Good thread discussion. Ball diffs all around are a pain to make them work properly. This is compounded in 4WD off-road for sure. Nick, Definitely get the ceramic diff balls for the main 3/32" size and 1/16" thrust if you are willing to pay. This is another one of those areas on our little cars that truly equal out to more money spent=higher quality (usually this is the case, however, Kyosho over prices their ceramic set so go to Acer). I was shocked to see how bad the stock steel diff balls were that Kyosho packed into the kit. I've used steel many times before, but never have they flat-spotted as quickly as they had in this car. I purchased ceramic sets for both the front and rear diffs in my ZX-5 and they've been doing great since November (last time I rebuilt my diffs). They are still as smooth as the day I put them together. That should tell you right away how long the rings will last. I still haven't bothered opening them up to see if the rings have grooved. When I finally do get around to opening them, I still have the other side to wear away.
Secondly, a lot can be said for how your diff is built. Like DCM already mentioned, not much lube is necessary for the main 3/32" balls. I follow Adam Drake's advice and put a small dab of the Losi or Associated clear grease on either side of each ball. That's it. The thrust is constantly spinning, so it needs to be packed with grease (as DCM mentioned as well). Now, I don't use Losi or Associated thrust grease. I think they aren't that good. Losi's is pure junk, and Associated black grease is 'ok'. Find yourself a high quality high temp./high pressure grease capable of keeping it's viscosity up to high temps. That way, the grease doesn't get slung out of the thrust right when you start to use your car. After your diff is built, I suggest doing the table break-in method. I like to hold one wheel stationary on the table while giving the car some throttle and let the other free wheel spin. I do this back and forth to both wheels 4 or 5 times. Check the diff afterwards to see if it loosened. Tighten it back up to where you'd like it, then run.
Lastly, I'm not sure how many of you have dealt with this problem over in the UK, but here in the states a lot of ZX-5 owners, including myself, were having a difficult time setting the slipper. Improperly set slipper clutches will destroy ball diffs quickly... and that means all the parts. If you don't melt the gear, you'll groove up your rings and flat-spot all the balls. The ZX-5 slippers seems to have way too 'grippy' of pads when brand new. I couldn't get it to slip, and the nut holding the slipper assembly together was only finger tight! I found the only way I could get it to work was to 'break in' my slipper clutch. I set it so loose that the nut could barely keep it together. I took my car out in my street where I would have more than enough traction and I'd constantly floor it from a dead stop. The slipper would slip about 10 feet or so, then I'd turn the car back around and do it again. This worked the pads in real nice and smoothed them out a bit. I only did this for about 5 min. or so. Make sure to take a look at your slipper every 30 seconds or so since this will generate a lot of heat on your spur and possibly melt the gear, but better the spur than a diff gear, in my opinion. After my slipper pads smoothed out, I could then adjust my slipper to slip only 2 feet or so and have it working perfectly where accelerating and landing from jumps didn't create the dreaded diff 'bark' noise.
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