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I'm am 99% done restoring an RC10 CE gold tub. I just rebuilt the transmission and I was checking how the differential feels by holding the wheels and spinning them in opposite directions. It has a bit of a "grinding" feel like sandpaper, and it's pretty stiff. It's been about 15 years since this car was in "normal" working condition, so I can't remember if this is normal behavior for the diff. Oh... and this is a 6-gear tranny, and from what I could tell all the gears and tranny parts were in great shape. I appreciate any input. Thanks!

Nope, not normal, means its time for a rebuild. Diff action should be nice and smooth when working properly.

I just took the tranny apart and cleaned everything just with a rag and checked the bearings and gear meshes. Everything that I could visually inspect seemed fine. What do I actually need to rebuild or replace? Do I need to use some kind of lubricant?

The balls may be pitted or flatspotted, may need to replace the balls. The diff rings can be freshened up by taking some 600 grit sandpaper and a nice flat surface, and moving them in a circular motion to smooth any grooves up.
You will also want to use some diff lube on the balls and rings.

blown5.0 wrote:The balls may be pitted or flatspotted, may need to replace the balls. The diff rings can be freshened up by taking some 600 grit sandpaper and a nice flat surface, and moving them in a circular motion to smooth any grooves up.
You will also want to use some diff lube on the balls and rings.

Thanks for taking time to respond. I very much appreciate your help.

Ok, I apologize in advance for what I know is ignorance on my part.... when you're talking about the balls and diff rings, those are the parts that are on the outside of the transmission case, right - directly on the spur gear? I was guessing that the grinding was something happening inside the transmission case, but based on what you're saying perhaps the issue is all related to the "outer" transmission parts? Am I following you correctly?

Should I be lubricating anything inside the transmission case? Currently I'm not doing that....

What blown5.0 was talking about was the ball diff inside of the transmission case. There is small balls that get pressed into the main diff gear. Then the rings go on the outside of those against the sides of the diff gear. Lubricate all of that with diff grease. Put some on the balls when you install them and put a little on the rings, and on the gear teeth themselves.

Sounds as though you have the older 6 gear transmission, Im not sure on those trannies if the internal gears were ran lubed or dry. The stealth transmission gears are ran dry.
What I recommended still applies to the diff though, which from you description sounds like the problem.
I would open the trans. just so you know everything inside is still in good shape while your at it, could even be a bad bearing or worn out bushings causing the gears to shift and bind internally.

Based just on what the manual says, I wasn't expecting to need to lubricate anything inside the transmission. I just now took the spur off completely and I confirmed that both wheels spin freely and independently, so that's making me think my issue is with the external parts with the slipper balls and rings.

As I mentioned, I just rebuilt the transmission yesterday - meaning I took it all apart and checked the bearings and made sure everything visually looked clean and in working order. Maybe I just need to go over the whole thing once more and really give those balls and rings (both inner and outer) a closer look and cleaning. I'll also probably go ahead and put some silicone diff grease on the internal gears as the previous poster suggested - I'm sure it couldn't hurt anything.

the diff on the 6 gear is both inside and outside the case, main gear on the outside, thrust bearing on the inside.
I would recomend cleaning the diff gear and thrust bearing well with denatured alcohol before re-lubing and re-assembling. q-tip to each hole to get all the gunk out, not just a rag to the surface.
Diff balls and plates for the main gear ar cheap enough that I usually just replace them.
The thrust bearing is getting hard to find, so I would clean the balls and retainer out very well and re-use it. sometimes you can get away with flipping the thrust rings over to use the other side if one side is scored or notched from the balls.

There is a post on here somewhere from eau rouge that explains diff building in more detail for a super smooth diff, but this should get you close. Nothing is going to get rid of the grit feeling if the parts are worn out though.

I cleaned the outer diff parts with motor spray and there was a big difference. After putting it back together, I think I still need to go back and clean it even more, but its much better already.

As a follow up question.... for the kimbrough spurs that have places for an inner and outer circle of diff balls (like 12 on the outer circle and then another 8 or so on the inner one).... do I need diff balls in all of those or just the outer ones?

just a single row is fine. Check to make sure the row of balls you choose ends up on the rings, I think some of those gears have so many holes, you could end up running them on the aluminum hubs if you are not careful.