too loose.
A nice thing with the new v2.5 cars is that once you assemble the module, you can adjust the mesh from the outside. This is how you should do it, not with the module disassembled

You'll learn how to do the mesh just by doing it. Beginners tend to go too loose with it- you really only need enough play for it to make a clicking sound when moving back and forth, you don't even need to really see the movement as long as it has a definitive click.

 

[Barmaleus]

@Glitch I've never done this process before, so bit scared to mess it up. But it looks like a good time to try it.
I believe those two big-headed bolts (that are used to adjust the mesh) must be thread locked. Looks like the process would be:

1. Take out those bolts, clean them
2. Apply Loctite, partially screw them in
3. Fully assemble the module
4. Adjust the mesh through those special holes and tighten everything up

Will be doing it tomorrow. Need to buy two bearings that hold the diff. One of the old ones is almost non-functional.

 

[Snacktime]

I just bought some 10-12m shims off amazon to fix my brand new 4S slipper play. I can't believe how sloppy the assembly was with all brand new parts. I was getting a lot of noise just setting the mesh, pretty sure the spur gear and motor are not 100% parallel.

 

[Dales396lt4]

With the old 4S system it is quite easy to bend the motor mount, causing it to not be parallel however, this K4s V2.5 has a center differential and new power module, it’s quite different. The old slipper clutches move side to side a good bit, really no need to shim them up. I’m sure it doesn’t hurt. I ran my V2 slipper clutch like that with a 6s system without any problems other than smoking the clutch all the time. Which is why I went to the new power module. Now I’m having problems keeping the spur gear on the center diff.

 

[Barmaleus]

Mmmm, tasty!

 

[Tex Koder]

Mmmm, tasty!

View attachment 363962

mmm.. creamy.

 

[332_RC]

I've seen that black gold before! Everything did its job

 

[Barmaleus]

I've seen that black gold before! Everything did its job

Is it time to replace the diff oil? This is how it looks after 2-3 months or so. Or it's fine to use as is for another few months?

 

[Snacktime]

Got to clean it out, if there is slack in the gear mesh then new internals. I would just put new diff fluid in and keep going if nothing is visually damaged.

With the old 4S system it is quite easy to bend the motor mount, causing it to not be parallel however, this K4s V2.5 has a center differential and new power module, it’s quite different. The old slipper clutches move side to side a good bit, really no need to shim them up. I’m sure it doesn’t hurt. I ran my V2 slipper clutch like that with a 6s system without any problems other than smoking the clutch all the time. Which is why I went to the new power module. Now I’m having problems keeping the spur gear on the center diff.

Building a street basher so I want the slipper for power drifting. Was really tempted to buy a 4s center differential but they are still fetching to much of a premium.

 

[332_RC]

Is it time to replace the diff oil? This is how it looks after 2-3 months or so. Or it's fine to use as is for another few months?

Clean out & replace the fluid. Some of that is the grease, it broke down. I've had mine do that especially in the composite diff cases. The grease gets hot & wears out & turns black. Common.

 

[Glitch]

@Glitch I've never done this process before, so bit scared to mess it up. But it looks like a good time to try it.
I believe those two big-headed bolts (that are used to adjust the mesh) must be thread locked. Looks like the process would be:

1. Take out those bolts, clean them
2. Apply Loctite, partially screw them in
3. Fully assemble the module
4. Adjust the mesh through those special holes and tighten everything up

Will be doing it tomorrow. Need to buy two bearings that hold the diff. One of the old ones is almost non-functional.

That's right! (Although I don't usually clean loctite off screws, seems to have a better hold that way)
I like to start from too tight and then spin the gear to back off the motor, then tighten it down. You don't need any "trick" or anything, the two popular ones (set as tight as you can with a pinion one gear up and then swap, and setting as tight as you can with a piece of paper in between), give you a mesh that is too loose.

 

[Barmaleus]

Sooo. I've:

• Installed new stainless steel grub screw to hold the pinion gear. Made sure it's sitting tight.
• Made sure the spur gear is held tight
• Adjusted the mesh to be tighter

Made a test, 2 battery packs. The car feels more responsive to the throttle trigger and accelerates a bit more "wildly", so to speak. I believe this is due to everything being tightened up in the center diff module.

Sadly, the strange sound did not disappear, so I'll have to dig deeper. The sound can be describe as a slight whine when I take my finger off the throttle and the car rolls passively. Also, when the throttle is somewhere around half pressed, a high-pitched whine appears. Something resembling Formula 1 from the 90's

The car itself feels fine to drive. Not sure what to make out of it. I guess I'll need to disassemble and look into front/rear diffs as well.

Could it be that the motor itself got damaged somehow?

Anyway. More wrenching and testing to do Gladly it's not a relic from the previous century and all the parts can be replaced. Oh, except those that are out of stock until Feb 31st 2026.

 

[Tex Koder]

Sooo. I've:

• Installed new stainless steel grub screw to hold the pinion gear. Made sure it's sitting tight.
• Made sure the spur gear is held tight
• Adjusted the mesh to be tighter

Made a test, 2 battery packs. The car feels more responsive to the throttle trigger and accelerates a bit more "wildly", so to speak. I believe this is due to everything being tightened up in the center diff module.

Good job on getting your Power Module back to ship shape.

Sadly, the strange sound did not disappear, so I'll have to dig deeper. The sound can be describe as a slight whine when I take my finger off the throttle and the car rolls passively. Also, when the throttle is somewhere around half pressed, a high-pitched whine appears. Something resembling Formula 1 from the 90's

The car itself feels fine to drive. Not sure what to make out of it. I guess I'll need to disassemble and look into front/rear diffs as well.

Could it be that the motor itself got damaged somehow?

Anyway. More wrenching and testing to do Gladly it's not a relic from the previous century and all the parts can be replaced. Oh, except those that are out of stock until Feb 31st 2026.

The strange whining sounds could be from a bearing.. either elsewhere in your drivetrain (hopefully)

or back in your motor (that's would be a bit more challenging to replace)

Should be simple to isolate..
Just disconnect the Center driveshaft
and go drive your rig around like normal (in basically RWD mode) and see if it makes the noises.
If so , then you know to focus on the rear of the vehicle..

 

[Barmaleus]

The strange whining sounds could be from a bearing.. either elsewhere in your drivetrain (hopefully)

or back in your motor (that's would be a bit more challenging to replace)

Should be simple to isolate..
Just disconnect the Center driveshaft
and go drive your rig around like normal (in basically RWD mode) and see if it makes the noises.
If so , then you know to focus on the rear of the vehicle..

Great thoughts, thank you! I'll keep digging.

Worst case, something blows up during bashing. Will be sad, but helluva fun

 

[Gedrick]

Thank you guys! Will tighten all up and check the diff fluid.

Also, lifehack. The diff "holder/blocker" part did not want to move. Zip tie to the rescue! Super easy to pull that part out.

View attachment 363668

Ah, that little plate - what I do is shove a flathead screwdriver into the gap (the same place you put the zip-tie) and twist it a bunch while pulling it towards you. I can post a vid later if you like. This also works remarkably well for getting out the power module - just gently slide it into a crevice and give it a twist.

 

[Barmaleus]

@Gedrick Yeah, I think I did it this way the last time. With the zip-tie method I finally have a use for them, tho